Effect of an ecdysteroid agonist, tebufenozide, on the completion of diapause in susceptible and resistant strains of the codling moth, Cydia pomonella

Codling moth Cydia pomonella (L.) (Lepidoptera, Tortricidae) larvae kept un der a short photoperiod enter a diapause which can be broken by exposure to 25 degrees C following an extended chilling period. We investigated the ef fects of the ecdysteroid agonist tebufenozide on the diapause of this speci es, both on a laboratory strain susceptible to insecticides and two strains selected for resistance to diflubenzuron or deltamethrin. The treatment in duced moulting by breaking diapause of the susceptible strain when applied at the end of the growth period, or by reducing the pre-emergence period wh en applied after chilling. The strains exhibiting resistance in the neonate instar were also resistant to the ecdysteroid agonist applied to diapausin g larvae, indicating that mechanisms of resistance remained active during t his arrest of development. High doses of tebufenozide were lethal for the m ost resistant insects, but neither reduced the pupation delay nor produced the larval-pupal moults which occur in the susceptible strain. As diapausin g larvae of C. pomonella can be easily collected, the expression of resista nce to tebufenozide both in the target instar, i.e. neonates, and during di apause may contribute to early detection of resistance to this insecticide in field populations.