Low energy reserves and energy allocation decisions affect reproduction by Mountain Pine Beetles, Dendroctonus ponderosae

1. Low internal energy reserves at the beginning of the breeding season may impose physiological constraints on an animal's reproductive investment and may alter the optimal trade-off between investment in reproduction and somatic condition. 2. Here we examine how the energetic condition of female Mountain Pine Beetles (Dendroctonus ponderosae) affects their reproductive investment. We starved beetles to simulate the decrease in energy that accompanies dispersal and tested whether starved beetles had decreased egg number and decreased egg size, or both. We further distinguished whether changes are due to physiological constraints or shifts in allocation between reproduction and somatic condition. 3. We found that starved beetles produced smaller eggs than non-starved beetles, but females were able to partially offset the energetic deficit by feeding at their breeding habitat. Starvation did not decrease the number of eggs beetles produced. 4. The number and size of eggs produced depended on whether females allocated energy to reproduction or to somatic condition. However, this life-history allocation decision was independent of the amount of energy beetles had at the beginning of reproduction. 5. Our results demonstrate the importance of assessing reproductive investment in the context of other life-history trade-offs. Specifically, since egg size in Mountain Pine Beetles was highly dependent on both the amount of energy remaining after dispersal and whether energy was allocated to reproduction or somatic maintenance, we expect both of these trade-offs to be under strong selection.