Local adaptation in cold tolerance in overwintering spruce budworm

Cold tolerance is an important trait that drives overwintering insect success and poleward range boundaries. As such it is frequently used in species distribution and population size modelling. However cold tolerance can evolve on both long and short timescales, is plastic, and that plasticity itself can evolve.  Here we examine these factors in the eastern spruce budworm, Choristoneura fumiferana, which is an outbreaking defoliator that can cause landscape-scale damage to the North American boreal forest.  This species spends the winter as a freeze avoiding 2nd instar larva which can significantly suppress its freezing point through the accumulation of large quantities of glycerol and significantly active ice-binding proteins.  Here we show that in response to cold exposure, northern populations accumulate significantly more glycerol than southern populations, and are also able to depress their freezing point to a much greater degree, suggesting that the plasticity of this cold hardiness has evolved among spruce budworm populations.  We then look at the enzymes involved in glycerol synthesis and find that more northern populations have significantly higher maximal enzyme activities, suggesting that they have much higher capacity for glycerol synthesis.  Our next steps are to investigate the cause of this increased enzyme activity, as wel as to build spruce budworm defoliation models that directly model local adaptation in cold tolerance plasticity.