White grubs in forest nurseries of the Carolinas

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Title:
White grubs in forest nurseries of the Carolinas
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English
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Johnston, H. R
Eaton, Charles B
United States -- Bureau of Entomology and Plant Quarantine
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U.S. Dept. of Agriculture, Bureau of Entomology and Plant Quarantine ( Washington, D.C )
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oclc - 778698612
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LIBRARY
ATE PLANT DOAM6 July 1939

United States Department of Agriculture
Bureau of Entomology and Plant Quarantine


WHITE GRUBS IN FOREST NURSERIES OF THE CAROLINAS

By H. R. Johnston and C. B. Eaton, l/
Division of Forest Insect Investigations




INTRODUCTION

For many years white grubs 2/ have been recognized in many
sections of the United States as serious pests of such farm products
as corn, timothy, and potatoes, entire crops of which have often been
completely destroyed by their destructive feeding. During recent years
these insects have been the major insect pests in pine-growing nurser-
ies of North Carolina and South Carolina, where, in many instances,
solid stands of pine seedlings in localized areas in certain nurser-
ies have been completely destroyed. In some nurseries of the Caro-
linas the annual loss caused by grubs has been variously estimated
at from 25 to 40 percent of the original stock of pine seedlings.

It is the purpose of this circular briefly to describe these
insects, the nature of their depredations, their general life history
and habits, and a method for controlling infestations in forest nur-
series.

DESCRIPTION OF THE INSECT

The adults, commonly called May beetles or June bugs, are more
or less robust, oval-shaped beetles, ranging in length from one-half
inch in the smaller species to almost a full inch in the larger ones.
They vary in color from light straw to very dark brown. The elytra,
or wing sheaths, which are hard shell-like covers for the membranous


1/ The investigations were conducted by the Bureau of Entom-
ology and Plant Quarantine in cooperation with the South Carolina
State Commission of Forestry and the North Carolina State Forest Serv-
ice. For many helpful suggestions and criticisms the writers are
indebted to B. H. Wilford, under whose direction this study was made.
Acknowledgments are made to R. J. Kowal, and W. L. Lowry, formerly
connected with this project, for assistance rendered in the course of
these investigations.


_2/ Phyllophaga spp;, order Coleoptera, family Scarabaeidae.





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hind wings, are smooth and shiny in some species, but in others they
are covered with short hairs. These insects become active about dusk
on warm spring and summer evenings and are often seen at night flying
about lights or feeding on the foliage of various plants. May beetles,
which are active at night, are not to be confused with the southern
green June beetles, which are active during the day, and are sometimes
seen about ripe fruits, upon which they feed.

The eggs of the May beetles may be found in the soil where they
are deposited by the female beetles. When first laid they are pearly
white and elongate, measuring about one-tenth of an inch in length,
but after a short period of incubation they become almost spherical
and assume a creamy color.

The larvae, or "grubworms," that hatch from the eggs are the
most commonly encountered immature form of the May beetle. They range
in length from one-eighth inch when first hatched to an inch when
full grown. The young larvae are almost transparent, but the older
larvae are dirty cream colored. The true white grub can be disting-
uished from most other similar grubs by the V-shaped anal opening
and by the presence of a double row of more or less conspicuous spines
along the median line on the underside of the last body segment.
True white grubs are found only in soil and are not known to inhabit
manure heaps or decaying refuse as do the grubs of the common carrot
beetle, 3/ which are often mistaken for the former. The larvae of the
southern green June beetle 4/ are frequently confused with white grubs,
but can be easily be distinguished from the latter by their character-
istic habit of crawling on their backs when placed on the surface of
the soil. The true white grub crawls in a normal position by means
of its legs when placed on the soil surface.

When the grubs become mature they enter a short resting period
in which they transform to pupae. The pupae are similar to the adults
in general size and appearance but are yellowish to light brown in
color, depending on their age. In this stage the insects are entirely
quiescent and cause no damage of any kind.

LIFE HISTORY AND HABITS

In the vicinity of Georgetown, S. C., the most common species
of Phyllop ha require from 2 to 3 years to complete their life cycles.
However, the generations overlap so that normally there is no marked
fluctuation in beetle abundance from year to year. Under natural
conditions a high mortality (85 to 90 percent) occurs from the egg to
the adult stage. Natural control is brought about by parasites,
insect predators, birds, rodents, fungous diseases, climatic factors,
and no doubt by the cannibalistic habits of the larvae.


