38 THE SPRING GRAIN-APHIS OR ' ' GREEN BUG." 



tions were unfavorable for the rapid increase of Allotria in 1908, 

 which conditions would prove favorable for Aphidius and also 

 unfavorable for its host, the Toxoptera. This infested area on the 

 department grounds in Washington has proved to be of considerable 

 interest, as the fluctuations of Toxoptera there, as well as those of its 

 parasite Aphidius and the secondary parasite Allotria, must coincide 

 with what is going on in similar places over the country, thus forming 

 small secluded breeding areas where Toxoptera survives throughout 

 the summer, more especially in the South. The area in question is 

 a depression covered chiefly by bluegrass, occupying perhaps half an 

 acre, surrounded on all sides except the south by shade trees (See 

 PL II, fig. 2.) It is rather more moist and therefore cooler in summer 

 than other portions of the grounds and in common with the rest is 

 kept closely mown. An underground steam pipe which affords heat 

 for a large number of greenhouses extends along the southern and 

 eastern margins ; the ground above this pipe is always much warmer 

 than the surrounding area during winter, the snow disappearing first 

 and the grass in that location starting much earlier in spring. So far 

 we have not found that these latter conditions have any influence 

 in enabling the Toxoptera to breed viviparously during the winter. 

 Even when the Toxoptera was excessively abundant here none could 

 be found in the bluegrass-covered grounds only a few yards away, 

 except in 1910, when it was quite numerous about the Washington 

 Monument some four blocks away. Because of its isolation — there 

 are no grain fields within miles on the Maryland side of the Potomac 

 Kiver and the department experiment farm at Arlington, Ya., has the 

 only grain for miles on the west side of the river — and because these 

 last had never suffered from Toxoptera attack, tins area became of 

 too much importance as a convenient field of observation and experi- 

 mentation to make an attempt at experimenting with the importation 

 of great numbers of Aphidius desirable. There is every reason for 

 believing that it is in similar favorable localities that Toxoptera 

 passes the summer months in the southwestern portion of the country, 

 where, as observations have shown, it is not able to withstand the 

 high temperatures of the open fields. 



Toxoptera has been studied throughout the summer in the South- 

 west with much difficulty, and not at all satisfactorily for the reason 

 that we have been unable to keep it under continuous observation in 

 the open fields. 



Except in cases of local outbreaks here and there over the country 

 there has been no serious injuiy to grain crops by the " green bug" 

 since 1907. Many additional localities for the species have been 

 added since then, however, and it now appears to cover almost the 

 entire United States, excepting perhaps New York and the New 

 England States. (See fig. 4, p. 19.) 



