34 BULLETIN" 416^ U. S. DEPARTMENT OF AGEICULTUEE, 



remaining on another leaf seven were dead and one alive following a 

 heavy dashing rain. 



The progress of infestation in cotton fields has been closely followed 

 on several occasions. The fluctuations in one of these fields, as indi- 

 cated by careful counts of infested and uninfested plants in large 

 series, was as follows: May 27, 57.5 per cent; June 10, 75 percent; 

 Jund 17, 33.3 per cent; June 25, 77.7 per cent, and June 27, 55.5 per 

 cent. A heavy rain fell on June 16, causing the reduction of 41.7 

 per cent; another fell June 26, causing the reduction of 22.2 per cent. 



Drought. — Long-continued drought works, at least indirectly, to the 

 detriment of the red spider. Perkins believed that the pest developed 

 fastest under hot, dry conditions, but also demonstrated that mites 

 are capable of living quite well under very moist conditions. Titus 

 (1905) states that infestation rarely becomes serious unless accom- 

 panied by long-continued dry weather. Worsham (1910) asserts that 

 dry and warm weather is essential for the maximum propagation of 

 the spiders and that only during a prolonged drought do their ravages 

 assume serious proportions. Ewing (1914) writes that it is during 

 July and August that the red-spider injury becomes most serious and 

 that these are the hottest and driest months in western Oregon. 



Investigations at Batesburg have further confirmed the rule that 

 the most rapid multiplication of the red spider is coincident with 

 periods of maximum temperatures and minimum precipitation. On 

 the other hand, the great rapidity of mite development reacts on the 

 species to its detriment. This reaction occurs in several forms. 

 First, the superabundance of the pest on hosts occasions the drying 

 of the foliage, so that the tissue becomes unattractive. This causes 

 widespread migratory movements, with the result that myriads of 

 the mites perish because of the intense heat of the soil or failure to 

 discover new hosts. Secondly, the concentrated massing of red 

 spiders at such times is to the benefit of their predatory enemies, with 

 the result that the predators in turn increase at their maximum rate. 



INSECT ENEMIES OF THE RED SPIDER. 



The investigations conducted at Batesburg have added substan- 

 tially to a knowledge of the insect enemies of the red spider. Three 

 insects were observed by J. C. Duffey (1891) to feed on red spiders 

 at the St. Louis Botanical Gardens in 1891. These were Scolothrips 

 sexmaculatus, a chrysopid larva, and Scymnus punctum. The latter 

 was observed to exert great control. Morgan (1897) states that a 

 very small black lady beetle (Pentilia sp.) was the only insect enemy 

 of the cotton mite noticed during 1893 in Louisiana. Perkins (1897), 

 in his account of the common red spider, mentions no predatory 

 enemies. Titus (1905) records chrysopid larvae, a species of Pentilia, 

 and other coocinellids as feeding on mites at several localities in 1905. 



