Biological studies on the citrus tree snail Drymaeus dormani (Binney), and the citrus rust mite Phyllocoptruta oleivora ...

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Title:
Biological studies on the citrus tree snail Drymaeus dormani (Binney), and the citrus rust mite Phyllocoptruta oleivora (Ashmead), as well as the effect of different acaricides on the citrus rust mite
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vi, 144 leaves : ill. ; 28 cm.
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Bledsoe, Michael Edward, 1951-
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Citrus rust mite -- Biological control   ( lcsh )
Drymaeus dormani   ( lcsh )
Gastropoda -- Florida   ( lcsh )
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bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

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Thesis:
Thesis--University of Florida.
Bibliography:
Includes bibliographical references (leaves 139-143).
Statement of Responsibility:
by Michael Edward Bledsoe.
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Typescript.
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Vita.

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University of Florida
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Full Text










BIOLOGICAL STUDIES ON THE CITRUS TREE SNAIL
Drymaeus dormani (BINNEY), AND THE CITRUS RUST MITE
Phyllocoptruta oleivora (ASHMEAD),
AS WELL AS THE EFFECT OF DIFFERENT ACARICIDES
ON THE CITRUS RUST MITE











By

Michael Edward Bledsoe


A DISSERTATION PRESENTED TO THE GRADUATE
COUNCIL OF THE UNIVERSITY OF FLORIDA IN PARTIAL
FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY




UNIVERSITY OF FLORIDA


1977










ACKNOWLEDGEMENTS


The author wishes to express his sincere appreciation to Dr. D. R.

Minnick, who served as chairman of the doctoral committee, for his coop-

eration in the pursuit of all investigations undertaken and the generous

donation of his time in counseling and guidance. Sincere thanks are'ex-

tended to Dr. C. W. McCoy for his guidance and assistance in the prepara-

tion of this manuscript. In addition the author wishes to thank Dr.

C. W. McCoy and Dr. L. G. Albrigo for their assistance in preparation

and interpretation of the scanning electron microscope series.

He also wishes to thank Dr. F. A. Johnson and Dr. L. C. Kuitert

for both serving on the doctoral committee and their guidance in em-

barking upon a degree in entomology.

The author thanks Dr. F. W. Zettler for his donation of time in ser-

ving on the doctoral committee. He wishes to express sincere gratitude

to Dr. R. J. Gouger of I.C.I. United States, Inc. for his help in the

field evaluations of acaricides and guidance in choosing an industrial

career.

He would like to thank Dr. Frank Martin, John Schullenberger, and

Tim Breeden, Department of Statistics, UF, Gainesville, for help in de-

signing, analyzing,and interpreting statistical tests used in this pro-

ject.

He wishes especially to thank Mrs. Marcy Lomax of Atlanta, Georgia,

for her patience, and donation of typing skills in the typing of this

manuscript.






Boykin Witherspoon, Jr., of Chevron Chemical Company sincerely is

thanked for his interest and financial support for completion of this

work.

Finally, he wishes to express his love and appreciation to his

wife, Janette Barger Bledsoe, for her continuous help and support in

every aspect of this research and dissertation.


i i i











TABLE OF CONTENTS


Page


ACKNOWLEDGEMENTS . . . .

ABSTRACT . . . .


LITERATURE REVIEW . .
Citrus Tree Snail .
Biology . .
Physical Requirements .
Predators, Parasites, and
Chemicals .
-Food Sources .
*Effects on Yield .
Citrus Rust Mite .
Egg . .
Larva . .
Adult . .
Sex Determination .
Rust Mite Dispersion .
Cultural Practices .
Chemical Control .
Injury to Citrus Fruit
Methods of Sampling .


Pathogens



. .


GENERAL INTRODUCTION . . .


CHAPTER I


BIOLOGICAL STUDIES ON THE CITRUS TREE SNAIL


Section 1. Seasonal Fluctuations of Snails i
Introduction . .
*Materials and Methods . .
Results and Discussion . .
Section 2. Effects of Relative Humidity on S
Activity . .
Introduction . .
Materials and Methods . .
Results and Discussion .. ..
Section 3. Effect of Snail Movement on Fruit
using Scanning Electron Microscopy .
Introduction . .
Materials and Methods . .
Results and Discussion . .
Control Area . .
Ambulatory Movement .
Areas of Snail Grazing .


n Groves .


nail




Microbi
. .

* .
* .
* .


. 12






Section 4. Examination of Snail Fecal Content .
Introduction . . .
Materials and Methods . .
Results and Discussion . .
Section 5. Determination of the Snail Feeding Potential
Introduction . . .
Materials and Methods . .
Results and Discussion . .
Snail Density and its Effect on Sooty Mold .
Snail Density and its Effect on Citrus Rust
Mite . . .

CHAPTER II BIOLOGICAL STUDIES OF THE CITRUS RUST MITE .
Section 1. Methods for Monitoring Citrus Rust Mites .
Introduction . . .
Materials and Methods . .
Hand Lens Method . .
Alcohol Emersion Method . .
Results and Discussion . .
Section 2. Extrinsic and Intrinsic Orientation of Citrus
Rust Mite on Valencia Orange . .
Introduction . . .
Materials and Methods . .
Method of Counting . .
Results and Discussion . .

CHAPTER III EFFECT OF DIFFERENT ACARICIDES ON CITRUS RUST
MTTP


Introduction . .
Materials and Methods . ...
1975 Field Acaricide Spray Tests .
1976 Field Acaricide Spray Tests .
Results and Discussion .
1975 Field Acaricide Spray Tests .
1976 Field Acaricide Spray Tests .
Comparison of 1975 and 1976 Evaluations .
Treatment Data . .
Chemical Standards . .


SUMMARY . .


. 135


LITERATURE CITED . . ... .. ... 139


BIOGRAPHICAL SKETCH .. ..


Page

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63

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92
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120
132
132
134


. 144


.


Ir L









Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy

BIOLOGICAL STUDIES ON THE CITRUS TREE SNAIL
Drymaeus dormani (BINNEY), AND THE CITRUS RULT MITE
Phyllocoptruta oleivora (ASHMEAD),
AS WELL AS THE EFFECT OF DIFFERENT ACARICIDES
ON THE CITRUS RUST MITE

By

Michael Edward Bledsoe

December 1977

Chairman: D. R. Minnick
Major Department: Entomology and Nematology

An arboreal snail, Drymeaus dormani (Binney), and several acaricides

were evaluated for their effect on the citrus rust mite, Phyllocoptruta

oleivora (Ashmead), on citrus in North Central Florida. D. dormani was

found to have the potential to suppress the citrus rust mite populations

by ingestion of the mites and their spermatophore as well as by

deposition of mucilage on the fruit surface. Studies using scanning

electron microscopy to observe the fruit surface, where snails had

grazed, demonstrated the removal of all microbiota. D. dormani feces

were determined to contain fungi, whitefly pupae, citrus rust mites,

citrus rute mite spermatophores, and other mites.

Snail grazing and motion were demonstrated to be dependent on 100%

relative humidity. D. dormani potential for removal of fungi and citrus

rust mites was determined.

Studies of citrus rust mite intrinsic and extrinsic orientation on

Valencia orange failed to demonstrate tree orientation, while orientation

on the fruit was greatest at the marginal (semi-shaded) areas.






In 1975, control of citrus rust mite by PP199 (0.02%) was com-

parable to dicofol (0.03%) standard while oxamyl and PP067 (0.04%) gave

nonacceptable control. In 1976, PP199 and oxamyl exhibited better

control of citrus rust mite than the chlorobenzilate standard. BanexR

gave poor control.







Chairman











LITERATURE REVIEW


Citrus Tree Snail

An arboreal snail, Drymaeus dormani (Binney), was first reported in

citrus groves in Florida by Sellards (1906). D. dormani has commonly

been called "Manatee snail" (Simpson, 1893; Sellards, 1096; Dailey,

1975), "Tree snail" (Sellards, 1906; Norris, 1955), "Citrus tree snail"

(Kramer, 1952), and "Bob Norris snail" (Lawrence, 1950).

L. J. Dorman first collected the snail near St. Augustine, Florida,

and Binney (1857) described the species as Bulimulus dormani Binney, in

honor of Dorman. B. dormani was found to be closely related to B.

masculatus and B. floridanis of New Grenada (Binney, 1859; Binney and

Bland, 1869).

The genus was changed to Liostracus dormani (Tryon, 1882). Pilsbry

(1899) assigned the citrus tree snail to its present genus.

The current hierarchical classification of D. dormani is as follows:


Kingdom Animal

Phylum Mollusca

Class Gastropoda

Subclass Pulmonata (air breathers)

Order Stylommatophora (terrestrial)

Family Bulimulidae


The range of the snail is limited in the U.S.A. to Florida (Grif-

fiths, 1949). Pilsbry (1946) stated that D. dormani was "an









apparent descendent of a Mexican species, and it probably migrated to

Florida via the Southern United States in Pliocene times" (p. 24). By

1895, following the description given by Binney (1885), the snail was

reported near the Manatee River, at Port Orange, Florida, and Oak Hill,

Florida, in Volusia County, and on Florida's West Coast. Simpson (1893)

reported finding several hundred dead shells in a heavy hammock north of

the Manatee River.

Very little practical interest was accorded the citrus tree snail

until Sellards (1906) reported that it was feeding on sooty mold on

citrus in Manatee County, Florida. It was brought to Sellards' atten-

tion by Mr. F. D. Waite who,in 1904,first noted the cleaned fruit and

leaves. The use of the citrus tree snail as a biological agent grew

from 1906 until Norris (1946) summarized existing knowledge on the

history of the snail from this time.

