12 Bulletin 328 



The elements of climate which, because of their large varia- 

 tions, influence insect life to a great extent, are temperature and 

 .moisture, especially the former. It is therefore, to be expected 

 that if weather has anything to do with the matter, temperature 

 and moisture must be playing a large part. Perhaps the influence 

 of these factors upon the relation existing between the plant lice 

 and their natural enemies has been best illustrated by a study of 

 Lysiphelbus tritici, a small hymenopterous (the order which in- 

 cludes the bees, ants and wasps) parasite of Toxoptcra grauii- 

 num, one of the most injurious of the plant lice attacking wheat 

 and oats. 



Under a constant temperature of 50" F. and an atmospheric 

 moisture ranging from "j^ to 100 per cent, 43 days were required 

 for the parasite to develop from egg to adult. Furthermore, 

 at this temperature, the number of healthy young produced is 

 gieatly reduced, in fact the insect hardly reproduces itself. On 

 the other hand, under a temperature of 50° F. and the same 

 atmospheric moisture the louse requires 24 days from birth to 

 maturity and reproduces, once that stage has been reached, nearly 

 one young a day for 2"/ days. 



At a temperature of 70 F. the parasite can complete its life 

 cycle in 10 days, and the average number of young ones from a 

 single pair of parents amounts to 56. This means that in one 

 month the offspring of a single pair would be more than 46,000. 

 On the other hand, at a temperature of 70° F. the louse reaches 

 maturity in 9 days and produces in the 1 1 days following about 

 29 young. At this rate in one month a single louse would pro- 

 duce less than 14,000 young. 



It is thus seen that while with a low temperature the lice can 

 carry on their activities practically unhindered by the parasite, 

 the advent of high temperature is likely to be followed by their 

 destruction by reason of the greater reproductive power of the 

 parasite. 



The studies of the efi:'ect of moisture are extremeh' limited, 

 but such evidence as has been collected indicates that so long" 

 as the atmosphere is not too dry to prevent the vigor of the food 

 plant and not wet enough to encourage the attack of parasitic 

 fungi, \-ariations in atmospheric moisture have little efl-'ect upon 

 either the louse or its parasite. 



No studies have been made to show the eft'ect of climate upon 

 the predaceous enemies. The lady-bird l>eetles and their larvcX 

 are the only important forms that attack the lice under low 

 temperatures and they are only rarely suftncienth- al)undant to 

 prevent an outbreak. 



