American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Combined effects of natural enemies and competition for resources on a forest defoliator: a theoretical and empirical analysis”

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Molly E. Gallagher and Greg Dwyer (Dec 2019)

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Resource competition & parasitoids drive budworm outbreaks in jack pine forests; climate change may upend the ecosystem

Parasitoids and plants control outbreaks of a very hungry caterpillar

Pollen cones at a collection site near Grayling&nbsp;MI in June 2015.<br />(Credit: Molly Gallagher)
Pollen cones at a collection site near Grayling MI in June 2015.
(Credit: Molly Gallagher)

Insect outbreaks can quickly damage or destroy thousands of acres of forests. Jack pine forests are frequently attacked by a species of very hungry caterpillar called the jack pine budworm. Jack pine budworm outbreaks occur every 6-12 years and continue for 2-4 years. Molly Gallagher and Greg Dwyer asked why these outbreaks happen, what causes them to end, and whether outbreaks might become more severe in the future as climate change progresses and the frequency of forest fires increases.

To understand the factors that determine budworm survival, Molly Gallagher and Greg Dwyer constructed mathematical models that included different ecological effects and interactions. To estimate model parameters, they collected data from budworm outbreaks in jack pine forests in Wisconsin and Michigan from 2012-2015. They measured the density of budworm caterpillars in different parts of the forest over time, and recorded the size and age of trees that were under attack. They also raised caterpillars in the lab to determine how many were infected with parasitoids such as wasps or flies, which lay their eggs inside their caterpillar hosts, and eventually kill their hosts.

By using their data to choose the best model, Dwyer and Gallagher showed that to best understand patterns of insect outbreaks, it is necessary to consider the effects of both parasitoid attacks and resource competition. At lower budworm densities, parasitoid attacks are more important, but when budworm density is very high, more caterpillars die due to a lack of plant resources. Simulations of a model including both of these factors result in realistic cycles of the budworm population.

In future work, the researchers plan to extend their model to include the interacting effects of climate variables and fire frequency on forest health. Jack pine forests are a major component of the North American landscape, and with changing pressures from insect outbreaks and climate stressors, their future remains uncertain.


Abstract

Explanations for the dynamics of insect outbreaks often focus on natural enemies, on the grounds that parasitoid and pathogen attack rates are high during outbreaks. While natural enemy models can successfully reproduce outbreak cycles, experiments have repeatedly demonstrated the importance of resource quality and abundance. Experiments, however, are rarely invoked in modeling studies. Here we combine mechanistic models, observational data, and field experiments to quantify the roles of parasitoid attacks and resource competition on the jack pine budworm, Choristoneura pinus. By fitting models to a combination of observational and experimental data, we show that parasitoid attacks are the main source of larval budworm mortality at low and intermediate budworm densities, but that resource competition is the main source of mortality at high densities. Our results further show that the effects of resource competition become more severe with increasing host tree age, and that the effects of parasitoids are moderated by strong competition between parasitoids for hosts. Allowing for these effects in a model of insect outbreaks leads to realistic outbreak cycles, while a host-parasitoid model without resource competition produces an unrealistic stable equilibrium. The effects of resource competition are modulated by tree age, which in turn depends on fire regimes. Our model therefore suggests that increases in fire frequency due to climate change may interact in complex ways with budworm outbreaks. Our work shows that resource competition can be as important as natural enemies in modulating insect outbreaks, while demonstrating the usefulness of high-performance computing in experimental field ecology.