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Pointing the way to pine beetle control, but at what cost?

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Pine beetle-killed trees in Summit County, Colorado.

Dartmouth scientists study pine beetle population dynamics

By Summit Voice

SUMMIT COUNTY — Dartmouth scientists say they may have found a pathway to keeping pine beetles in check, showing that their populations fluctuate between extremes, with no middle ground.

“That is different from most species, such as deer, warblers and swallowtail butterflies, whose populations tend to be regular around some average abundance based on food, weather, and other external factors,” said Matt Ayres, a professor in the Department of Biological Sciences at Dartmouth and senior author on the paper. “They don’t appear and disappear in cycles. Rather, they exist in two stable equilibrium states—one of high abundance and the other of scarcity.”

Once the population pendulum swings toward the high end, it won’t quickly or easily swing back, Ayres explained.

According to the new study, forest managers might be able to keep pine beetle populations at the low end of the scale by boosting competitor and predator beetle populations — but they don’t address how that could affect the overall equilibrium of forest ecosystems, especially those where older trees need a change agent like bark beetles to spur regeneration.

The Dartmouth study focused on southern pine beetles, a cousin to the same insects that have decimated millions of acres of forest in the West, shedding more light on the population dynamics of the pests.

The research identifies the presence of bark beetle competitors and predators (specifically two other beetles) as the predominant limiting factor that can keep the bark beetles at a low, stable equilibrium. The authors suggest that the presence of these competitors and predators could be encouraged as a control strategy.

“The pine beetles produce pheromones, chemical signals, that attract enough competitors and predators to prevent outbreaks,” said Sharon Martinson, a member of the research team and first author on the new paper. “Leaving more dead trees in forests can provide habitat for competitor beetles that rarely kill tree, and for predators that eat both beetle species.”

The authors suggest that other pest species with catastrophic impacts may also have natural dynamics that include a tipping point between the bipolar population states. By learning what factors control those tipping points, impacts on ecosystems can be averted through monitoring and occasional intervention strategies.

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