August, 'OS] JOURNAL OF ECONOMIC ENTOMOLOGY 261 



In its northern spread the Cotton Boll Weevil {Anthonomiis 

 grandis Boh.) had reached the northern bonndary of I'exas at the 

 end of 1904. In February. 1905, the temperature dropped to 1°F. 

 at Dallas and 1J:°F. at College Station, Texas. As a result, I am in- 

 formed by Prof. A. F. Conradi that it seemed to have been killed out 

 entirely north of Dallas and the spread of the previous season was off- 

 set, while as far south as College Station so few hibernated success- 

 fully that but comparatively little damage was done the following 

 season. The advance of the weevil was also given a decided set-back 

 in Louisiana the same winter, though only in the northern part can 

 this be attributed to Ioav temperature. Again in June, 1906, the 

 agents of the Bureau of Entomology were unable to find weevils 

 which had hibernated successfully in Dallas, Ellis and Navarro coun- 

 ties, Texas, which had been infested for three or four years, following 

 a minimum of 12°F.^ 



It is also interesting to note that from the first, the boll weevil and 

 other southwestern insects have spread much faster eastward than 

 northward. - 



About 1903 the Morellos Orange Fruit Worm {Anastrepha ludens 

 Loew) was introduced from ^Mexico and became established near 

 Brownsville. Texas. Prof. A. F. Conradi, state entomologist of Texas, 

 advises the writer that it had become quite abundant in this region, 

 out since the freeze of February, 1905, when a minimum of 22°F. 

 occurred, he has been unable to find any evidence of the pest. 



It is probable that the absolute minimum temperature is not the 

 controlling factor in limiting the northward spread of insects, for 

 many individuals would always survive in sheltered situations, and 

 these absolute minima occur at very 'long intervals. But it would 

 seem evident that where the average-annual-minimum temperature is 

 below that at which a species can exist, that it will never become 

 abundant. Inasmuch as the extreme cold of winter is usually in 

 spells of short duration, the average-annual-minimum temperature of 

 any locality is probably a better index of the effect of winter temper- 

 ature there than the average mean temperature, average dail.y mini- 

 mum, etc. Were thermograph records available for the different sta- 

 tions, a summation of the temperatures below a certain point might 

 possibly be more accurate, for it must be remembered that, as Bach- 

 metjew has shown, an insect may be killed by more protracted cold at 



iFor further discussion see a forthcoming bulletin of the Bureau of Ento- 

 mologj-, '-Some Factors in the Natural Control of the Mexican Cotton Boll 

 Weevil." 



^Webster, et al, Bulletin 60, Bureau of Entomology, p. 130. 



