Resistance to Diflubenzuron in the Codling Moth (Lepidoptera: Tortricidae)

Codling moth,Cyndia Pomona ella (L.), from pear in the Bedford, Oreg., area showed resistance to the chitin synthesis inhibitor diflubenzuron in laboratory and orchard studies. In the orchard this was shown in 1984 and 1985 as 54.5-63.6% damage at harvest compared with 0.8-2.0% damage occurring in another similarly treated orchard. Concentration-mortality studies later in 1985 and early 1986 showed about a 2-fold difference in susceptibility (LC30= 52.6 and 59.1 mg [All/liter versus 37.9 and 28.2 mg [All/liter) between the two populations based on the response of eggs of the F3. and F4. laboratory-reared generations to residues on the treated fruits. When results from exposure of eggs to a single discriminating concentration of diflubenzuron (112.3 mg [All/liter) were compared, the diflubenzuron-resistant population showed a significantly higher survival (≤29.2-fold). Studies in the same orchard in 1986 again showed a lack of control (35% damage) with a normal concentration. Higher concentrations of 2 times and 4 times normal showed 15% and 8% damage, respectively. This is the first report of resistance to diflubenzuron in the codling moth. This occurrence of resistance is unusual because it appeared the first time this population was exposed to diflubenzuron without previous exposure to related chemicals, whereas the population in another similarly treated orchard retained a high degree of susceptibility after approximately 8 yr of exposure to diflubenzuron. The causes or mechanisms for this develpment of resistance to diflubenzuron in the codling moth remain to be determined.