THE RED SPIDER ON COTTON. oo 
CLIMATIC CONTROL. 
Although climatic influences exert an immediate reducing effect 
over limited areas from time to time, the hardiness and the wide- 
spread occurrence of the species insure the survival of sufficient num- 
bers to reinfest localities that may have been thus partially freed. 
Owing to the fact that there is such a continuous succession of over- 
lapping broods and that every stage from egg to adult occurs simul- 
taneously, it is obvious that the most extreme weather factors can 
not be expected completely to eradicate the pest. 
Climatic conditions do, however, influence the development of the 
red spider to a marked extent, and this influence may be either detri- 
mental or beneficial. In the occurrence of the seasonal cycle the 
status of the red spider invariably undergoes a series of fluctuations. 
Beginning in January (see fig. 8, p. 26) we find the pest maintaining itself. 
In February no pronounced gain is ever made, although the develop- 
ment of the species may undergo some slight advance or setback due 
to weather. During March there is a gradual trend toward an opti- 
mum, but the pest usually suffers one or more retarding checks from 
adverse weather conditions. Through April and May the progress 
of infestation 1s usually most rapid, so that by the ist of June develop- 
ment has nearly reached its maximum. Usually some time in June 
or early July a sudden decimation occurs which reduces the status to 
normal or below. This reduction is followed by a reaction, so that 
some time between the middle of July and the last of August infesta- 
tion again increases. From that time until October, usually, develop- 
ment is subject to fluctuations varying considerably in extent, but 
the autumn period is characterized by a rather abrupt diminution 
until, by the end of November as arule, the minimum again is reached. 
Rainfall—During times of little rainfall and high temperature 
reproduction goes on very rapidly; on the other hand, long, heavy 
rains work havoc. In spite of the fact that the mites inhabit the 
underside of the leaves, many are washed off by rains and others are 
destroyed by the upward bombardment of sand particles. In fact, 
it appears true that a few heavy rains, especially if they continue for 
some time, reduce, temporarily at least, the degree of infestation. 
Although a temporary reduction of the pest 1s occasioned by heavy 
rains, many of the washed-off adults may be carried considerable 
distances in the surface water at these times, only to revive upon 
stranding, and to establish new colonies remote from the scene of 
their rearing. ‘Thus rain has the effect of greatly decreasing the per- 
centage of infestation, while at the same time considerably extending 
distribution. In one instance a violet leaf, heavily coated with soil, 
was examined and there remained, out of a recent large colony, only 
one female, one primary nymph, and numerous eggs. Of eight females 
543880°—Bull. 416—17-—_ 5 
