Adaptation of an outbreaking insect defoliator to chronic nutritional stress.

During insect outbreaks, the high number of individuals feeding on its host plant causes a depletion of the food source. Reduced availability and decreased quality of nutrients negatively influence life-history traits of insects driving them to develop adaptive strategies to persist in the environment. In a laboratory experiment with three repetitions, we tested the effect of chronic nutritional stress on spruce budworm performance during three generations to determine the adaptive strategies employed by the insect to deal with a selection pressure produced by low-quality diet. Our results show that all tested life-history traits (mortality, developmental time, pupal mass, growth rate and female fecundity) but female fertility were negatively influenced by the low-quality diet simulating food depletion during outbreak conditions. However, especially females in the third generation under chronic nutritional stress show an adaptive response in life-history traits when compared to those reared only one generation on low-quality diet. Larval developmental time significantly decreased and pupal mass, growth rate and fecundity significantly increased. The study demonstrates the capacity of spruce budworm to react to chronic nutritional stress with adaptations that may be caused by epigenetic parental effects. This information can help to understand the course of an outbreak especially at peak densities and during the collapse.