3/ Ligyrus gibbosus (Deg.).
4/_ Cotinis nitida (L.).






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Oviposition, or egg laying, begins early in the spring and con-
tinues until late in the summer. The eggs are usually deposited
singly in the soil at depths of from 2 to 5 inches within cavities in
the center of small balls of earth.

About 3 weeks after they are laid the eggs hatch and the tiny
first-instar grubs emerge and burrow through the soil, feeding on
decaying vegetation and the tender roots of living plants. From 40
to 50 days later the grubs shed their skins, molting the first time,
and become active, vigorous feeders. From 6 to 8 weeks after the first
molt many of the larvae shed their skins the second and last time to
become third-instar grubs, in which stage they spend the first winter.
Others, however, overwinter in the second instar and molt the fol-
lowing spring or summer.

During the winter the larvae are relatively inactive, but they
resume their feeding early in the second spring and cause considerable
damage to the young seedlings.

For the species that appear as adults early in the spring it
has been found that during mid-August of the second summer about 75
percent of the larvae cease feeding and dig deep into the soil, where
they make oval-shaped earthen cells in which the pupal stage is passed.
About 2 weeks later the pupae transform to adults, which remain in
the soil until spring. The remaining 25 percent continue to feed
throughout the second summer and pass the second winter as third-instar
larvae. During the third summer they pupate, pass the third winter
in the adult stage, and emerge from the soil during the fourth spring.
Thus about 75 percent of a single generation may complete the life
cycle in 2 full years and 25 percent in 3 full years.

Species that make their first appearance as adults in midsum-
mer probably vary from the above in that they pass the third winter
as third-instar grubs, which pupate in the spring and emerge from the
soil the following summer.

In the coastal plains of the Carolinas the larvae show a slight
tendency to burrow deeper into the soil during the winter, but many
grubs can be found near the surface throughout the year. During the
first year of development the larvae may migrate laterally as much as
5 to 7 feet through the soil, while second-year grubs are perhaps
slightly more active and may migrate as much as 6 to 9 feet. Inasmuch
as the total distance of migration of the larvae ranges between 11 and
16 feet, there is little likelihood of a nursery becoming infested
by grubs migrating into the seedbeds from adjoining infested areas.

The adults are active from early in the spring until late in
the fall. Some species begin to emerge during the first week of
April and continue to appear until midsummer, whereas other species
make their first appearance in June or July and continue their ac-
tivity until late in September. At dusk on warm evenings they emere
Semere




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from the soil, fly about for a few minutes, and alight on the foliage
of plants, where they mate and feed until dawn, at which time they
leave the foliage and return to the ground. Some species, especially
those that appear in the spring When early morning temperatures are
low, drop directly to the ground underneath the host plant and burrow
into the soil. Other species, which emerge from the soil later in
the summer when morning temperatures are relatively high, glide or
fly from the host plants and alight on the ground some distance away.
The majority usually land within 100 feet of the host plant, but some
individuals travel 300 feet or more before alighting. The beetles
apparently do not select any particular site on which to land, alight-
ing indiscriminately in woods, fallow areas, and seedbeds, where the
female beetles deposit their eggs to begin another generation.

Twenty-three species of Phyllophaga have been collected near
forest nurseries of the Carolinas. Fortunately only three species 5/
were found associated with forest nurseries in sufficient numbers
to be of economic importance. It should be kept in mind, however,
that all species of May beetles are probably potential nursery pests.

PLANTS DAMAGED

White grubs injure a wide variety of plants, including corn,
timothy, potatoes, wheat, strawberries, grass in lawns, and tree
seedlings, particularly coniferous nursery stock. Slight damage to
ash and tulip poplar seedlings has been observed, but black locust
in heavily infested seedbeds was apparently uninjured. In south-
eastern forest nurseries pine seedlings bear the brunt of the ravages
of these pests. Longleaf pine, because of the nature of its root
system, is probably capable of withstanding greater grub injury than
loblolly, shortleaf, or slash pine, hut white grubs attack all these
species without discrimination.