Biology. Binney (1878),giving a general description of land snails,

stated they normally lay their eggs in the soil and were phytophagous.

Griffiths (1949) gave an estimate of 40 or more eggs played in mass by

the citrus tree snail. This was later defined as 40 to 400 eggs per

egg mass (Muma, 1955). The newly hatched snail measures 2 mm or less

(Norris, 1946; Griffiths, 1949).

The snails were believed to take up to two years to reach full

maturity (Tryon, 1882) until Muma (1955) recorded mature snails in one

year or less. Fully mature shells reach 3 cm in length (Griffiths,

1949).

Lawrence (1950) stated that the snail was hermaphroditic, laying

its eggs in the soil and leaf mold at the base of citrus trees during

the summer rainy season. Griffiths (1949) placed oviposition in the








early summer months over a period of six to eight weeks, possibly

triggered by onset of warm weather.

Morphology of the shell was described by Sellards (1906) as

smooth, white or coneous white, with about four bands of brown spots.

Old shells often have corroded surface, the bands becoming indistinct

or absent.

Physical requirements. The order Stylommatophora or terrestrial

air breathing mollusks have learned to control water loss by frequenting

areas of high environmental humidities (Edney, 1960). Boycott (1934)

found only 12 obligatory hydrophiles and eight species of xerophiles.

Hunger (1964) stated that water is primarily lost through general

evaporation from the moist skin, but some loss resulted from the con-

tinued pedal secretion of mucus for locomotion. The land snails are

active at night or just after rain (Binney, 1878). Binney also mentioned

the epiphram, a semitransparent membrane-like structure, which is

secreted by the snail during hibernation to attach the shell to substrate

and help reduce water loss.

Griffiths describes the onset of hibernation in December coinciding

with winter cold. Activity is reinitiated in the spring. He also

suggested that breaking of dormancy may have to do with tree growth

and availability of water.

The "resting stage," that period when the snail is withdrawn into

the shell, is generally divided into two categories. The first is for

the purpose of hibernation. Hibernation is usually for overwintering

and the mouth of the shell is sealed by secretions of calcareous

material and hardened mucus termed the "epiphragm" (Hunter, 1964). The

second type of "sealing off" occurs daily during unfavorable conditions.








Howes and Wells (1934) referred to it as an estivation period. During

estivation, a "mucous veil" of dried mucus seals the opening. The use

of the term estivation here is unfortunate since the closing off is

short term and may occur at any time of the year.

Mature citrus trees six years old and older are the only ones from

which the snail can receive the proper abiotic conditions for survival

(Kramer, 1952). Cover crops were suggested by Kramer (1952) as a means

of increasing the shade and relative humidity in the grove undergrowth

for snail migration. He also suggested this would increase humidity in

the trees.

Hunter (1964) discussed the ability of stylommates to maintain tem-

perature homeostasis by withdrawal into the shell. The migration of

snails toward a preferred temperature or habitat was discussed by Getz

(1959). He showed the snails possessed a temperature and humidity

referendum.

Calcium is necessary for development of snail shells. Experiments

on land snails in captivity show that the thickness and weight of the

shell is directly dependent on the amount of calcium in food supplied

(Boycott, 1934; Robertson, 1941). Some snail species are limited to

high calcium content soils (Boycott, 1934).

Predators, parasites, and pathogens. Muma (1955) suggested

factors that cause mortality of D. dormani. Egg mortality was due

to desiccation and to small earthworms. Newly hatched snails were

preyed on by a predatory snail, Euglandina rosen Ferrusac. Additional

mortality was attributed to desiccation. Young snails were found to be

parasitized by two Diptera, Johnsonia elegans Aid. and Johnsonia sp.

probably frontalis Aid. Euglandina rosea also was found to prey on

young snails (Muma, 1955).








Predators of mature snails include birds, mice, and again, E. rosea.

Two disease-like conditions were described as green body and brown

body. Two parasitic flies, Sarcophaga lambens and S. morionella, were

associated with mature snails, as were Hippelates dissidents (Tuck)

and megaselia sp. (Muma, 1955).

Chemicals. The first study of the effects of chemicals on the

citrus tree snail was undertaken by Norris (1946). Zinc sulfate or

sulfur was shown safe to snail colonies. Kramer (1952) warns of dusting

or spraying snail infested trees with the exception of sulfur. He

also states that fertilizer applied to the base of citrus trees during

summer months should be avoided because of its toxicity to young snails.

The nitrogen portion of chemical fertilizers is toxic to the snail

causing immediate withdrawal into the shell and death (Muma, 1955).

He indicated that zinc and copper were repellent to the snail because

no feeding occurred on leaves dipped in these solutions. Boron and

manganese displayed no toxic or repellency effects, while arsenic

treated leaves were readily fed on and resulted in death. Muma also

recommended sulfur, lime-sulfur, and to a limited extent oil in con-

junction with snail culture.

Food sources. Binney (1878) described the land snail as a phyto-

phagous with no mention of any other food source. Algae was reported

as a food source by Pilsbry (1946). Sellards (1906) first recorded

D. dormani feeding on sooty mold on citrus in Manatee County, Florida.

Kramer (1952) stated that the snail consumes neither scales or mites

but, due to the deposited slime trail, makes conditions unfavorable to

these pests. Large lichens and entomopathogenic fungi on white fly

were reported unaffected by the citrus tree snail (Griffiths, 1949).









Melanose infestation serious enough to cause economic damage was

not found in snail groves from 1946 to 1948 (Griffiths, 1949). Grif-

fiths speculated as to the possibility that the snails may eat the

spore normally found on the tree trunk.

Effects on yield. Griffiths (1949) in a study of the effect of the

snail on yield production compared to nonsnail trees was unable to show

an increase or decrease in production costs or yield in the groves.

Muma and Selhime (1963) considered the purple scale, Florida red scale,

Chaff scale, Glover scale, dictyospermum scale, citrus rust mite, Texas

citrus mite, and citrus red mite on snail and nonsnail trees. They

reconfirmed Griffiths' findings that no difference could be found from

snail versus nonsnail trees. Sprayed plots, however, maintained lower

infestations than the unsprayed snail plots with exceptions of Florida

red scale and citrus red mite. Muma and Selhime (1963) also found that

sprayed plots produced a higher quality fruit with a higher yield than

the unsprayed acreage.

Citrus Rust Mite

The citrus rust mite, Phyllocoptruta oleivora (Ashmead), is con-

sidered a serious pest in all humid citrus growing regions of the world

(Delucchi, 1975). This species was introduced into the Western Hemisphere

from Southeast Asia (Yothers and Mason, 1930). The citrus rust mite be-

longs to the family Eriophyidae. It is tetrapodili having only four

legs located anteriorly near the mouth. The eriophyids characteristi-

cally have elongated bodies annulated with small spines or furrows giv-

ing a segmented appearance (Krantz, 1975). A complete generation can

be completed in six to eight days during warm seasons in subtropical

regions (Delucchi, 1975).








Egg. The egg of the citrus rust mite is round, white or pale

yellow, with a smooth surface approximately .03-.04 mm in diameter

(Swirski and Amitai, 1958). Others and Mason (1930) referred to de-

tection of a folded larvae visible within the egg a few hours prior to

hatching. Both Yothers and Mason (1930) and Swirski and Amitai (1958)

found the egg incubation period to be 3.1 days.

Larva. Swirski and Amitai (1958) described the first and second

stages of the citrus rust mite. The first stage is white and measures

about .08 mm in length. The second stage ranges from .10to .12 mm and

is pale yellow. Developmental times varied according to temperature

with 3.1 days at 32.60C and 10.7 days at 25.10C (Yothers and Mason, 1930).

Adult. The adult citrus rust mite is generally wedge-shaped, pale

yellow, and 1/200 in.long (Muma, 1965). The adult becomes light brown

to brown with age (Swirski and Amitai, 1960). The female is 0.15-0.16

mm in length and lives up to 16 days at 260C (Swirski and Amitai, 1959)

while the male is only 0.13-0.14 mm in length (Keifer, 1938; Swirski and

Amitai, 1959).

Sex determination. Others and Mason (1930) described the citrus

rust mite as parthenogenetic when they were unable to find any males.

Ebling (1959) found 39% males present in units of 144 individuals.

Swirski and Amitai (1960) recorded males to be present throughout the

year with increase in spring and decrease numbers in autumn.

Independent observations were made by Oldfield et al. (1970),

Sternlicht (1970), and Sternlicht and Goldenberg (1971) as to the occur-

rence of spermatophores in Eriophyidae-and the females self-fertilization

(laying eggs resulting in offspring of both sexes, as opposed to un-

fertilized females which lay parthenogenetic eggs bearing only male








offspring). This type of fertilization is common in tetrapodeliform

acari (Shevtchenko, 1957; Hall, 1967; Oldfield et al., 1970; Sternlicht

and Griffiths, 1974).

The use of spermatophore is a primitive method of insemination

where a male deposits a spermatophore (sperm sac) on the substratum

(Chapman, 1971). P. oleivora spermatophore consists of a base, a stalk

approximately 10 p in length, with an expanded apical head (12 P in

diameter), and capped with a spherical sperm bearing case approximately

3 v in diameter (Oldfield et al., 1970; Sternlicht and Griffiths,

1974).

The male P. oleivora produced approximately 16 spermatophore per

day (Oldfield et al., 1970). The sperm capsule of the spermatophore

of P. oleivora is removed and taken into the female (Oldfield et al.,

1970). The attraction of the spermatophores for virgin females of

Eriophyes sheldoni Ewing was noted (Sternlicht and Goldenberg, 1971).

