Scientists have developed a way to understand how nonnative insects might behave in their new environments—including which might wreak havoc.
About 450 nonnative, plant-eating insect species live in North American forests. Most of these critters are harmless, but a handful cause problems in their new environment, attacking trees and causing more than $70 billion in damage each year.
The problem is, scientists often don’t know which insect will emerge as the next harmful invader.
A team of researchers, drawing largely on the evolutionary history of insect-plant interactions, has created a model that could help foresters predict which insect invasions will be problematic for conifers, and help managers decide where to allocate resources to avoid widespread tree death. Their work appears in Ecology and Evolution.
“What makes the bad invaders so special? That has been the million-dollar question, for decades,” says Patrick Tobin, an associate professor in the University of Washington School of Environmental and Forest Sciences and one of the project leaders. “This has the potential to profoundly change how we predict the impact of nonnative species and prioritize limited resources used to mitigate these impacts.”
The new model can quickly evaluate whether a newcomer insect, even before it gets here, has a high probability of killing a population of North American trees. To use the model, all that’s needed is information about the insect’s feeding method (wood, sap, or leaf feeder, for example) and what trees it feeds on in its native range. The model will then determine whether any North American trees are at risk of dying from it.
The research team focused on nonnative insects that utilize North American conifers—cone-producing trees such as pine, cedar, fir, and spruce. They identified nearly 60 of these conifer-specialist insects, then built an exhaustive database of information about each one, including life-history traits and characteristics of the trees they attack. Six insects emerged as “high impact,” meaning they have killed large swaths of otherwise healthy native trees.
For example, the balsam woolly adelgid—a wingless, sap-sucking insect from Europe that infests and kills fir trees—has left more than 100,000 acres of dead trees across the Pacific Northwest. Another, the red pine scale from Asia, has decimated New England forests by sucking on the thin inner bark of trees.
An insect’s history
So what causes a select few nonnative insects to become the most destructive invaders?
“In the past, research has focused on aspects of the insects themselves, but we realized that wasn’t the case at all,” says lead author Angela Mech, who completed this work as a postdoctoral researcher at the university and who will continue this work as an assistant professor at the University of Maine.
Whether a nonnative insect takes hold and becomes destructive has more to do with the evolutionary history between the new (North American) host tree and the insect’s native host tree from its home region, Mech explains. Molecular tools that allow scientists to construct comprehensive phylogenies (or maps) of how tree species evolved was key to the team’s breakthrough.
For example, if a pine tree in Asia and another in North America diverged tens of millions of years ago, the North American pine likely wouldn’t have retained defenses against an insect that only lives with the pine in Asia. Alternatively, two pines on both continents that share more evolutionary history and diverged more recently might still share similar defenses.
The new model helps identify the evolutionary “perfect storm” for conifers, where the invasive insect still recognizes the new tree as a food source, but the tree hasn’t retained adequate defenses to keep the invader in check.
The researchers are building a similar database and model for nonnative insects that utilize hardwood trees, such as maple, oak, and ash. Both the conifer and hardwood tree databases will be publicly available for other scientists to use.
They are also partnering with the Davey Tree Expert Company to develop a mobile app that a forester could use to determine potential insect threats if a species of tree is planted in a specific location.
Coauthors are from the US Geological Survey; Arkansas State University; the Davey Tree Expert Company; the University of Nebraska-Lincoln; Dartmouth College; the University of Georgia; Stony Brook University; the US Forest Service; Colorado State University; and the University of Wisconsin, Madison.
Funding came from the US Geological Survey’s John Wesley Powell Center for Analysis and Synthesis, the Nebraska Cooperative Fish and Wildlife Research Unit, the University of Washington, the US Forest Service, the National Science Foundation, and the National Institute of Food and Agriculture.
Source: University of Washington