Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices

m

LIBEARY

OF THE

U, S, Department of Agriculture
Reserve

Class I

Book .Z-nSSC

&A

«<

«

^Si

IIII

Circular No. 57.

United States Department of Agriculture,

BUREAU OF ENTOMOLOGY.
L. O. HOWARD, Entomologist.

THE GREENHOUSE WHITE FLY.

(Aleyrodes vaporariorum Westw.)

By A. W. Morrill,
Special Field Agent.

The damage by the greenhouse white fly to tomato, cucumber, and
man} r other plants growing under glass easily places it in the front
rank of greenhouse pests. In many cases it would be impossible to
grow certain crops in forcing houses without the aid of remedial
measures. A specific instance is on record where, in a western Massa-
chusetts town, the attacks of this insect resulted in the total loss of a
greenhouse crop of tomatoes and cucumbers valued at $4,000.

Fig. 1.— Aleyrodes vaporariorum: a, egg; 6,. young larva; c, pupa, top view; d, pupa, side view;
e, adult— c, d, e, about 25 times natural size; a, b, still more enlarged (a-d, after Morrill, Tech. Bui.
Mass. Exp. Sta.; e, original).

20127-05

During the past few years many appeals for remedies have been
made to the editors of our horticultural journals, to many State exper-
iment stations, and to the United States Department of Agriculture;
yet, although it is not, in reality, a difficult insect to control, the
remedial methods which should be followed are far from being gener-
ally known among the many greenhouse men who suffer from its
depredations.

HISTORY, ORIGIN, AND DISTRIBUTION.

The records of the greenhouse white fly date back to 1856, a when
Prof. W. O. Westwood, of England, recognized it as a previously
und escribed species. The first published record, so far as is known to
the writer, of the existence of the insect in this country was in the
year 1870, when Dr. A. S. Packard, at that time Massachusetts State
entomologist, reported it as occurring in abundance on tomato plants
at Salem, Mass. h

Two places have been suggested as the original home of this
species, viz, Mexico and Brazil, but while presumably the origin is
tropical American, there is no definite information on this point. Of
more importance from a practical standpoint is its present distribu-
tion. Besides its occurrence in Europe, Canada, and Mexico, it is
known to be widely distributed in greenhouses throughout the eastern
United States, and without doubt it occurs more generally than the
published records show. We have specific reports of its occurrence
in greenhouses in the States of Maine, New Hampshire, Massachusetts,
Connecticut, New York, New Jersey, Penns} T lvania, Illinois, Indiana,
and Michigan, also in the District of Columbia.

FOOD PLANTS.

The insect under consideration is notable for its very general feed-
ing habits, having already been recorded as breeding on over 60 dif-
ferent kinds of plants. Of these the following are of the most eco-
nomic importance: Aster, chiysanthemum, salvia, lantana, fuchsia,
coleus, ageratum, primula, geranium, heliotrope, rose, eggplant, bean,
melon, lettuce, cucumber, and tomato. The two last named suffer the
most serious injury from this insect, perhaps more than the other
greenhouse plants together, although not infrequently there are
reported serious losses in greenhouses devoted to one or more of the
other plants mentioned.

DESCRIPTION AND LIFE HISTORY.

The mature white flies of both sexes are four-winged insects scarcely
more than 1£ mm. or three-fiftieths of an inch in length. The adult

« Gardeners' Chronicle, p. 852. & Agriculture of Massachusetts for year 1870.