This was later confirmed with Aculus cornutus (Banks) (Oldfield et al.,

1972).

Rust mite dispersion. Bodenheimer (1951) found the citrus rust

mite primarily on the outer canopy with the highest numbers near the

crown of the tree during the warm season. During winter months these

mites concentrate on the undersides of the citrus leaves and the in-

terior portions of the tree (Yothers and Mason, 1930; Swirski, 1962).

Hibernation was suggested by Bodenheimer (1951) as a means of over-

wintering severe conditions.

Positive phototaxis was demonstrated by Yothers and Mason (1930),

but the eriophyids avoided the direct sunlight. A semi-shaded preference

of the citrus rust mite on citrus fruit was mentioned by Watson and








Berger (1937). They found rings of rust on citrus fruit corresponding

to the aforementioned semi-shaded areas.

Citrus rust mite populations are extremely variable from tree to

tree and from various parts of the same tree. This variation in popula-

tion distribution was substantiated by Osburn and Mathis (1944).

Swirski (1962) felt that the recorded discrepancies were due to a lack

of knowledge of density and its relationship to time and space.

Pratt (1957) reported two peaks of infestations annually during the

summer months. He attributed this to the number of hours at dew point.

Rasmy et al. (1972) was not able to support his correlation to relative

humidity, but was able to show a relation to temperature.

Cultural practices. Cover crops, the cultivation of annual crops

in citrus groves, were studied by Osburn and Mathis (1944). They felt

cover crops helped maintain a humid condition that supports parasites

and especially fungi that attack the citrus rust mite. They were

unable to support this hypothesis, finding that cover crops had very

little affect on relative humidity, temperature, parasites, and amount

of fungi present. However, they did state that clean culture stimulated

tree growth and gave an overall impression of a healthier tree.

The condition of the citrus tree has been suspected of affecting

rust mite populations. Hamstead (1957) demonstrated a correlation

between rust mite populations and high nitrogen levels in leaves. Leaf

age was shown to cause variations in populations (Mohamed, 1964). Muma

(1965) attributed fluctuations in population to leaf drop and wind.

Cultural practices such as hedging or thinning of citrus trees were

found by Swirski (1962) to improve conditions for the citrus rust mite.

Overhead irrigation was shown to cause a sixfold mite popula-








tion increase (Rasmy et al., 1972). Tree planting distances, especially

relating to tree crown distances, were related directly to rust mite

density (Swirski, 1962).

Chemical control. Others and Mason (1930) gave an account of

tobacco and whale-oil soap being used to eliminate the russeting damage.

Sulfur was introduced as a miticide,but was found (Speare and Yothers,

1924; Griffiths, 1950) detrimental to fungicidal activity by reducing

entomopathogenic fungi attacking the citrus rust mite. Scheduled treat-

ments were applied in the spring, summer, and the fall. Numerous acari-

cides such as chlorobenzilate, ethion, sulfur, and dicofol can be used

for control of P. oleivora (Florida Citrus Spray and Dust Schedule,

1977).

Injury to citrus fruit. Citrus rust mite has been reported to be

responsible for three visable types of fruit injury, namely sharkskin,

russet, and bronzing (Griffiths and Thompson, 1957; Albrigo and McCoy,

1974). McCoy and Albrigo (1974) demonstrated that injury to the sur-

face of citrus fruit by P. oleivora is restricted to epidermal cells.

Sharkskin which is found on grapefruit, lemons and limes (Yothers and

Mason, 1930; Griffiths and Thompson, 1957) is occasionally found on

oranges. This is characterized by severe damage at an early age.

Further fruit growth results in cracking of the dead epidermis in pat-

ches which may slough off, leaving a smooth injured periderm (Albrigo

and McCoy, 1974). Russet damage, which occurs prior to fruit maturity,

results in additional fruit growth,which breaks up dead epidermis,and

subsequent wound periderm formation beneath the epidermis. The cracks

result in an unpolishable rough texture, while the oxidized cell con-

tent gives the fruit the rust color (McCoy and Albrigo, 1974). Fruit








bronzing is damage to the surface of citrus fruit when little additional

fruit growth will occur. P. oleivora feeding causes epidermal cells to

die and turn brown, but the cuticle does not crack. These fruit will

take a polish because the cutin and waxy layers remain intact (McCoy

and Albrigo, 1974).

Methods of sampling. The square method of sampling was described

by Others and Miller (1934) as a piece of paper with a half inch square

hole cut in it. The paper was placed over the surface of the citrus

leaf and mites counted by viewing through a 10X hand lens. A linen

tester with a defined field of 0.5 in. x 0.5 in. was used to establish

P. oleivora infestation by Osburn and Mathis (1944). Pratt (1957),

Johnson (1960), and Simanton (1960) used 10X hand lens to count citrus

rust mites per field of view. A stereoscopic microscope at 18X

magnification was used to count mite populations on leaf samples (Dean,

1959). Later that year, a brushing machine was used to gently brush the

mite population off the surface of leaves by Dean and Sleeth (1959),

then by Bailey and Dean (1962).

A method of removal of all citrus rust mites from the fruit

surface was described by Muma (1965). He washed the fruit in an alcohol

bath while still on the tree. Another method of rust mite sampling was

described by McCoy et al. (1971). An index for the number of mites

per leaf was determined by counting the mites within four microscope

fields, two on the upper and two on the lower leaf surface, at 10OX

magnification.

Allen (1976) has recently developed an attachment which fits a

10X hand lens and defines a 1 cm2 field.













GENERAL INFORMATION


The citrus industry in Florida prior to 1904 was confined to the

Northeastern part of the state. During this period of time D. dormani

was believed to have significant importance on the health of the citrus

tree. With the advent of synthetic pesticides following World War II,

coupled with changes in cultural practices and a general southerly

movement of the industry in Florida, a decline in the citrus tree snail

has resulted. Along with this decline there have been increases in the

citrus rust mite, disease problems, and a greater dependence of the

gorwer on the use of pesticides. This study was conducted to determine

the role the citrus tree snail has in relationship to the following:

1. The seasonal fluctuations of snails in citrus groves,

2. Effects of relative humidity on snail activity,

3. Effects of snail movement and feeding on fruit microbiota

using scanning electron microscopy,

4. Examination of snail fecal content,

5. Determination of the snail feeding potential.

Additional studies of the citrus rust mite were conducted on the

following:

1. Method for monitoring citrus rust mites,

2. Extrinsic and intrinsic orientation of citrus rust mite on

Valencia orange,






13

3. The effects of various candidate acaricides on control of citrus

rust mite.













CHAPTER I
BIOLOGICAL STUDIES ON THE CITRUS TREE SNAIL

Section 1. Seasonal Fluctuations of Snails in Groves


Introduction

No information is available on D. dormani distribution on citrus,

therefore studies were conducted to determine the distribution of D.

dormani within a citrus grove in Florida's northern citrus growing re-

gion from 1976-77.

Materials and Methods

A map of the grove was made indicating the position of each tree

in the grove. During the winter months the snail colonies migrate to

the dead wood and protected areas of the trees. Burlap feed sacs, one

per tree, were placed in the lowest fork of the tree trunks, during the

summer months, to create artificial snail harborages for the hibernating

snails. The snails then could move under these sacs during the winter

for protection.

On March 18, 1976, and April 7, 1977, while in hibernation, the

snail colonies were examined on each tree. Population and percent mor-

tality counts were made under each burlap sac and within dead wood.

The number of viable snails and total shell counts were recorded as V/T,

that is those viable (V) out of the total (T) shell count.

Results and Discussion

The snail population recorded in 1976 showed that this grove con-

sisted of a pocket of snail trees surrounded by nonsnail trees (Table 1).








Table 1. Drymaeus dormani population counts for 1976. Population de-
termination was made during winter hibernation of the snail
during 1975-76. Examination of deadwood and under artificial
snail harborages gave indications of snail populations.



Row

Column 1 2 3 4 5 6 7 8 9


X X
4/6 X
X X
X
X X
X

X
X
X X
X
X X
X X
X

X X
X

x X
X X
x x
x X
x
X X
X
X X
X


X

55/66
X
X

X
X
X
X
X
X
X
X
X
5/6
X
X
X
X
X
X
X

X
X
X


41/53
46/47
89/89
101/101
X
54/55
39/45
56/70
73/73
49/60


X
15/15
9/9
14/15
X
X
X
X
X
X
X
X
X
X
X
X
X
X


96/97
X
87/88
98/98
X

54/56
X
46/48
X


X
X
14/15
26/27
X
X
X
X
X
X
X


X


X
X


91/91
54/60
73/73
104/104
130/130
45/50
71/77
36/90
70/71
111/111
X
35/37
47/49
X
X
X
X
X

X
X
X
X


X
13/16
50/51
36/40
55/55
60/61
77/77
30/31
X
X
X
16/81
15/20
X
16/17
X
21/21
X
20/21
X
X
X


44/50
16/20
73/73
58/58
47/47
50/50
66/67
X
X
111/111
69/70
81/81
17/19
X
X
X
18/19
X
19/19
X
X
X
X
X
X


36/36
X
40/54
X
X

58/58
60/60
X
X
X
X

X
X
10/12
X
X


X
X
X


X
X X


X = Citrus tree present but no snail population
V = Number of viable snails
~T= Total number of snails
Space = No tree





16


Table 1. Continued.


