The bulb scale mite


Material Information

The bulb scale mite
Physical Description:
Doucette, Charles F ( Charles Felix ), 1898-
United States -- Bureau of Entomology and Plant Quarantine
U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine ( Washington, D.C )
Publication Date:

Record Information

Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 30271744
oclc - 778790110
System ID:

Full Text
E-507 July 1940



By Charles F. Doucette, Division of Truck Crop and Garden
Insect Investigations


Ir recent years many stocks of narcissus bulbs have been found in-
fested with a very small mite, the bulb scale mite (Tarsonemus approximatus
narcissi Ewing). Some reduction in the quantity of blooms may result from
infestation, and the flowers may be so distorted and stunted that they are
valueless. Under some conditions the bulbs go through their seasonal devel-
opment with the mite populations remaining at a very low level, and injury
or symptoms of attack are not observed; hence a grower is not likely to
detect the mite's presence in his stocks. Frequently, however, conditions
become favorable for rapid development of the mites, and the populations in
the bulb may increase very rapidly and cause very serious injury.

Infestations may be present in any region to which narcissus is
adapted. The mites have been reported in many producing areas and have
undoubtedly been carried to many localities on bulbs moving in regular
commercial channels. The mite is reported as present in stocks in England,
and bulbs from Holland have been found infested. No varieties of narcissus
are known to be immune. The mite also attacks several other bulbous species
of the family Amaryllidaceae.

Injury Caused by the Bulb Scale Mite

Symptoms of injury do not become apparent until the populations in the
bulbs become excessive, and consequently in places where cool growth condi-
tions prevail the mites may he present for a long time without being detected.
In general low temperatures retard development, moderate temperatures ac-
celerate development, and high temperatures probably kill most of the mites.
The most rapid development takes place between 600 and 800 F. Temperatures
of 950 or above, if continued for several days, probably kill all the mites.
Below 600 the mites exist but do not develop rapidly, and the populations in
the bulbs remain more or less stationary at a very low level.



The mites feed principally on the epidermal surfaces of the bulb
scales and of the leaves in the "neck" area of the bulb. The top, or neck
area, of the bulb is the focal center of an infestation. During the growing
season the mites concentrate in this spot, their activity extending upward to
the point where the leaves separate, usually above the soil-surface line.
Wherever sufficient space is available between scales and between leaves the
Diites work their way in. The new shoots developing inside the bulbs during
storage period are at first enclosed in thin sheaths, which separate as
the tips of these shoots approach the tops of the bulbs, thus allowing the
mites to enter. This stage of bulb development comes late in the summer
(late August and early September), when temperature conditions are partiu-
larly favorable for the mites. If the favorable conditions continue for any
extended length of time, the new growth may be seriously affected. It is
this particular injury which is most important, as it causes the reduction
in number of flowers and the distortion and stunting of the flowers that do
develop. The mites feed in the neck of the bulb, and as the leaves and
stems grow away from this point the areas of feeding become dry, resulting
in scarlike yellowish-brown streaks. Frequently there are short transverse
cracks in the scarred areas, and distortior and twisting of a varying degree
are usually associated with this scarring, for the uninjured side of the stem
or leaf grows fast whereas the mite-injured portion is slowed down in growth.
Often where infestation is less severe the only symptom evident may be a
slight twist of the flower stems amounting to about one-fourth or one-half a
turn between the bulb and flower.

The injury by this mite may be confused with that caused by mosaic
disease and the bulb nematode. All leaves of plants affected by mosaic
disease show streaks and irregular areas of pale green or yellow, giving a
striped or mottled appearance. The mottled areas are sometimes roughened,
but do not have the scarred tissue as do the mite-injured leaves. Injury by
the bulb nematode is recognized by the presence of small thickenings in the
leaves called "spikkels." These are easily detected by drawing the leaves
between the thumb and forefinger. Pale green spots and bands on narcissus
foliage may also result from frost injury after the leaves emerge and also
from improperly timed hot-water treatment. However, the surface of these
areas is usually not roughened.

On the flowers, mite injury causes pale-yellow, often thin, and
somewhat sunken, streaks, or the entire flower may be deformed and stunted.

On dormant bulbs there are no external indications of the presence
of these mites. Infestation within the bulbs is evidenced by moderately
c~ark yellowish-brown areas on the normally white scale tissue. A bulb must
be cut open from neck to base and the scales and shoots separated in order
ihtthese discolored areas may be seen. Usually these areas occur where
rLere is naturally a slight separation of the scales, and the discolored
areas are mostly likely to be found on the edges of that scale which rep-
resents the flower stem of the preceding spring.


