STATE kPLAN F BOARD
E-490 October 1939
United States Department of Agriculture
Bureau of Entomology and Plant Quarantine
CONTROL OF.THE GLADIOLUS THRIPS ON CORMS DURING STORAGEi1
By C. A. Weigel and R. H. Nelson.
Division of Truck Crop and Garden Insect Investigations
The gladiolus thrips feeds, both in the mature and immature stages,
on all parts of the growing gladiolus plant and also on the corms while
in storage, provided the storage temperatures are favorable to thrips
activity. Infested corms show a russetting of the affected areas, and
the young rootlets are killed. This retards subsequent growth and results
in the production of inferior flowers. The thrips is brought into storage
at harvest and, unless the infested corms are kept at low temperatures or
treated, it will continue to feed and multiply throughout the storage period.
When infested corms are planted the thrips will continue to breed and feed
on the young sprouts, and the infestations continue to develop with the
Prevention and Control
After digging, the corms should be stored in as cool a place as
possible, preferably between 32 and 50 F. Temperatures below 32 should be
avoided, as injury is likely to occur. When the corms have dried enough to
be cleaned they may be treated. It is best, however, to defer treatment
until a killing frost has destroyed all gladiolus foliage out of doors.
Thrips disappear after a killing frost, and there is little danger then
of the thrips coming into the storage or warehouse from the outside. It
is also advisable that all discarded corms and other refuse removed from
the corms during the cleaning process be promptly disposed of by burning.
It is preferable, wherever possible, that all stocks on hand in a
given storage room be treated at one time. Furthermore, all corms that are
subsequently acquired should be treated before they are stored or placed
with other clean stocks.
Several methods of treating corms are now available which, if properly
applied, will kill all thrips without causing injury to the acorns. Unless
a grower finds it necessary to soak the stocks in corrosive sublimate or
semesan to control scab or other rots, a dry treatment such as fumigation
I A general account of the gladiolus thrips and its control is contained
in Circular E-462, issued in January 1939.
seems preferable. Three different fumigants may be used for this purpose,
namely, naphthalene flakes, calcium cyanide, or ethylene dichloride-carbon
tetrachloride mixture. To save extra handling and expense, any of these
treatments is most easily applied as soon as the stocks have been cleaned.
The methods selected will depend largely upon the circumstances and facili-
ties of the individual grower.
Naphthalene flakes.--This material is now being most generally em-
ployed by growers during the storage period because it is readily available,
cheap, and safe to both the user and the corms, even if an excessive dose is
accidentally used. One pound of flakes (equal to one quart) should be used
per 2,000 corms. The naphthalene flakes are sprinkled over and among the
corms in the trays in which they are stored and then covered with a light
canvas or wrapping paper.
The naphthalene should remain with the corms for about 4 weeks, after
which time the excess flakes should be shaken out. If the treatment is ap-
plied late in the fall or early in the winter, the flakes may be left with
the corms for 2 months or longer without harm. The treatment may be used at
any time during the winter, preferably between early in November and early
in March, because the corms are then dormant. Treatment late in the spring
should be avoided because growth activity is then underway, and the naph-
thalene will then injure the young shoots and roots and retard the resultant
corm growth in its development.
A particular advantage of naphthalene is that it can be used with
the corms in the storage rooms without danger to inhabitants in nearby
or attached dwellings. Any thrips that may be outside the containers or
bulb trays will die before they can reinfest the corms, owing to the long
exposure to the fumes present in the room. Do not use a covered tin or
equally tight container, because the corms are likely to "sweat" or sprout
during the treatment.
Calcium cyanide.--This material, as well as the ethylene dichloride-
carbon tetrachloride mixture discussed below, is preferable only for growers
who have storage houses of very tight construction or who are equipped with a
fumigation chamber especially built for such purposes. Calcium cyanide
is extremely poisonous and should be used only by persons who are competent
and thoroughly familiar with the hazards involved in handling it.