Unlike the grubs, the adults of the different species of
Phyllophaga differ in their host preferences. Some species feed al-
most exclusively on pines, while others prefer oaks, and still others
feed indiscriminately on a wide variety of host plants. The trees on
which May beetles most frequently feed in the Carolinas are pines and
oaks; but they were collected from such diverse hosts as hickory, black
gum, persimmon, sweetgum, black locust, chestnut, willow, elm, witch-
hazel, Deutzia, blackberry, pecan, alder, gallberry, and running rose.

TYPES OF INJURY

In the Carolinas the chief damage is caused by the feeding of
the grubs on the plant roots. The adults seldom become sufficiently
numerous to cause serious damage to the foliage of the plants on which
they feed, as they do in certain other parts of the country. Injury
to pine seedlings is caused by the feeding of the larvae on the lateral


5/ Phyllophaga prununculina (Burm. P. luctuosa Horn, and
P. forsteri Burm,






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and tap roots of the plants. If continued long enough, this results
in the death of the seedlings. Grub-damaged plants can be detected
by the abnormal appearance of the needles or by exerting a gentle pull
on them. An injured plant can be easily pulled up, whereas a healthy
plant will remain firm. Severely injured plants are usually sickly
or dying in appearance.

Four general degrees of damage are recognizable (fig. 1), but
variations from these types are to be expected. (1) Injury may be con-
fined almost entirely to the lateral roots (fig. 1, B). This type
of injury is most prevalent in mid summer and is usually the work of
first-year larvae in the early stages of development. Plants injured
in this manner may survive if no further feeding occurs and the soil is
sufficiently moist. (2) Continued larval feeding may result in the
destruction of the lateral roots and a portion of the tap root (fig.
1, C). (3) The lateral roots and the bark and cambium of the tap root
may be eaten away (fig. 1, D). Seedlings damaged to the extent de-
scribed in the last two classes have no commercial value, although they
may survive in the seedbeds if no further feeding takes place and the
soil remains moist. (4) The tap root may be severed just under the sur-
face of the ground (fig. 1, E), in which case the seedling invariably
dies. The last three types of damage are usually caused by the feeding
of second- or third-instar grubs and may be found at any time from
early in the spring until late in the fall, although they are most
prevalent from midsummer to late fall when first-year grubs are most
active. Root injuries of the types illustrated are usually accompanied
by a wilting of the tops and browning of the needles. However, if the
grub infestation is particularly severe during cool, moist periods,
all four degrees of damage may occur unaccompanied by immediate changes
in top appearance.

PERIOD OF HEAVIEST DAMAGE

As mentioned above, grub damage in southeastern nurseries may
occur at any time during the growing season, from early in the spring
until late in the fall; but most severe injury occurs from mid-August
to late in October. This damage is the result of feeding by first-
year grubs. Therefore, from the standpoint of remedial control, most
concern must be given to the current season's brood of grubs which
hatch from eggs laid after the seedbeds are planted.

CONTROL MEASURES

Selecting the Nursery Site

As a means toward preventing damage caused by white grubs,
soil that is free from these insects should be selected for nursery
sites. Certain localized areas may be heavily infested, whereas
other areas which are equally well adapted to the production of seed-
lings are almost entirely free from grubs. Much damage can be pre-
vented if soils that are already infested are avoided.






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Whether or not a prospective nursery site is infested with
grubs can be determined by searching for the adults on the foliage
of plants growing on the area. Such inspections should be made on
warm summer nights at intervals over a period of a few weeks. The
presence of an abundance of beetles indicates future trouble from
grubs.

An estimate of grub population can also be obtained by screen-
ing soil samples to recover larvae. Taking 1-cubic foot samples of
soil at regular intervals of 40 feet proved to be a fair index to
grub population in a South Carolina nursery.

Injecting Carbon Disulfide into the Soil

Of the numerous methods tested, the injection of carbon di-
sulfide into the soil at the rate of 1 pint per 100 square feet of
soil surface appears to be the best available means of control for
white grubs in forest nurseries. Although this chemical may not erad-
icate all the larvae, when used judiciously it is an effective control
measure and causes negligible injury to pine seedlings. Under most
nursery conditions excellent control of larvae may be expected in the
upper 6 inches of soil where most of the grubs are found during the
period of severest injury. The depth of penetration of the chemical
depends somewhat on the nature of the soil, for the highest grub mor-
tality is obtained in sandy or sandy loam soils of a loose, porous
nature.

