48 WHITE FLIES INJURIOUS TO CITRUS IX FLORIDA. 
difficulties in effectively utilizing artificial checks, spraying and fumi- 
gation, are greatly increased. As the infested area in a newly infested 
grove or locality becomes larger the rate of spread by flight increases, 
aided by secondary centers of infestation which become established 
by various means. 
Winds. 
Light winds are an important adjunct to flight in the local distri- 
bution of adult white flies, but strong winds are ordinarily of slight 
consequence. The effect of light winds is shown by the influence 
of almost imperceptible movements of the air on the direction of 
migrations. This is especially noticeable in the vicinity of China 
and umbrella trees during a season when adults are emerging in abun- 
dance. The principal effect of the movement of the air under such 
conditions is not in carrying the insects, but in causing the flight 
energy of the insect to be expended in one general direction rather 
than to be wasted in zigzag lines with comparatively little real pro- 
gression. Other conditions being equal, the adult white flies migrate 
in greatest abundance when the atmosphere is calmest, and con- 
versely show the least tendency to migrate in strong winds. It 
is possible that isolated infestations may sometimes result from 
spread of adults by strong winds, but it is seldom that there is not 
a more plausible explanation obtainable. With the white fly pres- 
ent in abundance for many years in Orlando, Fla., and other towns 
and cities in important orange-growing sections of Florida, the fact 
that there are still many noninfested citrus groves within a radius 
of 5 miles of nearly all such centers of infestation is in itself an indi- 
cation of the minor influence of winds in this connection. Strong 
breezes or winds exert some check on the spread of adults by causing 
them to cling tenaciously to their support, as pointed out by Frof. 
H. A. Gossard. 1 
VEHICLES, RAILROAD TRAINS, AXD BOATS. 
In towns in Florida where the citrus white fly occurs and China 
trees and umbrella trees are abundant it is a matter of common 
observation that during the periods of migration large numbers of 
adults alight upon automobiles, carriages, wagons, and railroad coaches. 
The authors have seen covered carriages with more than 100 adults 
resting on the inside of the top and sides. In driving through a 
heavily infested citrus grove in late afternoon at certain seasons, 
hundreds of adults may be observed on the carriage (PL VI, fig. 2). 
Newly infested groves show the first infestation so frequently on trees 
close to a driveway or road that conveyance of the citrus white fly by 
means of carriages, wagons, and automobiles must be considered one of 
the most important methods of spread from town to surrounding 
1 Bulletin 67, Florida Agricultural Experiment Station, p. 13. 
