The joint evolution of diapause and insecticide resistance: a test of an optimality model.

J.-P. DELAND Post Office Box 165, MacDonald Campus, Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9 Abstract. The goal of this study was to assess whether selection for insecticide resis- tance resulted in evolutionary change in diapause propensity in the obliquebanded leafroller, Choristoneura rosaceana. Caterpillars that diapause under bark in midsummer were as- sumed to escape exposure to insecticides. Estival larval diapause was modeled as a bet- hedging strategy that reduces the risk of reproductive failure in a stochastic seasonal en- vironment and decreases mortality due to insecticides. The proportion of larvae entering estival diapause in populations within a local geographic area was predicted to increase in orchards treated with insecticides after midsummer. As predicted, insects from populations exposed to insecticides evolved a higher propensity to diapause than individuals from insecticide-free populations. A second prediction of the model was that the proportion of diapause in a population should be negatively correlated with survival of the non-diapausing larvae to insecticides applied after midsummer. This prediction was not supported; there was a positive correlation between diapause propensity and the estimated survival to the insecticides. A simulation model indicated that this unexpected correlation did not result from a non-equilibrium situation in which diapause propensity and physiological resistance evolved at different rates across orchards exposed to different insecticide regimes. A positive across-population correlation between physiological resistance and diapause suggests pleio- tropic effects of the resistance allele(s). Such pleiotropic effects could result in a correlated response of diapause following strong directional selection for physiological resistance and may explain the inadequacy of the optimality model to predict the evolutionary trajectory of diapause propensity across the insecticide-treated populations.

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