Row

Column 10 11 12 13 14 15 16 17 18


1 45/50 50/53 X X X X
2 X X X X X X
3 45/50 15/20 5/5 X X
4 X X 41/50 1/1 X X X X X
5 58/58 4/9 42/42 14/15 X X
6 78/81 17/25 X 17/25 X X
7 X X X X X X X
8 X X X X X
9 10/12 X X
10 18/18 11/19 X X
11 9/10 X X X X
12 X X X X X
13 33/40 43/43 X X X X
14 X 53/55 X X X X X
15 X 40/40 X X X
16 X X X X X X X
17 X X X X X X X
18 14/15 16/17 X X X X X X X
19 17/19 16/16 X X X X X
20 19/19 15/18 X X X X X
21 X X X X X X X X
22 X X X X X X
23 X X X X X X X X
24 5/6 X X X X X X
25 X X X X X X X
26 X X X X X X X X
27 X X X X X X X
28 X X X X X X X
29 X X X
30 X X X X X X








There were 93 snail trees recorded. These trees had a total population

of 4,137 viable snails and 283 dead snails. The dead snails represented

6% of the population, and are believed to have died during hibernation.

According to visual observations this snail population in 1976 is

believed to have declined from the previous year. In 1975, snail trees

could be identified from several feet away by the unusual shiny appear-

ance of the leaves. During 1976, this characteristic was not evi-

dent.

The snail population census was taken again in 1977 in the same

grove (Table 2). A total of 227 viable snails were located on 74

trees. This represented a 95% reduction in the snail population from

the previous year. Mortality within the snail harborages were also

greater. Where 6% mortality was recorded in 1976, the 1977 census

indicated 20% of those snails reaching the snail harborages died. In

1976, there was an average of 45 snails per snail tree compared to an

average of three snails in 1977. This reduction is believed due to

two major factors. The first factor was a change in cultural practices.

The grove originally was maintained with a ground cover, a summer grass,

which was believed by the owner to increase the humidity on the ground

and in the tree. This practice was replaced by one of total tillage

"clean culture." Snail migration from tree to tree and egg deposition

in the soil are believed to depend on the ground cover. Also, a tree

pruning program was initiated during this time. All trees were hedged

with mechanical hedgers in an east-west pattern. This hedging opened

up the trees to the drying effects of wind and sun, as well as made the

snails more vulnerable to bird predation.








Table 2. Drymaeus dormani population determination was made during win-
ter hibernation of the snail during 1976-77. Examination of
deadwood and under artificial snail harborages gave indications
of the snail population.



Row

Column 1 2 3 4 5 6 7 8 9


X 2/3
X X
X 3/4
X X 0/4
X X
X X
X X 2/2
2/2 X
X X
0/1 7/11
3/6


X X X
X X
X
X X X
X X X
X X X
X X X
X X
X X X
X
X X X
X X
X X


6/8
X
13/15
0/2
X

5/2
X
9/11
X


X
X
0/2
6/9
X
X
X
X
X
X
X


4/7
X
9/19
13/14
9/10
X
5/5
12/14
2/4
1/1
X
8/10
0/2
X
X
X
X
X

X
X
X
X


X
X
X
X
5/4
X
2/4
X
X
X
X
1/1
X
X
X
X
X
X
X
0/1
X
X


3/6
X
X
X
0/1
X
1/1

5/7
7/9
0/2
X
0/3
X
X
X
X
X
0/1
X
X
X
X
X
X


1/1
X
7/10
X
X
5/7
X
4/7
X
X
3/4
X

X
X
0/3
X
0/1


X
X
X


X
X X


X = Citrus tree present but
V = Number of viable snails
T = Total number of snails
Space = No tree


no snail population








Table 2. Continued.


Row

Column 10 11 12 13 14 15 16 17 18


X
0/1
0/1
1/3
0/3
1/3
X
X


X
x
x x
X X


X X
X


9/9
X
4/4
X
14/16
0/2
1/1


1/1
X
X
9/13
X
X
0/3
X
0/1
X
3/3
X


1/1


1/2
0/1
0/3
X
x

x
X

X


1/1
x
X
5/5
X
12/17
1/3



0/1

X
0/1
X
X
X

X
0/1
0/1


X


X X X
X X
X 1/2 X
X X X X

x x x x

x x x x
X X X X
X X X X
X X X

X X X X


x x x x
x x x x
x x x
X X X






x
x x x x
X X X
X X X X
X X X X
X X X X
X X X X
X X X
X
X X X X


X
x

x
X

X


X
X X









The second major factor believed responsible for the decline in

the snail population was the severe freeze during January, 1977. This

freeze resulted in crop losses, and up to 95% defoliation of trees. It

is possible that the snail harborages were not sufficient to protect

the snail colonies.

A combination of the aforementioned factors is believed to have

accounted for this radical population reduction. Within two years, a

thriving snail culture was reduced to near extinction. This reduction

was accomplished without the use of any pesticide sprays. It simply

reflects the fragile balance needed to maintain a snail culture.

Observations made during the summer following the winter popula-

tion counts of 1977 reflected the population decline. A total of eleven

snails were located within the entire grove area. Further population

reductions were possible, due to an extremely dry spring. Unless cul-

tural practices conducive to snail growth are resumed, this grove will

no longer harbor a viable snail culture.

This survey indicates that the cultural practices once used to

help maintain a snail culture played an important role. These practices,

however, are now in conflict with modern techniques. Ground cover is no

longer used because of water loss and nutritional consumption by the

grasses. Dead wood,once used by the snail for overwintering, now is

removed and burned. Dense tree canopy is reduced by row pruning to

allow for easier picking and grove maintenance. All of these techniques,

once common in citriculture and important to snail survival,are no

longer practiced.








Section 2. Effects of Relative Humidity on Snail Activity


Introduction

No information has been reported on the effect of environmental

conditions on the citrus tree snail's movement. Therefore, studies

were conducted to obtain information on the relationship of snail ac-

tivity and relative humidity. This information is necessary to de-

termine if relative humidity is a limiting factor in snail activity.

If snail activity is directly dependent on relative humidity, it would

be possible to monitor for periods favorable for the snail and to deter-

mine the hours of snail activity.

Materials and Methods

A naturally occurring population of the citrus tree

snail, D. dormani was located within a grove in Orange Lake, Florida.

The grove was unsprayed and was maintained with a cover crop or ground

cover. It was observed that the snails were generally active at night

or during heavy rains. Twenty snails were located to determine the

snail activity and relationship to relative humidity. Ten quiescent

snails on each of two adjacent trees were located and marked on the

dorsum of the shell with an identification number using Day-glowR paint.

Application was made with a one milliliter tuberculin syringe. A

spring type clothespin was placed adjacent to each snail and was marked

with the same number as the snail.

A battery operated ultraviolet lamp, model U.L.V.-56, illuminated

the Day-glowR pigment which facilitated locating the snails at night.

Relative humidity, time, and percent snail activity (percent of snails

active per unit time) were recorded every half hour from 9:00 p.m. un-

till 8:00 a.m. The experiments were terminated at 8:00 a.m. because








of solar light decreasing the effectiveness of the artificial light.

Relative humidity was measured with a Bacharach sling psychrometer.

The clothespins were placed adjacent to the snail following each read-

ing to help relocate the snail at later time intervals. This experi-

ment was replicated three times over a three year period.

Results and Discussion

The first experiment on August 27, 1975, showed a correlation in

line slope between the increasing percent relative humidity and the

increasing percent snail activity (Figure 1). In this case a slope (M)

of M=2 was found for both items. It is interesting to note that as the

relative humidity reached 100%, the first trace of snail activity was

noted. The dotted line demonstrates this phenomenon. The accompanying

snail activity line then proceeds at the same rate of increase as did

the percent relative humidity line, that is the slopes (M) were the same.

A similar relative humidity and snail activity slope correlation

can be seen with the April 28, 1976, observation (Figure 2). The first

traces of snail activity began with the relative humidity reaching 100%.

Again there are the similar slopes of the lines, in this case M=4.3.

It would appear that the increase in relative humidity may predispose

or effect the ensuing snail colony's activity pattern. Similar line

slopes for relative humidity and snail activity were found on two of

the three replicates.

On July 18, 1977, the final field observation was made (Figure 3).

A unique situation occurred where 96% relative humidity was maintained

for seven hours prior to reaching 100%. Even under this extended dura-

tion of high humidity the first snail activity did not begin until 100%

relative humidity was obtained. This would indicate a need for 100%





































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relative humidity or saturation for initiation of snail activity. In

each of the three observations over the three years, 100% relative hu-

midity was obtained before the initiation of any significant snail ac-

tivity. The July 18, 1977, observation resulted in a snail line slope

of M=3. The resulting line slope of the percent relative humidity was

M=0 until 3:30 a.m. when it began increasing. Saturation was reached

at 3:45 a.m. The slope formed from 96% to 100% relative humidity was

M=3, the same as with the snail activity line slope. Again the correla-

tion of the slopes was demonstrated.

The problem with locating the snails during light hours made snail

activity determination extremely difficult. For this reason no attempt

was made to determine the relationship of the termination of snail ac-

tivity with a decreasing relative humidity. Smith, 19761, (unpublished

data) was able to demonstrate a rapid decline in snail activity when

the relative humidity fell below 100%, again supporting the need for

100% relative humidity for activity.

The citrus tree snails were determined to be active only during

periods of 100% relative humidity. This was demonstrated in all three re-

plicates. D. dormani,which is dependent on water, has evolved to an

arboreal habitat and utilized available water sources found during 100%

relative humidity. Locomotion is believed limited to this condition

due to excessive pedal secretions used to maintain adhesion to the sur-

face. If sufficient water was not present the snail would quickly

desiccate. During showers the snails have been observed to be active

throughout the day. This would suggest the dependence of the snail on


Smith, B. 1976. Graduate Student. Department of Entomology and
and Nematology, I.F.A.S., (University of Florida).








water and not on nocturanl conditions. The author wishes to acknowledge

the possibility that the snail may be dependent upon free water which

develops on the leaf surface. Further studies examining this environ-

mental factor are needed.


