Very hungry caterpillars thwarted by tree diversity

Satellite images and tree rings show how mixed forests keep the pests away

August 27, 2020
For Immediate Release

Contact: Heidi Swanson, (202) 833-8773 ext. 211, gro.asenull@idieh

The diverse mix of tree species in North America’s rugged Border Lakes region, which extends from northern Minnesota into northwestern Ontario, has shielded it from severe outbreaks of forest tent caterpillars, says a new study published in the Ecological Society of America’s open-access journal Ecosphere.

A forest tent caterpillar, Malacosoma disstria. Photo courtesy of Marian Goldsmith; CC BY-SA 2.0 

Forest tent caterpillars (Malacosoma disstria) can defoliate large swaths of forest – and eventually kill trees, if outbreaks continue year after year – but the structure of forests in the Border Lakes region has provided a measure of protection. “It’s not like the aspen parkland of Alberta or the fir forests of New Brunswick and Maine circa 1980. It’s in-between, with lots of stand compositional diversity and intermingling, so the defoliators don’t cycle with nearly the intensity that they do elsewhere,” says Barry Cooke, a co-author of the paper and researcher at the Great Lakes Forestry Centre in Sault Ste. Marie, Ontario.

The study encompassed Minnesota’s scenic Boundary Waters Canoe Area Wilderness and Ontario’s Quetico Provincial Park. In both areas, hardwood trees like aspen and sugar maple provide ample edible foliage for the caterpillars. But conifers like spruce and pine, which the caterpillars avoid, are also abundant. Cooke and his colleagues analyzed satellite images going back to the 1980s, as well as tree ring records, and found that over long periods of time the strong presence of conifers in the Quetico and the Boundary Waters interfered with caterpillars’ outbreak cycles, which can reach a fever pitch in areas where the larval colonies enjoy limitless access to hardwood host trees. In fact, even in more heavily managed areas with more hardwoods, Border Lakes forests overall were mixed enough to avoid hard-hitting caterpillar impacts.

The Boundary Waters Canoe Area Wilderness. Photo courtesy of Michael Schwartz; CC BY-NC-ND 2.0.

Communities that rely on timber harvesting need tree growth to be consistent and predictable, as do land managers using trees to store atmospheric carbon – which means avoiding damage from pests like forest tent caterpillars. Maintaining tree species diversity may be an effective way to manage pest-induced stress and mortality, especially in forests where multiple pests have unique tree species preferences. “If we diversify entire states and provinces, do we damp out pest population oscillations to the point where no one cares? My guess is that that is the case. That’s where the evidence is pointing,” says Cooke.

But deciding how to manage such large areas over long periods of time requires enormous amounts of data, especially for pests whose outbreak patterns are so erratic. “It’s important to understand the conclusion that spatiotemporal ecology requires more sampling than what we’re used to seeing in classical studies,” says Cooke. “But, of course, all ecology is spatial, and all ecology is temporal. So this means we basically need more data to do ecology well.”

Journal article:

Robert, Louis-Etienne et al. 2020. “Forest landscape structure influences the cyclic-eruptive spatial dynamics of forest tent caterpillar outbreaks.” Ecosphere. 


Louis-Etienne Robert1,8, Brian R. Sturtevant2, Daniel Kneeshaw3, Patrick M. A. James1,9, Marie Josée Fortin4, Peter T. Wolter5, Philip A. Townsend6, Barry J. Cooke7

1Département de sciences biologiques, Université de Montréal, Montréal, Québec, Canada; 2Institute for Applied Ecosystem Studies, Northern Research Station, USDA Forest Service, Rhinelander, Wisconsin, USA; 3Centre d’étude de la forêt (CEF), Université du Québec à Montréal, Montréal, Québec, Canada; 4Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada; 5Department of Natural Resource Ecology and Management, Iowa State University, Ames, Iowa, USA; 6Department of Forest and Wildlife Ecology, University of Wisconsin, Madison, Wisconsin, USA; 7Great Lakes Forestry Centre, Canadian Forest Service, Sault Ste. Marie, Ontario, Canada; 8Great Lakes Forestry Centre, Canadian Forest Service, Sault Ste. Marie, Ontario, Canada

Author contact:

Barry Cooke: ac.adanacnull@ekooc.yrrab



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