PRESS BULLETIN 156
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
THE CITRUS WHITEFLY IN THE FALL
By E. W. Berger
While the best time for introducing the fungus parasites of the whitefly
is during the period of summer rains, one of the best times to spray with
contact insecticides is during the fall. Spraying with contact insecticides
can be employed with advantage during October or November. Do not begin
to spray until ten days or two weeks after the whitefly has quit, or nearly
quit, flying. By that time the eggs deposited by that brood will have hatched.
It is the young larvae that are most easily killed. By mid-winter these
larvae will have developed to the fourth or older stages, and are not then
so easily destroyed. In addition to the larvae being larger and stronger in
winter, the spray employed' may be less effective, because of the cooler
weather. The advantages of spraying for the whitefly early in the fall are:
1. The larvae are young and easily killed.
2. There are few rains to interfere with spraying.
3. The larvae are killed before they wax fat on the trees, and so do not
drain the trees so long as when they are not sprayed until winter.
If there is not sufficient whitefly present in the fall to produce visible
sooting of the trees, spraying may be postponed until winter; when, of
course, a stronger insecticide must be used.
Having sprayed thoroughly at the time above noted, it will not be neces-
sary to spray again before spring. Ten days or two weeks after the spring
brood of whiteflies have quit swarming (that is, by the time the eggs have
hatched) the spray may be applied effectively.
Our experiments show that 90 to 95 per cent. of the younger larvae (up
to the thickened condition of the fourth stage) have been readily killed by
1 pound of whale-oil soap to 6 gallons of water. Solutions as dilute as 1
pound to 9 gallons of water have given good results. Failures to get good
results in fall may have been due to cooler weather at the time of applica-
October 1, 1910
tion, since most liquids are less penetrating and less active chemically at
lower temperatures. It, therefore, appears desirable to use whale-oil soap
stronger in fall than 1 to 6-say 1 pound to 4 gallons of water-except when
the temperature approximates that of summer. Other spraying solutions, such
as miscible oils and proprietary brands, may be employed at the strengths
recommended by the manufacturers.
As a general rule, it is not best to attempt to introduce and spread fungus
after the summer rains have ceased. After this time the fungi spread but
little and are of but little value in checking the brood of larvae then on the
leaves. Sometimes, however, fungus may be introduced in the fall in order
to give it an early start neit spring, should favorable conditions arise at that
time. The writer has, as late as October, successfully introduced fungus which
resulted in an effective growth by the middle of the next summer.
Groves Treated With Fungus
Those who have been depending upon fungus to check the whitefly, and
have allowed a cover-crop of some kind to grow tall and rank in order to
help fungus growth during the period of rains, should cut this down and use
it as a mulch, or plow it under as soon as it is evident that the summer rains
have ceased. Otherwise this cover-crop may extract sufficient moisture from
the soil to prove disastrous to the trees and fruit. Growing vegetation of all
kinds gives off large quantities of water through its leaves. This water is
absorbed from the soil by the roots of the cover-crop. In this way the
moisture in the soil is depleted. During the period of rains, when there is
superfluous soil moisture, the cover-crop can do no harm, and by bringing up
moisture from the soil it adds to the moisture of the atmosphere in the
grove, which is an advantage to the friendly fungi.
Should the fungi not have checked the whitefly sufficiently by October or
November, the grower should not hesitate to spray with whale-oil soap or
other contact Insecticide as previously directed. Spraying solutions recom-
mended for controlling citrus diseases should not be used in whitefly-infested
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