Description and Development

The mites are exceptionally small and are not discernible without thc
aid of a microscope. The adult mites are only 1/125 of an inch long. Groups
of considerable numbers look like a scattering of fine grains of very light-
colored sand. White six-legged larvae are hatched from the white oval-shaped
eggs and develop until they assume a quiescent form. During this quiescent
larval stage changes in the mite occur which are equivalent to those occur-
ring in the pupal stage in insects, after which the eight-legged adult mites
emerge. Both sexes are light tan in color. The complete development takes
place on the plant, and many of the mites may go through their complete life
without moving a distance of one-half inch from the point where they emerged
from the eggs. The females have a tendency to deposit eggs in narrow
crevices, and the larvae seek similar places before they enter the quiescent

Spread among growing bulbs is relatively slow, a few feet being the
usual limit during a season. However, in storage, where the bulbs are
concentrated in trays or similar containers, infestation may become wide-
spread in a stock even though the initial infestation may have been rela-
tively very small.

Injury is associated with high populations of mites in the bulbs.
Approximate counts made of the mites in dormant bulbs have indicated the
presence of several thousands in individual bulbs previous to planting.
It has also been demonstrated that populations can build up to serious
numbers within a period of approximately 2 months, under favorable condi-
tions. If the conditions continue to be favorable, these numbers are prob-
ably increased considerably during the period of early growth of the plant.
However, when the environment of the planted bulb is not favorable, the popu-
lations drop to a few hundred per bulb, a level much below that which causes


To control this mite, dependence-must be placed on thermal treatments
Eradication should be the objective and is easily attained if proper pre-
cautions are observed. Bulb producers who have been following a regular
thermal treatment schedule for their stocks have had little trouble with
this pest. Immersion of the bulbs in hot water at 1110 F. for 1 hour, in
addition to the time required for the bulbs to reach that temperature, re-
sults in complete kills of all stages of the mites, with sufficient allowance
for any variations which might occur in ordinary procedure. Treatment of
bulbs for nematode infestation or as a precaution against such infestation
naturally is effective against the mites, for the standard duration of
treatment recommended for this pest is 4 hours at 1110. Normally 20 to 40
minutes is required for the bulbs to reach the treating temperature after
they have been placed in the hot water. This period added to the suggested
treating period of 1 hour makes the total immersion time required approxi-
mately 1 hours. This period of treatment is also sufficient to kill all
stages of the ordinary bulb mite (Rhizoglyphus hyacinthi Bdv.; and the


larvae of the narcissus bulb fly (Merodon equestris (F.), the lesser bulb
fly (Eumerus tuberculatus Rond.) and other bulb flies (Eumerus spp.)

Treatment of Large Quantities of Bulbs

Equipment for hot-water treatment is in general use by most narcissus
bulb growers. Rectangular tanks are customary, the heating of the water
being accomplished by steam, either through coils or by direct injection into
the water. Circulation of the water is accomplished by motor-driven pro-
pellors. To hold the bulbs, specially constructed wire-mesh containers or
standard-type shipping crates are used.

Treatment of Small Quantities of Bulbs

If there is only a small quantity of bulbs to be treated, a washtub
or similar container may be used satisfactorily. Continuous attention is
required to maintain the water temperature within the desired limits, but
where only one or two treatments are needed the expense of special equipment
is avoided. When this type of equipment is used, the bulbs should be placed
in open-weave sacks (onion type) or in cheesecloth bags, and these should
be placed loosely in the water. Tight packing should be avoided, because
it hinders the circulation of the water among the bulbs. At the start the
water in the container should be at a temperature of 1130 or 1140 F. The
immersion of the bulbs in the water will lower the temperature to the desired
treatment point, 1110. Small quantities of hot water (1500 to 1800) added
at frequent intervals will maintain the temperature at the desired point with
only slight variation. Actual trials when the air temperature was 750 to
800 have demonstrated that additions of approximately 1 pint of hot water
(1600 to 1800) at 10-minute intervals maintained the temperature of the water
in a tub with less than one degree variation above or below the treatment

Regardless of the type of treating equipment used, it is essential
than an accurate thermometer, graduated in single degrees, be used to
determine the temperature of the water.

Vapor-heat treatment is likewise effective as a control measure for
the mites. However, this requires special equipment and is economically
practical only for large-scale operations. A vapor-heat treatment of 2 hours
at ll1 in addition to the time required for the bulbs to become heated to
that temperature, will give complete kill of all stages of the bulb scale

Time of Treatment

The most desirable time to treat narcissus bulbs is from 3 to 4 weeks
after digging, assuming that the bulbs are dug at the normal time, shortly
after the foliage dies. The wet bulbs should be spread out immediately

5 -

after treatment to permit rapid drying, otherwise damage by molds and
similar detrimental organisms will be likely to occur.

As it is not advisable to apply hot-water treatment to bulbs which
are to be sold, bulb producers should treat their planting stock from which
bulbs are to be selected for sale the following season. In addition proper
precautions should be taken to avoid contamination from any other stocks
that might be infested.


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