Section 3. Effect of Snail Movement on Fruit Microbiota
Using Scanning Electron Microscopy

Introduction

No information was available on D. dormani effect upon the micro-

biota of citrus. Therefore, this study was undertaken to characterize

the microstrata of the gruit surface and the effect of D. dormani on it.

Materials and Methods

Scanning electron microscope work was done at the Orlando location

of the U.S.D.A., Horticultural Research Laboratory, Orlando, Florida.

All D. dormani used were collected in Orange Lake, Florida, from the

Parson Brown variety of Citrus sinensis (L.) Osbeck. Green fruit, 6.5

to 7.0 cm in diameter, were collected in Lake Alfred, Florida, and were

selected on the basis of high citrus rust mite spermatophore counts.

A glass aquarium (1' x 2' x 1 1/2') used to house the snail was

maintained at relative humidity (R.H.) of 90% + 2% by filling the

bottom with one inch of water. D. dormani were placed on five oranges

suspended by cotton strings attached to the top of the aquarium. The

string was tied to paper clips partially opened and inserted into the

fruit.

Snail trails were recognized easily by a wet silvery sheen left on

the fruit surface. Each trail where feeding had occurred was marked

with a felt tip marker for identification. Twelve 4 x 4 x .05 mm samples

were carefully removed with a single edge razor blade from the surface

of the fruit in each of the following three areas: control areas (not








visited by snails); ambulatory areas (areas visited by snails without

feeding); and grazing areas (areas visited by snails where feeding had

occurred). Each of the labeled samples were placed into a solution of

50% Gluteralaldahyde and 50% phosphate buffer at pH 7.0 for two hours

(Anderson, 1966) and then reduced to 2 x 2 x .05 mm samples. Samples

were left in osmium for two additional hours, removed, washed, and

placed into a phosphate buffer (pH 7.0) for approximately 18 hours.

The following day the samples were subjected to both an acetone

dilution series and a Freon TF dilution series which consisted of 5,

10, 20, 30, 40, 50, 60, 70, 80, 90, 95, 100, 1002, 1003% washes for ten

minutes each. Samples were critically point dried using a Freon 13

critical point dryer.

The specimens were placed on one centimeter cylindrical studs and

held in place by a conducive silver glue. A Hummer II sputter coater

gold plated the sample's surface. A JEOL/JSM-35 scanning scope was

used to review and to photograph the surface. Surface photographs were

taken with a polaroid camera horizontally mounted to the scanning

electron microscope.

Results and Discussion

Control areas not affected by the citrus tree snal are covered

generally with microbiota (Figures 4-9). The ambulatory areas demon-

strated the ability of D. dormani to encrust with mucilage the surface of

the fruit. Spermatophores were pressed to the surface and their sperm

sacs ruptured. Mycelia were encompassed generally within the mucilage

(Figures 10-17). Grazed areas, one centimeter wide, were void of micro-

biota and were covered generally with a thin mucilage veil (Figures

18-22).








Control area. The surface of the fruit is a stratum for the

life processes of many mite and fungal species. A careful examina-

tion of Figure 4 displays the exuvia of a citrus rust mite, a citrus

rust mite spermatophore, a fruit stoma, and some of the mycelia found

on the surface. By viewing exuvia and spermatophore together a per-

spective of size became evident. Stomata were found scattered over

the surface of citrus fruit.

Oldfield et al. (1970) characterized the spermatophores of erio-

phyoidea. As shown in Figure 5, this spermatophore, though not erect,

can still serve to exemplify the base, stalk, expanded apical head, and

sperm sac. The functional posture of the spermatophore is upright or

erect. The base of the spermatophore is shown at 34000X magnification

to demonstrate its structure and attachment to the surface of the fruit

(Figure 6).

Other areas of the surface of the orange peel (Figure 7) again

serve to show the intertwining of mycelia and the presence of spermato-

phores on the surface. An erect spermatophore and citrus rust mites'

eggs are shown as normally associated with the surface of citrus (Fig-

ures 8 and 9).

Ambulatory movement. For the context of this work ambulatory

movement refers to the translocation of snails from one area to another

under its own incentive and energy without feeding.

The terrestrial snails secrete mucilage from several small pores

at the ventral portion of the cephalic end of the foot which help form

a suction to the surface (Dr. F. Thompson, personal communication). The

thin layer of dried mucilage less than 1 V thick is visible in Figure

10. The mucilage has entirely coated the traveled surface of the fruit.
































Surface of a citrus peel, demonstrating the
general condition of a complex mycelial matrix
and stoma (bottom-center) which are associated
with the surface. An immature citrus rust mite's
exuvia is viewed to the entire left, and a
spermatophore (lower-center). 1000X magnifica-
tion.


Figure 4.









































I C_1~1~
f:- ~g



















Citrus rust mite spermatophore. Evident
in this photograph is the base, stalk, ex-
panded apical head, and a partially removed
sperm sac.




















Base of a spermatophore. 34000X magnifi-
cation.


Figure 5.























Figure 6.






33




















... .

















Figure 7.



























Figure 8.


Mycelia covering surface of citrus fruit.
The spermatophore (center) is in its erect
posture. 1000X magnification.

























A close up of Figure 7 displaying the
sperm sac atop the expanded apical
head of the spermatophore. 7000X mag-
nification.




35
































Mycelia normally associated with the
surface of the citrus fruit. Three citrus
rust mite eggs (top, left and bottom) are
shown. 400X magnification.


Figure 9.




37

































'.




-F








The mycelia is sealed generally in mucilage while the stomata extend

above it. In this photograph a close observation discloses a spermato-

phore just below the stoma (Figure 10). The spermatophore has been

flattened, and slightly elongated by the weight of the snail and embedded

in the mucal crust (Figure 11). The sperm sac was ruptured expelling

the sperm. The sperm sac was believed damaged by the snail, but the

author acknowledges the possibility that it may have been previously

damaged.

The action of glattening the spermatophore and rupturing the sperm

sac are both important, in that either would be sufficient to prevent

sperm transfer in the citrus rust mite. This is the first definitive

example that the ambulatory movement of the tree snail has any effect

on the citrus rust mite sex determination. Should sufficient spermato-

phore become damaged, there would be an increase in the ratio of males

in the next generation.

The slime trail as it hardened often broke up into small irregular

shaped platelets ranging from 20 p to 5 p or less across (Figure 12).

If disturbed the platelets apparently collapsed or fell off taking with

it the mycelia and debris. This left a clean fruit surface. The

formation of these platelets could possibly be due to stress from the

mycelia, surface drying and shrinkage or simply surface stress due to

preparation for scanning electron microscope (S.E.M.) work.

The ability of the slime mucilagee) to adhere to various surfaces

inherent to the fruit surface were examined. The fruit stoma (Figure

13) was generally free of mucilage adhesion to its surface. The surface

adhesion of the mucilage to the citrus rust mite spermatophoe is

demonstrated in Figure 14. The sperm sac is ruptured and a



















Figure 10. Area where the citrus tree snail moved across
the citrus fruit's surface. A thin mucilage
veil was deposited on the surface encrusting
some of the mycelia. 1000X magnification.























Figure 11. Spermatophore. The structure is flattened
and the sperm sac ruptured. 5400X magnifi-
cation.




40


















Figure 12.


Area traversed by snail not accompanied by grazing.
Note the mucilage platelet formation on the
surface. 720X magnification.


Fruit stoma and mucilage.


Figure 13.


1000X magnification.















pg-








continuous layer of material is coating the spermatophore surface.

There is a definite affinity of the mucilage to its surface. This ad-

hesion of the slime to the surface of the spermatophore aids in seal-

ing it to the surface,making it inaccessible to the female popula-

tion. D.dormani mucilage was not found adhering to the surface of

citrus rust mites. An immature citrus rust mite was found free of

mucilage (Figure 15).

The effect of the snail trails on the citrus rust mite egg was

observed. No visible damage or adhesion was detected (Figures 16

and 17). Apparently the mucilage did not adhere to the egg surface.

Even more surprising is the fact that the weight of the snail did not

seem to damage the eggs, as many of those observed showed no signs of

dehydration or compression.

Areas of snail grazing. The surface of the fruit where grazing

occurred was found covered with mucilage. This was,as expected,be-

cause of the necessity of the slime for adhesion of the foot to the

surface (Figures 18 and 19).

If the snail was a selective feeder and removed mycelia only,

nonmycelial objects would be expected to be found on the surface fol-

lowing feeding. On the other hand, if the snail was an indiscriminant

feeder engulfing everything in its search for fungi, the surface should

be clean except for mucilage. As is evident in Figure 18, the surface

was totally void. of all matter with exception to the deposited mucilage.

Areas where citrus rust mites and their eggs, immature whitefly,

spermatophore, as well as other mites and foreign surface material

were previously found, now completely were devoid of any microbiota.

This would suggest the ingestion of these items. An examination of the


















Figure 14.


Spermatophore with mucilage encrusted on its
surface and its ruptured sperm sac from an
area of nonsnail grazing. 11000X magnifica-
tion.


Figure 15. Immature citrus rust mite.
fiction.


3000X magni-







































41
77P
b6%--m


















Figure 16.


Citrus rust mite egg in area where the snail has
traversed without feeding (ambulatory area).
Mucilage failed to adhere to the egg's surface.
1000X magnification.


Figure 17.


Close up of Figure 16.
2400X magnification.


Citrus rust mite egg.





47































































f i



















Figure 18.