3

white flies, as well as the scale-like larvae, are provided with sucking
mouth parts. In a short time after the emergence of the adult from
the pupa case, the body, legs, and wings become covered with a white,
waxy substance which gives this, as well as other species of the genus,
a characteristic floury appearance. The adults feed nearly contin-
uously during their existence. If deprived of food, they will rarely
live for a longer period than three days under ordinary temperature
conditions. The longest recorded length of life of one of these insects
in the adult condition is thirty-six days, but it seems probable that the
average length of adult life is much greater than this instance would
indicate. The largest number of eggs which an adult white fly is
positively known to have deposited is 129, but this number is probably
below the average. Indeed, the specimen which produced this num-
ber of eggs with little doubt deposited over 50 others which were not
recorded. The number of eggs deposited per day by an adult female
white fly in a laboratory has been found to average very nearly four.
Probably in the warmer temperature of a greenhouse this number is
greater by one or two eggs per day. These observations, even though
falling short of showing the normal increase in numbers of this spe-
cies, emphasize the importance of a remedy which will, above all,
destroy the adults and check at once the rapid deposition of eggs. A
peeularity of the egg-laying habits of this and some other species of
white fly is the tendency to deposit the eggs in a circle while feeding,
using the beak as a pivot. These circles, when completed, are about
1^ mm. in diameter and usually contain from 10 to 20 eggs each. On
the more hairy leaves groups of eggs of this kind are less frequently
met with than on those which are more nearly smooth. The majority
of the adults are found upon the upper and newer leaves of the food
plant. They are almost invariably found upon the underside of the
leaves, and it is here that nearly all the eggs are deposited, although
many are found upon the tender stems and leaf petioles and a very
few scattering ones on the upper surfaces of the leaves.

The eggs are distinguishable with difficulty by the naked e} T e, being
but one-fifth of a millimeter, or one one hundred and twenty -fifth of an
inch, in length. They are more or less ovoid in form and suspended
from the leaf by a short, slender stalk. With ordinary greenhouse
temperatures the eggs hatch in from ten to twelve days. The newly
hatched insect is flat, oval in outline, and provided with active legs and
antennae. It rarely crawls farther than one-half inch from the empty
eggshell before settling down and inserting into the tissue of the leaf
its thread-like beak. After feeding for five or six da}^s, the insect is
ready to molt its skin. The second and third stages are much alike,
except in size, and differ principally from the first stage, in that the
legs and antennae are vestigial and apparently f unctionless. These two
stages occupy from four to six days each.

!: !l

The so-called pupal stage, up to the time when growth ceases, is in
reality the fourth larval stage, the fourth larval skin enveloping the
true pupa. The pupae and empt} r pupa skins are quite conspicuous
when the insects are abundant. Their outline is similar to that of the
larvae, but they are thicker and box-like, about three-fourths of a mil-
limeter, or three-hundredths of an inch, in length and provided with
long, slender wax rods or secretions which are useful in distinguishing
this from nearly allied species of the white fly.

The entire stage from the insect's third molt to the emergence of
the adult form lasts from twelve to sixteen days in the laboratory 7 and
greenhouse. The adult emerges from a T-like opening, leaving the
glistening white pupa case attached to the leaf. At first the wings of
the adult are crumpled close to the body, giving them a peculiar
appearance. In the course of a few hours the wings unfold and the
insect has then completed its development, which has extended over
nearly five weeks, if under the ordinary temperature conditions of a
greenhouse.

CLOSELY RELATED FORMS.

In addition to the one here discussed, there are but two other species
of white flies which are likely to be met with in the greenhouse. A
white fly found infesting citrus plants would be likely to be the
orange white fly (Aleyrodes citri Riley and Howard), while one infest-
ing strawberry plants, either in the greenhouse or in the field, would
probably prove to be the strawberry white fly (Aleyrodes packardi
Mori.). The latter species resembles the one commonly found in green-
houses, but fortunately its list of food plants is much more restricted,
it being apparently unable to subsist on the tomato.

APPEARANCE OF INFESTED PLANTS.

As already stated, the upper leaves of a plant are preferred by the
adult females for the deposition of their eggs. Thus there is a slow but
continuous migration of adults upward to keep pace with the unfolding
of the leaf buds. On thoroughly infested plants we find on the upper-
most leaves only adults and freshly laid eggs; a little lower on the
plants we find eggs in the process of hatching; and, finally, on the
lowermost parts of the plants we find discolored, shriveled leaves with
many pupae and emerging adults and few, if any, unhatched eggs or
young larvae. The larvae and pupae secrete little globules of honey-
dew, so named after the material of a like nature secreted by plant
lice. These globules usually either drop or are forcibly ejected, and,
falling on the upper surface of leaves directly below, give them a
glazed appearance. This is frequently followed b} r the growth of a
soot} 7 fungus which hastens the complete destruction of the leaf.

