THE EED SPIDER ON COTTON. 31 



From Table VI it is seen that the shortest period of submergence 

 that will suffice to kill all red spiders is 9 hours. All individuals sur- 

 vived a submergence of 2^ hours, and 50 per cent revived after an 

 exposm-e of 8 hours. 



Ohservance of water dispersal in the field. — The actual dispersion of 

 red spiders by surface water has been observed repeatedly in the field. 

 In Plate VII a typical case of this sort in a cotton field is illustrated. 

 The primary source of dispersion consisted of cultivated violets 

 growing in a dooryard, indicated by A. Upon becoming over- 

 crowded a migration took place from these plants which brought 

 many of them across the street {B) and into the cotton field (so indi- 

 cated), where they became concentrated at {G), the point nearest to 

 the violets. This infestation, Umited to one end of the field, is indi- 

 cated by ringed stalks. The heavy rains dislodged great numbers of 

 red spiders and carried them along in the little streams which ran 

 between the cotton rows. This surface water converging at the 

 lowest point of the field, a large percentage of the stranded mites 

 revived and reestabhshed themselves upon the cotton plants imme- 

 diately at hand. Thus, at D there began a secondary development 

 which tended to spread throughout the field. Naturally this dispersal 

 is repeated with each heavy storm, with the result that these concen- 

 trations at the lower points of the fields become more and more 

 severe. 



Not all of the mites which are conveyed by the surface streamlets 

 reach the gromid through the effect of rains, as many are dislodged 

 by falling leaves, through the weakening effect of heavy infestation, 

 or by early frosts. 



PLANT TO PLANT TRAVEL. 



It is commonly the case during times of severe infestation that 

 mites spread directly from one plant to another through the inter- 

 lacing of branches. Such dispersion is facilitated by the close plant- 

 ing of cotton and, inversely, is discouraged by wide spacing. It is 

 difficult to say whether dispersion is accompHshed more by means 

 of ground travel or by leaf to leaf travel. 



WIND DISPERSAL. 



Several investigators have suspected that red spiders may be con- 

 veyed considerable distances by heavy winds, but it remained for 

 Mr. E. E. Mungor, of California, to conduct serious tests during 1913. 

 He employ(!d sticky fly paper at different heights above the ground 

 and at varying distances from the; source, which happened to be a 

 badly infested almond orchard. Mr. Hunger found quantitii^s of 

 mites on the sticky surface placed under the following conditions: 

 Twelve feet from tlu? ground and 100 feet from the orchard, ?>() f(>et 

 from the ground and 2.')0 feet from the orchard, and 50 f(u;t from the 



