THE EED SPIDER ON" COTTON. 25 



sible to tlie mites which may be crawUng about in search of green 

 plants, and, hkewise, the basal leaves favor greatly the reestablish- 

 ment of red spiders which may become stranded while being carried 

 in surface water. 



Appearance of spring adults. — The date which marks the maturity 

 of the progeny of the females which wintered through from the pre- 

 ceding fall is subject to some variation. In 1911 the first spring 

 brood was maturing as early as March 11, while in 1912 the first 

 females did not appear until about April 17. In both 1913 and 1914 

 this brood matured about March 25. 



Time of violet devastation. — Usually toward the end of May the 

 cultivated violet plants in the Southeast become so completely over- 

 run with red spiders that they are kiUed to the ground. The first 

 indication of damage to violets is the presence of russet spots on the 

 leaves. As the infestation increases the appearance of violet borders 

 is as though they had been fire swept. Those who see them invari- 

 ably beUeve they have been killed by drought. (PI, IV, fig. 4.) 

 When violets grow within a few hundred feet of cotton they may 

 constitute the immediate source of infestation. 



The average date of maximum injury to cultivated violets, as shown 

 by the observations of the last four years, is found to be May 25. This 

 is important in the seasonal history of the red spider, since the death 

 of so many violet plants precipitates the migration of the mites, with 

 the result that the pest becomes established on many new hosts. 



Tijne of estahlisTiment on cotton. — The earliest date for the estab- 

 lishment of the red spider on cotton varies somewhat from season to 

 season. The time depends largely on the date of appearance of the 

 crops, since we frequently find females established on seedlings which 

 are only three or four days old. Mites have occasionally been seen 

 established on cotton as early as the 1st of May, but for the seasons 

 1911, 1912, 1913, and 1914 the average for the first records on cotton 

 is computed to be May 20. The lower (sandy) section of South 

 Carolina averages one-half month earher in the pest's appearance 

 than does the upper (clay) section, June 25 marking the time of the 

 beginning of damage for the former and July 10 for the latter. 



Time of severest infestation. — The progress of infestation in a given 

 field depends on several interrelated factors, such as temperature, 

 precipitation, natural enemies, etc. Furthermore, there are succes- 

 sive waves of infestation occasioned by the favorable and the inimical 

 natural agencies. In 1911, at Batcsburg, a cotton field was seen 

 very acutely infested on June 12, while in 1913 fields were seen badly 

 infested as late as Septeml^er 6. Most of the severe infestation, how- 

 ever, occurs during July and August. By averaging the dates of oc- 

 currence of a considerable number of acute cases in our files we obtain 

 July 20 as the date of lieaviest infestation. 

 rK4;iH()°— Bull. 410—17 4 