When overcrowding of the young occurs, this fungus growth finds
favorable conditions for its development on the under surface of the

leaf, resulting in the destruction of many of the immature insects-
Owing- to the interference with the respiratory processes of the leaf,,
both by the bodies of the insects themselves and by the fungus growths
due to them, badly infested plants have a tendency to wilt when exposed
to the sun's rays. In seriously infested greenhouses the leaves of the
plants gradually die, the lower leaves first, and if unchecked the insects
greatly impair the value and vitality of the plants, even though they
do not actually cause their total destruction.

PREVENTIVES.

The importance of preventive measures in combating the white fly
in greenhouses is not due to the inefficiency of properly applied reme-
dies, but to the fact that in many cases the tiny depredator is unob-
served until considerable injury has been accomplished. With little
trouble and expense one may, in a large measure, preclude the possi-
bility of this and other pests appearing in the winter in greenhouses
which are unused during the summer months. The introduction of
the insect into noninfested floral establishments may be prevented by
avoiding the introduction of infested plants unless first subjecting
them to a fumigation in a tight fumigating box, based on the directions
hereafter given for entire greenhouses. Vegetable houses, which are
not used during the summer months, allow of a practice which not only
greatly reduces the chances of the white fly appearing in the house
during the growing season, but may result in the house being success-
fully kept free from thrips and other insect pests during the whole or a
considerable part of the season. The practice referred to consists in
removing from the house all vegetation, even the smallest weeds, and
fumigating the tightly closed greenhouse with hydrocyanic-acid gas at
the rate of 5 or more ounces per. thousand cubic feet of space for a ten
hours' exposure.

REMEDIES.

Fumigation with hydrocyanic-acid gas. — This has been found to
be the most successful means of controlling the white fly in green-
houses. Its success in this case is due to the susceptibility of the
adults and larvae of these insects to a comparatively long exposure to
a small amount of the gas. Mairy experiments have been conducted
with a view to determining the usefulness of this gas against the
greenhouse white fly, the amount of gas to be generated, and the
length of exposure necessary to produce the best results. a Experi-
ments of this kind thus far have been with tomato and cucumber
plants, but as these plants are among those most liable to injury from
improper fumigation with hydrocyanic-acid gas, a wide range of use-
fulness is indicated by the success thus far obtained.

«Conn. Station Bui., No. 140; New Hampshire Station Bui. , No. 100; Mass. Station
Tech. Bui., No. 1; Maine Station Bui., No. 96; Can. Entomologist, XXXVI, p. 35;
American Gardening, XIX, p. 741.

6

Amount of potassium cyanide to use and length of exposure. —
Experiments and practice have shown that the white fly is destroyed
in all except two stages (egg* and late pupal) by an amount of potas-
sium cyanide which is extremely small as compared with the amounts
generally recommended for other insect pests. As small an amount
as 0.005 gram" per cubic foot of space, or between one-fifth and one-
sixth ounce per 1,000 cubic feet, for three hours' exposure, has been
used with success, b while as large an amount as 1 ounce per 1,000 cubic
feet for an "all-night exposure," in a house containing infested toma-
toes, has been reported to have given, in one instance, a like result/
On the other hand, Mr. E. C. Rittue, of the Bureau of Plant Industry
of the Department of Agriculture, in attempting to control the
white fly infesting tomatoes in a greenhouse on the grounds of the
Department, found that 0.01 gram per cubic foot, or one-third ounce
per 1,000 cubic feet, slightly injured the plants when the exposure
exceeded thirty minutes. This treatment for thirty minutes destroys
only the adults. The greenhouse is a new one and, judging from the
great difference in the results obtained there and in other houses
whose fumigation has been recorded in various publications, it is
tighter and does not allow the gas to escape as readity as does the
average forcing house.

