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.
