Which invasives threaten coniferous forests? Scientists can now predict

Published: January 2, 2020 · Updated: January 6, 2020

white pine on georgian bay Leene/shutterstock

Chalk up a win in the battle against invasive species. A team of biologists have discovered patterns that can be used to predict which potential new invasive insect species pose the biggest threat to North America’s coniferous forests. Their paper, published this past fall in Ecology and Evolution, concludes that it’s the trees’ own evolutionary biology that plays the biggest role in determining which species are most susceptible to “high impact” attacks by non-native species, rather than the invasive insects’ biological traits.

“That was surprising,” admits Dr. Kamal Gandhi, a professor of forest entomology at the University of Georgia, and one 15 co-authors of the report.

The researchers discovered three aspects of native trees’ biology to focus on to determine which species are most likely to be severely impacted. The first is the “divergence time” between when the North American species’ began to evolve separately from the invasives’ native host plant. The further apart the divergence time, the less likely North American trees will have defences against the invader.

Shade- and drought-tolerance are also a factor in determining how capable a tree species is in fending off attack, with shade-intolerant and drought-tolerant species less likely to be impacted by invasives. Being able to withstand drought, obviously enough, makes you a strong tree. But apparently trees that need open space to grow (as opposed to ones that thrive in the shaded forest understory) do best against invasives.

The third factor is whether or not the trees are already exposed to a “coevolved congener” – a species relatively closely related to the invader. “If there is a native insect in the same family as the invasive species, the likelihood of an impact is much lower,” says Gandhi.

Their conclusions were pretty stark, with the models predicting that various “non-native insect[s] not yet present in North America will have a one in 6.5 to a one in 2,858 chance of causing widespread mortality…if established.”

With the database created, the next step is to analyze potential new invasives for their destructive potential. “We can look at the insects that are frequently caught at ports of entry and determine which have the greatest likelihood of causing a high impact,” says Gandhi.

The same research team has now begun compiling a database of hardwood species’ and potential invasive insects that could prey on them. Both sets of data will be made available for other scientists to use.

Not only do the findings present a new weapon for fighting invasives, the researchers are also working with an ally in Davey Tree Expert Company. Together they are developing an app that will help foresters identify potential threats to species they’re planting.

How to protect your trees from invasive insects

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