Adaptation of an outbreaking insect defoliator to chronic nutritional stress

Abstract

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.

Interguild interactions between a non‐native phloem feeder and a native outbreaking defoliator

“…Our adult specimens were in poor condition because branches were harvested at the end of the season, when budworm adults were dead; therefore, we were unable to sex the adults. These effects may indicate either reduced growth as a result of overcrowding (Quezada‐Garcia et al ., ) or a skewing of sex ratio in favour of more males (which are smaller than females), which has been reported to occur in the laboratory when larvae were reared on suboptimal diet (Quezada‐Garcia et al ., ). Unbiased estimates of sex ratios for local populations are difficult to obtain because of gender‐specific behavioural differences that influence trap catch data.…”

Phenotypic variation in food utilization in an outbreak insect herbivore

“…Furthermore, Quezada‐Garcia et al . () found that when spruce budworm is exposed to chronic nutritional stress, pupal mass shows an adaptive plastic response that allow the insect to increase its mass after 2 generations. Phenotypic plasticity seems to be more advantageous for the insect to persist in changing environments.…”

“…Despite the fact that the spruce budworm is one of the most studied forest pests, its adaptive capacity is poorly understood. Recent studies have determined that this insect has the capacity to adapt its life‐history traits such as larval developmental time, growth rate and fecundity in a short period of time under chronic nutritional stress (Quezada‐Garcia et al ., ). The physiological mechanism behind this adaptive response was described by Quezada‐Garcia ().…”

“…One of the most important drivers of natural selection is food quality. Indeed, it has been observed that food quality has a substantial value to insects’ fitness and evolution (Slansky, ; Kolss et al ., ; Quezada‐Garcia et al ., ) by affecting development (Stockhoff, ) and reproductive success (Boggs & Freeman, ). Variation in food nutritional quality is therefore a component of selection pressure that may promote plastic changes in physiological systems (Nylin & Gotthard, ; Stotz et al ., ).…”

Do Offspring of Insects Feeding on Defoliation-Resistant Trees Have Better Biological Performance When Exposed to Nutritionally-Imbalanced Food?

“…This result suggests that offspring of parents that fed on resistant trees had a higher capacity for increasing their fecundity and fertility compared to offspring of parents that fed on susceptible trees when reared on good quality food. These life-history traits are highly inheritable [ 26 ] and may be implicated in the adaptive process of this forest pest [ 27 ]. During outbreak episodes, foliage quantity and quality are significantly reduced by repeated defoliation over several years [ 28 ].…”