46 WHITE FLIES INJURIOUS TO CITRUS IN FLORIDA. 
occurs. That these are principally those which have bred upon 
China and umbrella trees is shown clearly by the fact that at Gaines- 
ville, Lake City, Tallahassee, and other points in the northern part of 
Florida, where other food plants are too few to produce noticeable 
numbers of migrating adults, the numbers are apparently not less 
than where both citrus trees and China and umbrella trees are exten- 
sively grown, as at Orlando. On this point, Dr. Berger states: 1 
"The principal food plants in Gainesville and north Florida are China 
and umbrella trees, there being only enough citrus, privet, and other 
evergreen food plants to bring about the restocking of the deciduous 
trees every spring." These considerations indicate very positively 
the main source of the enormous number of migrating adult flies on 
trees in midsummer, sometimes observed between the middle of 
May and the middle of June. These adults are the second brood of 
the season and the first to mature on the food plants mentioned. The 
newer growth of these trees is, as has been shown, very attractive to 
the adult flies, and if there is an abundance of it comparatively few 
migrate. The third brood, composed mainly of individuals of the 
second and third generations, matures over a more extended period, 
in general covering the months of July and August in different sec- 
tions of Florida. 
Estimates of the number of adult citrus white flies breeding on 
umbrella trees and on citrus trees as given under the subject of food 
plants have shown that a single umbrella tree of medium size may 
produce as many adult white flies by midsummer as could be pro- 
duced on 7 acres of orange trees. The maturity of so many adults 
on single trees, and their migration therefrom in search of a more 
desirable food supply than China and umbrella trees afford in mid- 
summer, cause the rapid spread of the pest throughout the towns, 
directly by flight of the adults and by mediums hereinafter dis- 
cussed into the surrounding country and from town to town along 
railway lines and watercourses. 
Dissemination hy flight when citrus trees only are concerned. — It has 
been shown under the subject of food plants that the citrus white fly 
does not ordinarily increase to the point of overcrowding on grape- 
fruit. Migrations of adults in noticeable numbers from solid blocks 
of these trees probably never occur under ordinary circumstances, 
and spread through such blocks or groves from the first point of infes- 
tation is very slow if no other food plants are concerned. The spread 
in groves of orange or tangerine trees or of both is more rapid, but 
not as much so as ordinarily considered. The white fly is rarely 
observed during its first 3^ear's appearance in a citrus grove. Atten- 
tion is usually first attracted to its presence through the blackening 
of foliage on one or a few trees. This blackening of foliage in itself 
1 Press Bulletin 10S, Florida Agriculture Experimental Station, February 13, 1909. 