This shows that the greatest difficult} 7 attending the use of hydro-
cyanic-acid gas, in greenhouses containing plants as susceptible to
injury by it as are the cucumber and tomato, is the difference in the
tightness of different greenhouses. It is consequently impossible
to give specific directions which will be suitable under all circum-
stances. A fumigation with hydrocyanic-acid gas which will kill
adults o\\\y is not effective for practical use in checking the multipli-
cation of the white flies. Rather than this, the method of control
described under the heading, "Treatment when the use of hydrocyanic-
acid gas is undesirable," is greatly to be preferred.

In most greenhouses, probably 0.007 gram of potassium cyanide for
each cubic foot of space for an exposure not exceeding three hours
represents the amount which will prove most effective for treatment
of the insects without injury to tomato or cucumber plants. In many
cases 0.01 gram per cubic foot has been found suitable for the same
exposure, but this should not be used except in loose greenhouses
where, after trial, a smaller amount is found ineffective. In all green-
houses when an attempt is to be made to control the white fly with
hydrocyanic-acid gas, it is advisable to first use not more than 0.005
gram per cubic foot of space for a three hours' exposure. If this
amount is sufficient for the house, none of the adults will recover after
the fumigation, though in the course of two or three days many more

a 28.35 grams=l ounce. &Mass. Station Tech. Bui., No. 1, p. 46.

'American Gardening, XIX, p. 741.

will emerge from the pupa cases. The larvee, when destroyed, as they
should be by the fumigation, change in two or three days from their
normal glistening, greenish color to a yellowish or brownish color.
When this result is not obtained by the first test, one or more further
tests should be made, increasing the amount of potassium cyanide
0.001 gram per cubic foot of space for each test, with three days inter-
vening to note results, until an amount is reached which is sufficient to
destroy the larva?, or until the tender leaves of the plant show injuries
as a result of the fumigation.

Tests, thus far, with other greenhouse plants likely to be attacked
by the white fly, according to available records, have all been for a
much shorter exposure than three hours and with a much larger
amount of potassium cyanide, but it is probable that in case plants
other than the cucumber and tomato require treatment for this insect,
preliminary tests in a fumigating box or in a small greenhouse will
show that the amount of chemicals and length of exposure recom-
mended for these two can be used without the slightest danger to other
plants. In most cases much larger rates of potassium cyanide per
cubic foot can be used.

Time to fumigate, prejxtration of greenhouse, and method of gener-
ating gas. — Fumigation of plants with hydroc} T anic-acid gas should be
at night and the foliage of the plants should be dry. The greenhouse
to be treated should be made as tight as possible, all entrances but one
closed and locked, and arrangements made to open a few ventilators
from the outside at the expiration of the period of exposure. A house
when fumigated should not be unnaturally tight as a result of rain
or snow, otherwise the greater amount of gas confined in it under
these conditions may injure the plants. The materials used for the
generation of the gas are 98 per cent potassium cyanide, commercial
sulphuric acid, and water, the proportions generally used being one-
half more acid (liquid measure) than potassium cyanide, and one-half
more water than acid. Having determined the cubic contents of the
house and the total amount of potassium cyanide, sulphuric acid, and
water to be used, these should be divided into parts representing each
25 feet of length of the greenhouse. Owing to the small amounts of
the acid and water, small receptacles must be used. Six or 8 inches
is a desirable height for the receptacles, while the diameter should be
as small as possible to use, preferably not more than 2^ inches. They
should be either of earthenware or glass. In many cases, ordinary
glass tumblers will be suitable, though the diluted acid should never
more than one-fourth fill the receptacle; otherwise the violent chem-
ical action which follows the introduction of the potassium cyanide
might result in the loss of considerable of the material. Each lot of
the potassium cyanide should be pulverized or broken up into small
pieces, wrapped in thin paper, and laid beside one of the receptacles,

these being placed at intervals of about 25 feet on the floor of the
house. In each receptacle lirst pour the proper amount of water and
then the acid. Beginning with the end of the house farthest from the
exit, drop into each receptacle, in succession, the package of potas-
sium cyanide, proceeding as quickly as possible toward the exit.
During the few seconds the operator is in the house after the genera-
tion of the gas is started, the breath should be held to prevent even
the least bad effects. Close and lock the door of the house and. after
three hours, partially ventilate it by opening the ventilators, pre-
viously arranged for opening from the outside. One ventilator for
every 25 or 30 feet, opened for ten or fifteen minutes, is sufficient to
protect the plants from possible bad effects of overexposure to the
gas. Before inhaling air in the house, however, the ventilation should
be more thorough, so that no odor of the gas, which is much like that
of peach pits, can be detected. The morning after the fumigation the
contents of the receptacles should be buried.