In applying the chemical it is important to avoid treatment of
soils that are too wet (above 15 percent in moisture content), as this
condition results in severe chemical burning of the pine seedlings.
Minimum plant injury is incurred when treatments are made in soils that
are in a suitable condition for cultivation, i.e., moist and friable.
The chemical remains in the soil at least 1 hour after it is applied,
therefore the treated areas should not be watered within 1 hour after
treatment, as this will cause chemical injury to the seedlings.

To obtain maximum control of the grubs with minimum chemical
injury to the seedlings, the following conditions must be complied
with:

(1) The soil, as just stated, should be moist and friable
(suitable for cultivation), and it should not be watered within 1
hour after treatment. The carbon disulfide should not be injected
into the soil immediately before or after rains, as treatment under
such conditions, when the ground is wet, results in severe burning
of the plants.

(2) The temperature throughout the entire upper 6 inches of
soil must be 780 F. or above.




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(3) The chemical should be injected into holes in the ground
3 to 4 inches deep, spaced 6 by 6 inches apart, at the rate of 1.2 cc.
per hole. Immediately after the chemical has been injected, the holes
should be filled with soil and the soil firmly packed.

Carbon disulfide is inflammable and should be handled with the
same care with which gasoline is handled.

Carbon disulfide may be injected into the soil with an ordinary
hypodermic syringe without the needle, or with an injector for forcing
liquids into the soil. 6/ This injector greatly facilitates treatment
and prevents loss of the chemical by evaporation.

This method of control can be used for treating local "spot"
infestations, or for general treatments of entire seedbeds. In mak-
ing treatments where only a few plants have been damaged, the chemi-
cal should be applied to the area immediately surrounding the infested
area as well as to the known infested area, in order to kill migrat-
ing larvae. General treatments should be made when grub-injured
plants are found throughout the seedbeds. The extent of damage can
be determined by the appearance of the plants, and by gently pulling
seedlings here and there in the seedbed as previously described.

Cost of Treatment

The cost of treatment depends to a large extent on the cost
of labor. Carbon disulfide can be purchased wholesale at a cost of
approximately $1 per gallon. At the rate of 1 pint per 100 square
feet, the cost of the chemical alone is 12- cents per 100 square
feet of seedbed. Assuming that 1 square foot of seedbed will yield
30 merchantable seedlings, the cost of treatment is slightly over 4
cents per 1,000 plants, exclusive of labor.

Mechanical Barriers

Plots of very valuable plants may be protected from white grub
infestations by surrounding them underground to a depth of 18 inches
with 18-mesh wire screen and providing covers of 1-inch-measure hard-
ware cloth for the tops of the seedbeds. This method of control,
however, may be too expensive and impractical for general nursery
practice.

SUMMARY

In southeastern forest nurseries the feeding of white grubs
(Phyjjlphaga spp.) on the roots of young tree seedlings results in
a heavy loss in the annual production. The southern pines, notably
longleaf, shortleaf, loblolly, and slash pines, suffer the most severe


6/ Eaton, C. B. An injector for forcing liquids into th.E soil.
U. S. Dept. Agr., Bur Ent. and Plant Quar., ET-127 (multigraphed).
1938.





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injury, although attacks on various hardwoods have been noted. Sev-
eral degrees of root injury, associated with grubs of various ages,
are recognizable.

The species most commonly found in this section have a life
cycle of from 2 to 3 years, the greater part of which is spent in the
grub stage. Both first-year and second-year larvae cause injury,
but the former, which hatch from eggs laid in the ground after the
seedbeds have been planted, cause the most severe damage late in
the summer. Early-season damage is usually caused by second year
grubs. The larvae are more or less inactive during the winter months
and show a slight tendency to seek the lower levels in the soil as
the temperature decreases.

Carbon disulfide injected into the soil at the rate of 1 pint
per 100 square feet seems to be the best available means of control
for these insects. Protection may be secured at a cost of slightly
over 4 cents per 1,000 plants, exclusive of the cost of labor for the
application of the chemical.











































Figure .-- Root injuries on pine seedlings as a result of feeding by
Phyllophaga larvae: A, Healthy root system; B, light damage (ends
of laterals and tap root cut); C, severe damage (all lateral roots
cut); D, severe damage (lateral roots cut and cambium stripped from
tap root); E, severe damage (tap root cut Just beneath the surface).





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