THE CITKUS WHITE FLY: SPREAD. 45 
easily greatly exceed one-fourth of a mile. Mr. W. C. Temple, of 
Winter Park, Fla., states that he has observed adults migrating into 
one of his groves on Lake Maitland under circumstances plainly indi- 
cating that they had traveled over the water for If miles. Dr. 
Berger has recorded an instance which presents strong evidence that 
adult white flies have heavily infested citrus trees through flights of a 
mile or more. On the other hand, there are orange groves within 
three-fourths of a mile of the city limits of Orlando, Fla., and within 
2 miles of the courthouse which have only so recently become infested 
that no blackening of the foliage has taken place, although the citrus 
white fly has occurred at Orlando for more than 10 years with migrat- 
ing adults in summer about as abundant as in any town in the State. 
As regards the capability of flight of the citrus white fly, it may be 
said to be undoubtedly more than a mile and perhaps several miles 
when aided by a gentle breeze. Distances of even a mile, however, 
are not usually attained except under certain circumstances which 
are largely preventable and which are discussed in the following 
paragraph. 
Cause of extensive migrations by flight. — Overpopulation of food 
plants, usually associated with the emergence of adults in large num- 
bers at seasons when the new and attractive growth is scarce or en- 
tirely wanting, is the main cause for migrations from citrus, cape 
jessamine, and privet. Migrations from China trees and umbrella 
trees, probably the most potent factors in the spread of the pest, are 
not due directly to overpopulation, so far as observed, since leaves are 
never overcrowded in a manner comparable to the overcrowding 
on citrus leaves. An average of 25 live larvae or pupae and pupa 
cases per square inch of lower leaf surface would represent an un- 
usually heavy infestation of a China or umbrella tree and is rarely 
exceeded, whereas an average of 50 or 60 per square inch is not un- 
usual for citrus leaves. In the case of China and umbrella trees, mi- 
grations are evidently due to lack of attractiveness of the foliage to the 
adult white flies at the times when the migrations occur. There are 
comparatively few live larvae and pupae on the foliage after the middle 
of August at Orlando. The greater part of these represent delayed 
emergence from the second generation of white flies and not the result 
of eggs deposited by the third brood of adults. This supports direct 
observations to the effect that the third brood of adults, which is con- 
cerned in the most extensive migrations, deposits practically no eggs 
on the China and umbrella trees. 
China and umbrella trees as a factor in dissemination. — Umbrella 
and China trees are extensively grown throughout the Gulf coast 
citrus-growing regions, and they are almost entirely responsible for 
the hundreds of millions of adults which in midsummer appear on 
the wing throughout most of the towns where the citrus white fly 
