Temperature and humidity relationships ofTetranychus desertorumBanks with special reference to distribution

The desert spider mite, Tetranychus desertorum Banks, has been reported as an important agricultural pest in Paraguay and the cotton belt of the southern United States. In California, however, it is found occasionally on weeds and becomes a pest of cultivated crops only rarely and in isolated localities. Experimental studies to ascertain the reason for these differences in economic severity were undertaken for the purpose of applying ecological principles to the theoretical question of why a pest is severe in one place and not in another as well as obtaining specific biological data for this species. Differences in climate, host plants, natural enemies, cultural practices, and intrinsic physiologies of the mite populations were considered as possible explanations for the differences in economic severity in the various parts of the distribution of this species. Of these, climatic differences appeared to be the most marked. It was noted that this spider mite is an important pest only in areas in which normal spring and summer rainfall exceeds one inch per month. The hypothesis attributing lack of economic severity of T. desertorum in central California to the aridity of this region was supported by laboratory studies demonstrating a higher rate of reproduction and a faster rate of development at high, as compared to low, relative humidity. This view was further supported by laboratory data showing Tetranychus telarius Linnaeus, an important spider mite pest in central California, to have a lower rate of reproduction and a slower rate of development at high, as compared to low, relative humidity.