A dosage of 5 ounces per 1,000 cubic feet of space should be used
with a fumigation period of 3 hours. Since the eggs are not killed, it is
necessary to fumigate the corms three times at 10-day intervals. The
stock should be held at a temperature of from 60 to 70 F. just prior to
and during the fumigation. They should also be held in this temperature
range during the intervals between fumigations so that any eggs that may be
on the corms will hatch in the interim. After the third treatment the corms
should be returned to cool storage.
The corms to be fumigated should be in open slat or wire-bottom shal-
low trays.- These should be stacked in such a manner that air can circulate
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between the individual trays. Not more than three-fourths of the space in
the room should be taken up by the corm-filled trays. An electric fan may be
used during the exposure and will aid in obtaining a better distribution of
the gas within the fumigation room.
In using calcium cyanide the following procedure should be followed:
(1) Calculate the cubical contents of the room to be used by multiplying
the width by the length and then by the height. (2) Weigh out the correct
amount of calcium cyanide, using 5 ounces per 1,000 cubic feet of space.
(3) Close tightly all openings into the room except the entrance. (4) Scat-
ter the material thinly over the floor as quickly as possible, beginning at
the far end and working toward the open door. (5) Close the door, post a
warning sign, and allow no one to enter until the fumigation period is over
and the room has been thoroughly aired.
Under no circumstances should this type of fumigation be done if the
storage room is connected to an occupied dwelling or animal shelter, as
disaster may result. In such cases a special fumigation room must be used
and the corms returned to a thrips-free storage after treatment. The latter
point is very important, because if thrips are present in the storage room
the corms will become reinfested after the fumigation.
Ethylene dichloride-carbon tetrachloride.--This mixture is a non-
explosive liquid. It is used at the rate of 14 pounds (5 quarts) to 1,000
cubic feet of space with an exposure of 12 to 24 hours.
The recommendations regarding temperature of the corms, stacking of
the bulb trays, calculation of the cubic contents of the room, measuring the
correct quantity of material, and precautions, as given for calcium cyanide,
should be followed here. However, only one fumigation is necessary, since
this material kills the eggs as well as the other stages of the thrips.
In using this mixture, place a flat shallow pan, or pans as neces-
sary, on top of the stacked bulb trays. Place sheets of blotting paper or
crumpled newspaper in the pans and pour in the measured fumigant. Close the
room and allow it to remain shut for the desired time. Be sure that the
quantity to be used has been correctly calculated, since a heavy overdose is
injurious to the corms.
The point regarding possible reinfestation, from thrips in the storage
room, mentioned under calcium cyanide must be kept in mind with this material
An electric fan may be used to aid in obtaining better distribution of
either of the above fumigants. As with the naphthalene flakes, avoid using
a tin or equally tight container because the corms are likely to "sweat"
and thus be induced into growth.
Corm Treatment at Planting Time
The use of naphthalene flakes during the winter is preferable whenever
practicable, but under certain conditions growers may find it necessary
UNIVERSITY OF FLORIDA
3 1262 09224 6809
to treat their stocks just prior to planting, especially when new stocks
are being acquired late in the spring or if an infestation is not discovered
until that time. Under these conditions the corms may be treated with
mercuric chloride (corrosive sublimate).
Mercuric chloride.-This chemical, when used in a solution containing
1 ounce to 7- gallons of water (1 to 1,000), kills all thrips on unpeeled
corms which are submerged for 12 to 17 hours. Since mercuric chloride
dissolves very slowly in cold water, it is advisable to first dissolve it
in a small quantity of hot water, then dilute to the correct amount with
cold water. Sufficient solution should be prepared so that the corms will
be completely submerged. Do not use the same solution a second time, but
make up a fresh one for each new lot of corms. Mercuric chloride is deadly
poison, and the greatest precautions should be taken in handling it. The use
of rubber gloves may be desirable. Since mercuric chloride is corrosive to
metals, it should be mixed and used only in glass, earthenware, or wooden
It should be remembered that corm treatment alone does not prevent
later reinfestation. It is very essential to isolate untreated corms
from treated stock and to avoid carrying thrips from one to the other on
clothing, tools, or containers. The refuse from cleaning should be promptly
destroyed, and all new stock should be treated before it is stored with any
corms that have been previously treated.