Figure 19.


Areas grazed by the citrus tree snail.
Absence of microbiota demonstrated the
ability of the snail to remove all sur-
face material. 1000X magnification.




















Area of snail grazing resulting in re-
moval of all surface material. 1000X
magnification.
















As-








snail fecal content, discussed in Section 4 of this chapter, was used

to determine what material the snail was ingesting.

In several areas, the author observed the absence of the mucilage

in areas traversed by D. dormani. This is possibly the normal course of

events where, under natural conditions, wind, abrasion, and rain may

cause the breaking away of the mucilage (Figure 20) until the entire

surface is cleaned and just the waxy surface of the fruit remains

(Figures 20 and 21).

The forward movement of the snail easily could provide areas where

the foot had traveled while not covered by feeding. Figure 22 demon-

strates this phenomena. One-half (the right side) of the photographed

surface is clean of surface material with the exception of mucilage.

The center left of the figure shows the area covered by the foot not

yet grazed. The reduction of mycelia in this area was due to the strings

of mycelia being pulled away by adjacent feedings. The remaining sur-

face showed no signs of mucilage and the flora still remained. This is

a good example of the transition from grazing to nongrazing and was

generally indicative of samples taken.

Close observation of exposed fruit surface, that is those surfaces

devoid of mucilage or debris, permitted observations into the effect

the snail might have on the fruit's waxy layers. No damage was evident

on the waxy layer of any of the surfaces viewed. The only effect the

snail had was to remove all surface material and/or to deposit its slimy

mucilage. As was expected, there was no damage to the surface of the

fruit. If damage had occurred it would have been manifested as scar

tissue on the fruit and observed in the field. Studies using the

scanning electron microscope failed to detect any surface damage. If


















Figure 20.


Stoma in area of snail grazing. No surface
damage to the fruit was observed. 1000X
magnification.


1000X magnification.


Figure 21.


Snail grazed area.




52































Figure 22. Photograph demonstrating the three conditions
covered in earlier figures. Snail grazed the
right side removing all surface material and
depositing mucilage. Center is where the snail's
foot traversed over the surface but no feeding
occurred. Left side is normal mycelia found on
fruit surface. 220X magnification.




54













! *
}.









damage had been associated with the feeding, this could negate easily

any possible good the snail could have accomplished. Further studies

as to the surface effects of the snail on the leaves are needed.


Section 4. Examination of Snail Fecal Content

Introduction

No information is available on the fecal content of the citrus

tree snail. To support the scanning electron microscopy findings, this

study was undertaken to determine if D. dormani ingests insects, citrus

rust mites, and their spermatophores.

Materials and Methods

Citrus tree snails were examined for their fecal content. Fecal

pellets 2 mm x 3 mm were field collected from a grove in Orange Lake,

Florida. Also snails were placed on citrus fruit and leaves for feed-

ing. Fecal pellets were collected from laboratory specimens as they

were deposited. The feces were placed first into watch glasses con-

taining distilled water, then 70% isopropyl alcohol. This was repeated

three times for each fecal pellet. The baths were used to remove any

foreign or living material from the surface of the feces. The fecal

pellets then were allowed to air dry prior to disruption by forceps

then by a sonic vibrator in 5 ml of distilled water. Suspended materials

were placed onto glass microscope slides. All samples first were

examined with a binocular microscope, then with a phase microscope and

photographed.

Results and Discussion

The preponderance of the fecal content consisted of strands of

various length fungal mycelia. Due to its disrupted condition, no








attempt was made to identify the mycelial species. It was assumed that

some of the mycelia was from sooty mold.

Further examination of the fecal content showed several species

of mites, some of which were still alive, whole white fly pupae, and

various parts of insect bodies. Lepidopterous wing scales and parts of

chitinous hexapod appendages were among the debris. Several specimens

of the citrus rust mite were found, none of which showed movement.

Some of the larger Prostigmatid mites were still alive when placed in

water for observation. The viability of these mites suggests the in-

ability of the snail to digest them supporting the hypothesis that the

snail is feeding on the fungi, but also consumes other foreign matter

by chance. However, it seems unlikely that the various mites could

escape from the encrusted fecal entrapment.

The mite and insect portion of the feces represented a small frag-

ment of total content. The white fly pupae were the most numerous with

citrus rust mites second.

Once it was determined the snail was ingesting material other than

mycelia, additional studies were undertaken to locate citrus rust mite

spermatophores in snail fecal material. Spermatophores were found in

the fecal pellets and demonstrated yet another means for the snail to

exert pressure on the rust mite sex determination. The spermatophore

removal by the snail could alter the sex ratio increasing the ratio of

males in the next generation. Reduction in the ratio of female P.

oleivora could be of significance in the suppression of the population

especially if other factors suppressing the citrus rust mite populations

were present.









Section 5. Determination of the Snail Feeding Potential

Introduction

Quantitatively speaking, no literature was found on the feeding

potential of the snail. Occasionally sooty mold was mentioned as a

food source but no other mention of the quality and quantity of inges-

ted materials is available. The first part of this study was directed

at quantifying the citrus tree snail's ability to clean given areas of

citrus. The second part extrapolates this rate to encompass the en-

tire citrus canopy.

Materials and Methods

Specimens for this study were collected at Leesburg, Florida. Due

to the severity of the winter only a very small population was located.

Twelve citrus tree snails collected were transported in glass mason

jars with a relative humidity of 98% 2% to Orange Lake, Florida, where

observations were made on Valencia orange.

A suitable tree with both citrus rust mite and sooty mold was

located. Nine, one cubic foot areas of peripheral leaves from two feet

to six feet off the ground were created by use of hedge shears and a one

foot carpenter's square. The shears were used to isolate an area of one

cubic foot on the tree.

Once the cubic unit was formed, all leaves from adjacent branches

were removed, leaving a six to eight inch leafless barrier around each

unit.

A standard 18 mesh metal screen 2" x 4" was wrapped about the stems

of the branches included in the cubic unit, to restrict the snails to

this area and to keep any other snails from entering. The snails

were unable to traverse the screen due to loss of contact with the








tree surface, thus maintaining them within the confines of the cubic

foot.

Tie wire was twisted around the screen to secure it. A clip-type

clothespin was placed just beyond the screen and outside of the cubic

unit to identify the particular cube. Yellow ribbon was also attached

to make locating the cubes easier.

Three of the cubes were randomly chosen to have one snail, three

with three snails, and three without any as controls. As the snails

were placed on the units, yellow Day-glowR numbers representing the

particular cube unit were applied to the dorsal surface of the shells

as in Chapter I, Section 2. A similar number was placed on the clothes-

pin for identification. For the first twelve hours (8:00 p.m. until

8:00 a.m.) the snails were observed hourly. An ultraviolet light,

model ULV-56, was used to see the Day-glowR painted snail and identify

it. Temperature, relative humidity, and periods of snail movement were

recorded every 15 minutes.

Pretreatment and posttreatment counts on citrus rust mite popula-

tions, as well as presence or absence of sooty mold, were collected just

prior to snail infestation, using a modified hand lens (Allen, 1976).

Ten leaves from nine different cubic feet of citrus were randomly chosen

and one 1 cm2 observation made on each leaf. Numbers of rust mite

P. olievora were counted as were positive or negative presence of sooty

mold. These observations were continued for six days.

Determination of snail hours each day was initially made by visual

observations and relative humidity readings. Later, snail hours were

determined by recorded humidity readings on a recording hygrothermograph

located near the test tree.








At the completion of this study all of the leaves were removed

from each cubic foot area. The total surface area for each leaf, av-

erage leaf surface, total leaf surface, and average leaf surface per

cubic foot were obtained by use of a portable area meter model Li-Cor

L1-3000 in sequence with a Wang computer, model Wang 600 Programmable

Calculator.

Results and Discussion

Snail density and its effect on sooty mold. Snail hours and per-

cent of mold present are indicated in relation of mold to the various

snail populations in Figure 23. In the controls without snails the

percent of mold present was a constant 97.14% level of infestation for

the entire six day period. Where one snail was permitted to graze over

a one cubic foot area of leaf surface, the entire surface was cleaned

at 26 snail hours (Figure 23). This was equal to just over five days

in this test. As would be expected, where three snails were placed on

a cubic foot of citrus, it was calculated to take 8.5 snail hours, approx-

imately one-third of the time necessary for one snail.

The following calculation shows the average surface (S) area

grazed by the snail per snail hours. Numerical values for these calcu-

lations are found in Table 3.

S = Surface area grazed per snail hours.

S= 3415.06 cm2 (average leaf surface area/ft3).
26 snail hours*
2
S = 131.34 cm /snail hours.

The average total leaf surface area (dorsal and ventral surface)

of the citrus tested equals 28.8024 cm /leaf (Table 3). Determination


Number of hours calculated necessary to clean the surface
of one cubic foot of citrus per snail (Figure 23).
































Figure 23.


The lines, X = 0, X = 1, and X = 3, refer
to % mold removed from the surface of one
cubic foot of citrus by 0, 1, and 3 snails,
respectively.

















































14.5 17.5 24.5 29

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of the average number of leaves grazed by D. dormani per snail hour (L)

is as follows:

L = 131.34 cm2/snail hour
28.8024 cmz/leaf

[ = 4.56 leaves/snail hour

Calculations of the quadratic equation describing the relationship

of percent sooty mold present by time are given in Table 4.