Time for subsequent fumigations. — A single fumigation, according
to the directions given above, will destroy practically all of the insects
except the eggs and some of those in the late pupal stage. Although
one such treatment might check the insects so that they would not cause
noticeable damage for weeks, in many cases it would be the part of
economy to give two more fumigations at times which a knowledge of
the life history of the white fl} T indicates would be most advantageous.
Knowing that the egg and late pupal stage of the insect are not to
any great extent affected by the treatment recommended, while all the
other stages may be destroyed, and knowing the duration of each
stage, we can outline a plan of treatment which will practically eradi-
cate the pest in the worst-infested greenhouses. Two subsequent
fumigations two and four weeks, respectively, after the first will sub-
ject to the gas all of the white flies in the house in stages wherein,
under ordinary circumstances, they are unable to withstand its destruc-
tive effects.

Treatment when the use of hydrocyanic-acid gas is undesirable. —
Fumigation with tobacco fumes, made by burning the refuse stems
and leaves, has no effect on the greenhouse Avhite fly beyond tempo-
rarily stupefying the adults. The adults may be destroyed, however,
by vaporizing in the infested house certain tobacco extracts which
are sold in liquid form. To accomplish this result preliminary tests
should be made, first using the amount recommended in the directions
accompanying the preparation. The attempt to control the greenT
house white fly by means of tobacco extracts alone has never, to the
writers knowledge, proven successful, while many cases of failure
have been reported. The frequent fumigation necessary to control
the insect when once it has become abundant would be impractical
and costly. However, in connection with syringing the plants with a

soap solution such a treatment may sometimes be of value, although
only when the use of hydrocyanic-acid gas is impossible or for some
reason undesirable.

Among the sprays, the best brands of whale-oil soap, used in the
proportion of 1 to 1\ ounces per gallon of water, have been found to
destroy all of the white flies except the eggs, a small percentage of
the nearly mature pupa?, and from 25 to 50 per cent of the adults
which escape the spray by flying from the plants. It is not advisable
to syringe tomato plants in greenhouses at any time, when avoidable, as
syringing interferes with pollination and produces a damp atmosphere
which promotes rot, but the injury by syringing may be as nothing
compared with that which is caused daily by the insects. When the
use of hydrocyanic-acid gas is impractical, an all-night fumigation with
a tobacco extract is recommended, followed during the next da} T by
a syringing with a solution of whale-oil soap or its equivalent.

Comparative cost of the treatment. — -Potassium c}'anide costs from
about 30 to 50 cents per pound, according to the amount purchased.
Commercial sulphuric acid costs from about 2i to 10 cents per pound.
The entire cost for a single fumigation of a greenhouse containing
20,000 cubic feet is between 20 and 30 cents. The labor required is
scarcely, if any, greater than for fumigation with other materials.
A single fumigation with a suitable tobacco extract would cost, in a
house of the same size, at least $ 1.50, and the cost of labor for the
syringing which is recommended to follow such fumigation would prob-
ably not be less than $1. The soap required would cost only a few
cents, though possibly as much as the materials used in a fumigation
with h} T drocyanic-acid gas.

CAUTIONS.

Hydrocyanic-acid gas is one of the deadliest poisons known, and
should always be handled with the greatest care. Never hold the
potassium cyanide in the bare hand when pulverizing, but wrap up
the lumps in two or more thicknesses of cloth before breaking with a
hammer. Dust or small pieces of potassium C3'anide should not be
thrown away in exposed places, but always buried, as should the con-
tents of the receptacles of the chemicals after the fumigation. Never
inhale air in a greenhouse after the generation of gas has begun,
and provide against anyone entering the house before it is properly
ventilated.

Approved:

James Wilson,

/Secretary of Agriculture.
Washington, D. C, February 15, 1905.

^*m