THE JOINT EVOLUTION OF DIAPAUSE AND INSECTICIDE RESISTANCE - A TEST OF AN OPTIMALITY MODEL

The goal of this study was to assess whether selection for insecticide resistance resulted in evolutionary change in diapause propensity in the obliquebanded leafroller, Choristoneura rosaceana. Caterpillars th at diapause under bark in midsummer were assumed to escape exposure to insecticides. Estival larval diapause was modeled as a bet-hedging st rategy that reduces the risk of reproductive failure in a stochastic s easonal environment and decreases mortality due to insecticides. The p roportion of larvae entering estival diapause in populations within a local geographic area was predicted to increase in orchards treated wi th insecticides after midsummer. As predicted, insects from population s 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 support ed; there was a positive correlation between diapause propensity and t he estimated survival to the insecticides. A simulation model indicate d that this unexpected correlation did not result from a non-equilibri um situation in which diapause propensity and physiological resistance evolved at different rates across orchards exposed to different insec ticide regimes. A positive across-population correlation between physi ological resistance and diapause suggests pleiotropic effects of the r esistance allele(s). Such pleiotropic effects could result in a correl ated response of diapause following strong directional selection for p hysiological resistance and may explain the inadequacy of the optimali ty model to predict the evolutionary trajectory of diapause propensity across the insecticide-treated populations.