The original percentage data were transformed by use of Arc-sin

tables to allow for computations with percentages (Table 5). An ex-

amination of the significance of the various sources of error showed

only the intercept and the day by snail (N-X) interactions as being sig-

nificant (Table 4). Statistically, factors such as time (N) or time2

(N2) had little if any effect in accounting for the reduction in the

sooty mold on the leaves. As was expected, the number of snails (X)

by time (N) interaction was significant. That is to say, by increasing

the snail population, over the six days you would get a reduction in

sooty mold. There was a reduction as opposed to an increase because the

value was negative for N'X interaction. It was possible to make a di-

rect comparison of the different cubic foot units, without having to

deal with the variation in leaf numbers (NLVS) because the NLVS inter-

action was not statistically significant. The calculated values for

Figure 23 are displayed in Table 6.

Sooty mold was calculated to be consumed at a rate of 131.34

cm2/snail hour. The basis for the linear representation was derived

from a quadratic equation where only the intercept and N-X interactions

were significant.

Snail density and its effect on citrus rust mites. The data fol-

lowed a Poisson distribution which is found generally when dealing with









Table 4. Quadratic Equation Variables for Sooty Mold Grazing
Test. These variables were examined for sources of error in the sooty
moTd grazing test. Only the intercept (Intercept) and Day by Snail
population interaction (N-X) were significant.


Parameter Estimate PR ITI

Intercept 97.1404 .0001**

NLVS 0.0622 .2315

N 0.0000 1.0000

N2 0.0000 .0000

X 2.9601 .8426

X2 0.5236 .9099

N*X -17.8688 .0495*

N2.X 1.4395 .2896

N-X2 2.4099 .4220

N2.X2 0.1192 .7757


** Highly significant (99%) .01

Significant (95%) .05

NLVS Number of leaves per cubic foot

N Number of days (or nights)

X Number of snails

R2 Value = .931642 (Very high accounting for sources of variation)
















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Table 6 .
time by % mold


Calculation of points for graphic representation of
present for 0, 1, 3 snails per cubic foot of citrus.


Number of Day Snail Hours/ Y Value
Snails Day


97.1404
97.1404
97.1404
97.1404
97.1404
97.1404
79.2616
61.4028
43.5340
25.6652
7.7964
-10.0724*
43.5340
-10.0724*
-63.6788*
-117.2852*
-170.8916*
-224.4980*


Y intercept = 97.1404


4.0
4.5
6.0
3.0
7.0
5.0
4.0
4.5
6.0
3.0
7.0
5.0
4.0
4.5
6.0
3.0
7.0
5.0


* Negative values of % mold were used only for determining
period of time necessary to reach total sooty mold removal.








random sampling of organisms in some medium, or insect and mite counts

in field plots (Steel and Torrie, 1960). For this and all Poisson val-

ues in these studies the transformation of Y = (X' + .5) was used

(Table 7).

The derived quadratic equation (Table 8) was used to give a repre-

sentation of the effects the snail populations had on the citrus rust

mites (Table 9; Figure 24). Control areas (X=0), after day one, dis-

played a constant value through the experiment. The reason for the

apparent initial decline in mite population was due to the variability

in average surface area throughout the experiment. The surface areas

of the nine cubic feet of test area (X1) were averaged as were the

averages of all the untransformed initial mite counts (X2). This al-

lowed for comparison of the various cubic units. Quadratic equations,

having basis in all of these factors,are affected by any averaging

(Figure 24). Thus by averaging X1 and X2 values,a decline in population

values is depicted. The average of these two values,X1 and X2,were

combined with the intercept value to yield the control values.

Y (x = 0) = Intercept .003 X1 + .0260 X2
(n = 1-6)

Y (x = 0) = 2.8613 .5122 + .2503
(n = 1-6)

Y (x = 0) = 2.599 (transformed data)
(n = 1-6)

To untransform the data to original state,the following
formula was used.

X' = Y2 .5

X' (x = 0) = 6.2548 (Table 7)
(n = 1-6)

Test areas using one snail per cubic foot (X = 1) displayed a

sharp decline in citrus rust mite populations for the first four days









Table 7.
by mite populati


Calculation of points for graphic representation of time
on for 0, 1, and 3 snails per cubic foot of citrus.


Number of Day Snail Hours/ X' Value Y Value
Snails Day


0 1 4.0 7.6870 2.5994
0 2 4.5 6.2568 2.5994
0 3 6.0 6.2568 2.5994
0 4 3.0 6.2568 2.5994
0 5 7.0 6.2568 2.5994
0 6 5.0 6.2568 2.5994
1 1 4.0 6.4674 2.6396
1 2 4.5 4.1997 2.1679
1 3 6.0 2.5905 1.7580
1 4 3.0 2.1455 1.6265
1 5 7.0 2.3940 1.7012
1 6 5.0 3.4287 1.9821
3 1 4.0 5.5319 2.4560
3 2 4.5 3.1975 1.9229
3 3 6.0 1.9812 1.5752
3 4 3.0 1.4962 1.4129
3 5 7.0 1.562 1.4360
3 6 5.0 2.2043 1.6445


Y intercept = 2.8613
X' = Untransformed values
Y = Transformed values


(X' = Y2-.5)








Table 8. Calculation of the Quadratic Equation for Snail Feeding
on Citrus Rust Mite. Refer to Table 4 for source of quadratic. Table
7 gives calculated values.


Y = 2.8613 .003X1, + .0266 X2 + 1.0896X .2993X2 1.1446 (X-N)

+ .1392 (X-N2) + .2914 (X2-N) .0361 (X2.N2)


X1 = Average initial surface area per cubic foot = 1707.43

X2 = Average untransformed initial mite count = 9.4111

X = Number of snails

N = Number of days (or nights)








Table '9.. Quadratic Equation Variables for Citrus Rust Mite Grazing
Test. These parameters were evaluated after statistical elimination of
several unsignificpnt parameters. All of the parameters proved signifi-
cant except the (X ) i.e., (number of snail) factor.

Parameter Estimate PR (T)


Intercept 2.8613 .0001**

SFAREA -0.0003 .0489*

I Count 0.0266 .0050**

X 1.0896 .0413*

X2 -0.2993 .1368

X.N -1.1446 .0022**

X-N2 0.1392 .0076**

X2.N 0.2914 .0251**

X2.N2 -0.0361 .0468**


** Highly significant (99%) .01

Significant (95%) .05

SFAREA- Source of error due to surface area

I count Pretreatment mite count

X Number of snails

N Number of days (or nights)

R2 value = .14986

















8.0a










t'-- ... ....- ....... .... ....................

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'.9
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foot of citrus and their ability to suppressitrus
IL 9. '








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4 8.5 14.5 17.5 24.5 29.5

SNAIL HOURS

Figure 24. Graphic representation of O, 1, and 3 snails per cubic
foot of citrus and their ability to suppress citrus:rust
mite populations on citrus.








(20 snail hours) which reduced the population by 83.7%. After this point

(Figure 24) there was an increase in the mite population. Field ob-

servations indicated that areas cleaned by the snail could become rein-

fested from immigrating rust mites. The immigration and migration of

mites by locomotion, wind dispersal,and rain could account for the rein-

festation of previously cleared areas.

This test demonstrated that even in this type of uncontrollable

mite infestation a high degree of citrus rust mite population suppres-

sion was attained briefly. Greater suppression was displayed by the

units containing three mites per cubic foot (X = 3). By day four, as

high as 90% reduction in the rust mite population was observed. This

again demonstrated the capacity of the snails for rust mite removal.

Having determined the amount of surface area covered by a snail

per snail hour (S), it was possible to determine the number of snails

(N) necessary to clean the surface of a tree (A) in any given number of

days (T).

The snail hours per night (h) were calculated by monitoring periods

of 100% R.H. and its daily occurrence. The value for S, surface area
2
cm cleaned by a snail per snail hour, was taken as an average S =

131.34 cm2/snail hour, as calculated earlier.

Calculations of the number of snails (N) that would be needed is

as follows:

N = 2A x 10,000
Txhxs

N = Number of snails needed

A = Surface area of tree (meter2)

T = Time (days) to completion

h = Snail hours/night

s = Area cleaned by snail per hour.








To calculate for A, total leaf surface area per tree, the follow-

ing formula was needed (Turrell, 1961):

Log A = C2 + N2 log a.

Constants

C2 = .994 on Valencia

N2 = 1.068 on Valencia

Variables

a age of tree

Log A = .994 + 1.068 log a.

In the calculation, 2A is used because Turrell's formula just

measures the dorsal surface area. Using this set of formulas and the

calculated averages wherever possible, the author arrived at some es-

timated number of snails needed per tree (Table 10).

The author estimated that because of the short, four month acti-

vity period of the snails, that a T value of 15 to 30 days should be

used. This means that the entire leaf surface of the tree should be

cleaned within this period. Several factors such as ambulatory move-

ment, and original snail distribution on the tree may tend to make the

T period longer than calculated. For this reason the snail values N

for 15 days were calculated.

The trees,evaluated in Chapter II, Section 1, dealing with snail

population per tree, were 25 year old trees. Reference to Table 10

shows that up to 168 snails per tree would be needed to clean totally

the tree surface in a 15 day period. Even if the lower value of

84 snails for 30 days is used, it is quickly realized that the 1976

populations were too low to afford the control needed. An average of

only 45 snails per tree were found. At this rate the earliest the









Table 10. Calculated minimal number of snails necessary to clean
an entire citrus tree per unit time.


Age of Tree (Years) Days to Completion Number of Snails
(a) (T) (N)

10 15 64

10 30 32

15 15 98

15 30 49

20 15 134

20 30 67

25 15 168

25 30 84

30 15 206

30 30 103


* For the above calculation
derived from the field da

S = = 131.34 cm2

h F= 5 hours per night


s the following averages were
ta.








entire surface could be cleaned would be 60 days, about half of the

entire active snail season. At this rate the snail grazing may not

suppress the biotic potential of the sooty mold and citrus rust mite.

