Feeding ecology of outbreak insect species

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Understanding the nature and relative importance of endogenous (density-dependent) and exogenous (density-independent) effects on population dynamics remains a central problem in ecology. The relative contributions of bottom-up and top-down effects (resources and natural enemies, respectively), particularly in herbivore populations, have been difficult to compare because data on population density variation have rarely been combined with resource measurements. Testing the hypothesis of whether or not variation in food quality plays a role in spruce budworm population dynamics represents the long-term objective of my research program. In the short-term, I intend to study trans-generational effects of food quality on outbreak insect defoliator population fitness and population dynamics using the spruce budworm-host tree system of interactions. If food quality changes contribute to population dynamics, I predict that the trans-generational mechanism is the driving force of that contribution. In contrast, if food quality does not contribute to population dynamics, I predict that trans-generational effects generate a locally adaptative phenomenon that is strong enough to render population dynamics independent of food quality variation. Climate change is expected to cause increases in temperature and in the frequency of extreme climatic events, including extreme variation during winter, such as frequent thaw episodes. I predict that these anticipated winter thaw episodes, which I intend to reproduce in the laboratory, would favor local adaptation to food quality deterioration, as would occur during the course of an outbreak, by exerting additional selective pressure on insect populations in favor of stronger, more adaptable insects to poor quality nutrition. Whether food quality has or has not driven population dynamics of spruce budworm in the past, I predict that these expected new winter conditions would affect the dynamics of spruce budworm populations or at least, their fitness. The hypotheses of this research project will be tested by rearing, over multiple generations, larvae installed on host branches that were covered with sleeve cages. Results from caged and uncaged populations will be compared. Local adaptation to food quality and effects of winter thaw will be investigated using field and laboratory reciprocal transfer experiments. This research program will contribute not only to a better understanding of the role of food in the population dynamics of outbreak insect defoliator species, but also to a better understanding of how climate change and food interact on insect population dynamics and fitness. Results of the current research program will provide information useful to develop forest management practices aimed at reducing insect economic impact. Also, the current research will allow the development of models that can predict the course of insect defoliator outbreaks in a changing environment due to climate alterations.