FACTORS INFLUENCING THE ACTIVITY OF DIFLUBENZURON AGAINST THE CODLING MOTH, LASPEYRESIA POMONELLA (LEPIDOPTERA: OLETHREUTIDAE)

Abstract Codling moth eggs were very sensitive to diflubenzuron (Dimilin), particularly when treated topically shortly after oviposition. Regression analysis showed a linear relationship between % hatch and the age at which the eggs were treated with diflubenzuron. The LC50 values for 0- to 21/2- and 3-day-old eggs were 1.1 and 17.2 ppm, respectively. Egg hatch was also inversely related to the length of time the chorion was in contact with the diflubenzuron solution. When the compound had dried on fruit or foliage, residual activity against the eggs was excellent and did not decrease markedly over a 10-day period. The surfactant, Tween 20, enhanced the contact ovicidal activity of diflubenzuron especially on older eggs. Tween 20 also improved the residual activity of diflubenzuron on foliage and immature apples but not waxy mature apples. Diflubenzuron incorporated into artificial diet was toxic to 1st- and 2nd-instar larvae. The LC50s for the two instars were 48.2 and 8.1 ppm, respectively. When 1st instars fed on diflubenzuron + Tween 20-treated foliage for 2 days, larval entry into apples was not impaired but subsequent larval survival was reduced significantly. However, neither fruit entry nor larval survival was affected when 1st instars were reared on apples which had been dipped in 500 ppm diflubenzuron - Tween 20. Adult codling moths dipped in or fed diflubenzuron solutions showed no marked adverse effects although marginal reductions in egg viability were observed.