OTHER PROBLEMS IN UNITED STATES 381 



of tolerably successful control, and it now only occurs in destructive 

 numbers in limited areas. This result is credited to the establish- 

 ment of a more effective sequence of parasitism, but owing to the 

 multiplicity of other factors involved absolute proof of the 

 contention is scarcely attainable. 



Other Biological Control Problems in the United States. Among 

 noxious insects which have accidentally entered the United 

 States in recent years, the alfalfa weevil, Hypera variabilis Hbst. 

 (Phytonomus posticus Gyll.), the European corn-borer, Pyrausta 

 nubilalis Hiibn., Oriental fruit moth, Cydia molesta Busek., and 

 the Japanese beetle, PopilUa japonica Newm., are especially 

 noteworthy. They continue to increase and spread to a degree 

 which affords conclusive evidence of their escape from effective 

 control. As W. R. Thompson observes (1928), the only factor 

 present in the native home of most introduced species and absent 

 in their new terrain is the parasitic factor. The probability, 

 therefore, is that the absence of parasites and predators is the real 

 cause of their increase. Attempts to restore the loss of balance 

 occasioned by the absence of the parasite factor form an important 

 part of the programmes of control with respect to the four pests 

 mentioned. 



The alfalfa weevil appears to have been first reported in 

 America in 1904, when it appeared in Utah ; by the end of 1912 

 it had also invaded Idaho and Wyoming, and by 1926 it had 

 spread into Washington, California, Montana and Nevada : since 

 then further States have come under infestation. The introduction 

 of several European parasites has resulted in one important 

 species, e.g., the Ichneumonid Bathyplectes curculionis Thoms. 

 becoming well established. During the years 1912 to 1914 

 liberations were made in the fields of Utah and colonisation 

 appears to have been easily effected, although the area of 

 dispersion remained extremely limited (Chamberlin, 1926). 

 Later the parasite spread rapidly, and after about eight years 

 its distribution was about coincident with that of the weevil. In 

 1925 it parasitised weevil larvse to the extent of 85 per cent, near 

 Salt Lake City, and to an even higher degree in some other places. 

 On the other hand, it attacks only the older larvae and is not 



