February, '16] PARIvER: WESTERN WHEAT APHIS 185 



During the hottest summer weather coccinellid and hymenopterous 

 parasites greatly reduce their numbers, but as cold weather comes on 

 the parasites lessen their activities and the aphids become so abundant 

 that their host plants are overloaded, forcing the plant lice to migrate 

 in search of fresh food. By this time the new crop of fall wheat is well 

 started and again the migrants find the ideal conditions which they are 

 seeking. Observations were made in infested grain fields during the 

 first week of November of the present season, and, whenever an in- 

 fested stool of volunteer wheat was found, the wingless lice could be 

 seen crawling away from it in all directions. Thus one good-sized 

 stool of volunteer grain will reinfest much of the new crop within a 

 radius of 10 to 25 feet and sometimes to greater distances. It is a 

 significant fact that, whenever an area in which the wheat has been 

 destroyed by the wheat aphis is examined in the spring, the old dried 

 volunteer plants from which the infestation started can always be 

 found. 



About October 15, true males and females are produced and egg- 

 laying is carried on until verj^ late in the fall. On December 2, 1914. 

 wheat plants were seen which were covered with egg-laying females 

 even though the thermometer had registered 11 degrees below zero 

 in that vicinity on November 15. The eggs are deposited for the most 

 part upon the leaf blade upon which the female is feeding, but are also 

 placed upon dried stems and upon the soil. The old volunteer plants 

 and bunches of blue joint have upon them the greatest number of eggs, 

 but many are also laid upon the small fall wheat plants by the crawling 

 migrants. 



Control 



Clean Tillage of Summer Fallowed Fields. — The life-history 

 and habits as just discussed would naturally suggest clean tillage of 

 summer fallowed wheat land as the most feasible method of control. 

 That this is the right method may be emphasized by the statement 

 that no wheat aphis injury has ever been found except on summer 

 fallowed land where volunteer grain and grasses have been allowed to 

 grow. Fall wheat on sod land in heavily infested localities has never 

 been injured even when adjoining fields have been completely de- 

 stroyed. 



It is a very simple matter to advise that the wheat aphis can.be 

 completely controlled by keeping summer fallowed fields absolutely 

 free from volunteer grain and grasses, but it is often a difficult and 

 expensive proposition for the farmer to put the advice into practice. 

 The ordinary method of procedure in the handling of summer fallowed 

 wheat land in Montana is to plow in the spring and to follow with a 

 varying number of diskings to kill vegetation and to conserv e moisture. 