The author believes that the 1975 snail population levels at

Orange Lake, Florida, were sufficient to suppress sooty mold and mite

populations in accordance with calculated levels, but no quantitative

data confirming this are available. Snail trees during that period

exhibited a clean shiny gloss which was not seen during 1976 or 1977

observations. The author also wishes to acknowledge that these cal-

culations are based on a small number of snails and that additional

testing would be needed to be conclusive.










CHAPTER II
BIOLOGICAL STUDIES OF THE CITRUS RUST MITE


Section 1. Methods for Monitoring Citrus Rust Mites

Introduction

Early researchers used linen testers and 0.5 in.holes cut in

paper to help define the field of view sampled in citrus rust mite

counts (Yothers and Miller, 1934; Osburn and Mathis, 1944). Later

researchers used the entire field of view of a 1OX hand lens (Pratt,

1957; Johnson, 1969; Simanton, 1960) to count mite populations. The

lack of uniformity of method and surface area examined by earlier re-

searchers dictated the need for a method of monitoring which delineates

the field of view. This study defines the field of view and relates

samples taken to the entire mite population on the fruit.

Materials and Methods

Valencia orange trees located in Orange Lake, Florida, were used

in this study. Fruit size ranged from 3.4 to 4.2 cm, and were cho-

sen randomly from a five acre plot.

Hand lens method. A 14X Bausch & LombR hand lens was used to

count the numbers of rust mites per cm2 on the surface of oranges. A

rubber stamp was used to stamp a square 1 cm area on the fruit. Three

areas were randomly counted on each fruit along its equatorial belt.

Diameter was taken with aluminum calipers and a metric rule. Circum-

ference was taken later in the laboratory by placing a fresh red line

on the fruit vertically over its equator, then rolling it along the








equator on white paper. A mark was left at two places on the sheet

denoting the fruit's circumfrence. The volume of the fruit was taken

in laboratory by placing the fruit into a 1000 ml graduate cylinder

and recording water displacement. Samples were multiplied by factors

derived from diameter, circumference and volume to give total popula-

tion estimate over the entire fruit.

Alcohol emersion method. At the completion of each set of lens

counts, the orange,while still on the tree, was placed into a clear

plastic bag 8 x 3 x 15 x .0015 in. containing 50 ml of 95% isopropanol.

The bag over the fruit was sealed by twisting it several times around

the branch bearing the fruit. The alcohol was vigorously shaken over

the orange for ten seconds to remove citrus rust mites. The bag was

removed from the fruit and sealed. The fruit was removed from the

tree marked for later identification.

In the laboratory the alcohol was agitated in the bag, and three

2 ml samples were pipetted into separate watch glasses. An OlympusR

binocular microscope at 25X was used to count the number of mites per

sample. The three 2 ml samples were averaged and multiplied by the

percent of the sample they represented to give total number of mites

from each fruit.

Results and Discussion

The average number of citrus rust mites on the equatorial belt

of ten fruit (Y) are shown in Table 11. These values were multiplied

by the estimated surface area calculated from fruit volume (Sv), fruit

diameter (Sd), and fruit circumference (Sc), which yielded estimates

of 51.25, 48.56, and 55.93 mites per orange, respectively. The alcohol

emersion method gave an average of 53.23 mites per orange (Table 12).

















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Table 12, Ten oranges were washed in separate bags containing
50 ml isopropanol. Three 2 ml aliquots were taken from each bag
and the number of rust mites counted and recorded (Sample 1,2,3).
These were then averaged (Average) and multiplied by the percent
of the sample they represented to give (Number of mites per fruit).


Alcohol Emersion Method



Fruit Sample Number
Number 1 2 3 Average Per Fruit


1 0 0 0 0 0

2 0 1 1 .66 16.50

3 1 0 0 .33 8.25

4 0 2 0 .66 16.50

5 0 2 0 .66 16.50

6 2 4 2 2.66 66.50

7 4 3 4 3.66 91.50

8 3 1 2 3.00 75.00

9 4 0 0 1.33 33.25

10 10 9 6 8.33 208.25

53.23*


* Average of 10 samples.








A linear correlation coefficient was used to determine that there was

a 99% correlation between the alcohol emersion method and the hand

lens data. A X2 test for expected values failed to demonstrate any

statistical difference between the average citrus rust mite populations

on the ten fruit.

This test, though lacking in replicates and large populations of

citrus rust mites, was carried out early in 1975 as a first indication

of the usefulness of the hand lens monitoring method for determining

citrus rust mite populations on fruit. This test did indicate a high

degree of correlation,but is inconclusive and needs additional repli-

cation. It is presented here only as on indication of the relationship

between the population of P. oleivora on the equatorial belt and the

entire citrus rust mite population which is believed to exist on the

fruit. Other factors such as seasonal variation would need to be con-

sidered as would greater mite populations. Citrus rust mite orienta-

tion on the equatorial belt of the fruit is examined in the next sec-

tion.


Section 2. Extrinsic and Intrinsic Orientation of
Citrus Rust Mite on Valencia Orange

Introduction

Watson and Berger (1937) described rings of russet damage corre-

sponding to semi-shaded areas around the fruit. They speculated that

this was due to an aggregative effect of the citrus rust mite and its

preference for these areas. Others and Mason (1930) demonstrated a

positive phototaxis but made mention of the mites avoidance of direct

sunlight. It was noted previously (Chapter II, Section 1) that the

sample about the citrus fruit equatorial belt was believed to be a









representative sample of the mite population on that fruit. To gain

a better understanding of this population an experiment was conducted.

This study examined the distribution of the citrus rust mite on both

the tree and on the fruit.

Materials and Methods

P. olievora was studied for population distribution patterns

on both fruit and tree quadrants at Orange Lake, Florida. All tests

were on Valencia variety C. sinensis with east-west row orientation.

Four blocks with two trees per block were monitored. One

tree per block was treated with water at 200 p.s.i. to run off while

the second tree was left untreated. Counts were made pretreatment

(day of application), and at one week intervals for six weeks.

Method of counting. Four fruit were chosen to represent each

of the four major compass directions. Citrus rust mite counts were

taken as in Chapter II, Sectionl, along the equatorial belt (a

plane through the fruit parallel to the ground) on each of the

four major compass directions. This gave a total of 16-1 cm2

samples/tree.


Results and Discussion

Initial efforts to analyze the data employed the use of an analysis

of variance (ANOVA) (Table 13) and tested for the various interactions.

There was no significant difference between the blocks and treat-

ments for the trees tested, permitting summation of data of the two

treatments (Tables 14-19). The significant difference displayed









Table 13. ANOVA for citrus rust mite orientation on fruit.



Source Df Sum Squares F


Blocks (B) 3 170.71 A
Treatments (T) 1 8.96 A
Error (B*T) 3 330.65
Tree Direction (TDIR) 3 61.53 A
(TDIR T) 3 192.55 1.96
Error B(B*TDIR) (B*TDIR*T) 18 590.45
Orange Direction (ODIR) 3 9.26 1.05
ODIR T 3 24.34 2.75*
ODIR TDIR 9 74.85 2.82**
ODIR TDIR T 9 19.18 A
Error C 72 212.65
Week (W) 5 888.70
TXW 5 50.27 1.13
TDIR W 15 441.27 3.3**
ODIR W 15 72.88 A
Residual 600


* Significant at .01 level.

** Significant at .05 level.

A Less than 1.0 (not significant).
















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by the orange direction by tree direction interaction (Odir Tdir)

suggested that no one direction on each fruit was dominant.

Tree direction was not significant indicating that no single tree

direction maintained a superior mite population. On the contrary, a

directional preference seemed to shift from week to week indicating a

possible interaction with some abiotic factor not accounted for here.

Totals (B) on Tables 14-19 display the shifting directional dominance

on the tree. Totals (A) demonstrate the averaged value of mites on

each direction of each orange.

To test for the relationship of the peripheral (that portion of

the orange furthest from the center of the tree), internal (that por-

tion of the fruit closest to the center of the tree), and the marginal

(semi-shaded) areas, the average of the mite populations relating to

these areas were compared (Table 20). The peripheral surface had

147.125 mites or 29.8% of the overall population. The internal sur-

face comparatively was close having 144.71 mites or 29.3% of the over-

all population.

The marginal area, which took into account that it was the product

of two sums, had an average population of 200.604 mites or 40.7% of

the total population. This is an increase of at least 11% over the

other two surfaces.

These data defend the earlier theories by Watson and Berger (1937)

that the mite populations tend to clump about the marginal aspect of the

fruit. No indication of a unilateral tree direction was discernable

and it is believed to be dependent on abiotic factors.



















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CHAPTER III
EFFECT OF DIFFERENT ACARICIDES ON CITRUS RUST MITES



Introduction

The purpose of this study was to evaluate rust mite control by

some newly developed chemicals and to determine their effectiveness

by comparison to known standards.


Materials and Methods

During the summer of 1975 and 1976, several compounds were evalu-

ated for control of the citrus rust mite, P. oleivora, on Valencia

orange trees. The compounds were mixed with water according to the

manufacturer's directions, and applied to run off. A brief description

of each compound is found in Table 21.

1975 field acaricide spray tests. Acaricidal tests were conducted

on Valencia variety of C. sinensis at Orange Lake, Florida. The

following chemicals and concentrations were evaluated:

PP199 at .005, .01, .02, and .04%

PP067 at .01, .02, .04%

Dicofol at .03% (commercial standards)

Oxamyl at .001%

A water control and an unsprayed control were incorporated into this

test.
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