OCR Text |
Show Insects May Develop Special Resistance Insects commonly controlled by insecticides have in several cases developed increased resistance re-sistance to the chemical commonly com-monly used to control them, reports re-ports Dr. George F. Knowlton, Extension Entomologist of the Utah State Agricultural College. In certain areas of the United States, the small codling moth larvae, or apple worms, have developed de-veloped resistance to 'chemical sprays, and have thus become more difficult to control. The San Jose scale, another orchard pest which occurs in Utah, has developed resistance to lime sulfur sul-fur spray, which was first used in 1914. Other insect pests, not occurring occur-ring in Utah, which have developed devel-oped resistance to insecticides include the California red scale, citricola scale, and black scale, which have become increasingly difficult to kill by hydrocyanic acid gas fumigation; the larvae of the screw-worm which have become more' difficult to kill with puenothiazine in areas where this has been used; and in seme places the citrus thrips, which have become harder to hold in check with a tarter em:;-tic-sucrose spray, first used in 1942, the ectorologist points out. Such -resistance developed by j serious insect pests has required ! additional research on control, necessitating increased dosages, more frequent treatment, or use of chemicals to which the particular par-ticular pest fs not resistant, Dr. Knowlton indicates. |