Differential Age Susceptibility of the Codling Moth (Lepidoptera: Tortricidae) to Chitin Synthesis Inhibitors and Thuringiensin

Abstract

The concentration-response of Cydia pomonella eggs and first instars to chitin synthesis inhibitors diflubenzuron, triflumuron, and CGA 112913 (2,6-difluoro- N -[[[4-[3-chloro-5-trifluoromethyl-2-pyridinyloxy]-3,5-dichlorophenyl]amino]carbonyl]benzamide), and the beta-exotoxin thuringiensin was determined in laboratory bioassays. Phosalone, the insecticide currently used for codling moth control in walnuts, was included in the tests for comparative purposes. Eggs were topically treated and larvae were fed surface-treated synthetic diet. Diflubenzuron and triflumuron were most toxic to 0- to 2-day-old eggs, and CGA 112913 and thuringiensin were nontoxic. In contrast, CGA 112913 was the most toxic compound to first instars: LC50 was 300-fold lower than LC50 for eggs. Triflumuron and diflubenzuron were 6- and 50-fold less toxic to first instars than to eggs, respectively, but still within the same range of toxicity as phosalone. LC50 of thuringiensin against first instars was also similar to that of phosalone. To determine the most sensitive period in the egg stage to triflumuron, hatch rates of 0- to 2-day-old eggs were compared with 2- to 4-day-old eggs. Because mortality of treated 2- to 4-day-old eggs was significantly lower than mortality of 0- to 2-day-old eggs, the critical stage for chitin synthesis occurs between 0 and 2 days. If applied to eggs, timing of sprays may be important.