198 BuLi^ETiN 87 



almost globular in form. The insect before dying attaches itself 

 rigidly to the leaf on which it was feeding. These parasitized in- 

 sects and the empty shells which remain after the adult parasite 

 emerges are very conspicuous and a farmer should learn to recog- 

 nize them. Each adult female of the parasites is capable of destroy- 

 ing around two hundred aphis individuals. That is, each adult 

 parasite will parasitize or deposit an egg in the body of each of two- 

 hundred or more specimens as long as the supply of aphis holds out. 

 The effectiveness of parasites, lady birds and other natural ene- 



'•^'ig. 25 — The convergent lady bird, an enemy of the cotton aphis, a, 

 Adult, b, Pupa, c, Full grown larva. (All greatly enlarged.) 



mies of aphis, is dependent on the weather. Cold weather during 

 the crop growing season is unfavorable for the natural enemies and 

 therefore favorable for the aphis. During warm or hot weather 

 the parasites are capable of multiplying so much faster than the 

 aphis that the latter are relatively unimportant among cotton pests. 

 The natural enemies of this species of aphis for some reason are 

 much more reliable in the control of the pest on cotton than on 

 melons and other crops. Owing to this indirect relation of the 

 weather to the effectiveness of the parasites it is a common belief 

 among farmers and gardeners that the hot weather destroys the 

 aphis. As a matter_of fact the aphis will thrive in our hottest mid- 

 summer weather and do much damage if for any reason the para- 

 sites and other natural enemies are not active. 



In 1914 and again in 1918 the cotton aphis was notably destruc- 

 tive in the Yuma Valley. In July, 1918- the attack was especially 

 severe and for a time threatened the complete destruction of the 

 crop. Soon after the first of August, however, hymenopterous para- 

 sites increased sufficiently to control the pest. Lady birds, which 

 are more conspicuous and therefore better known as enemies of this 



