Invasive, like the emerald ash borer and the redbay ambrosia beetle, wreak havoc on native ecosystems, kill hundreds of millions of trees, and cost hundreds of millions of dollars per year in lost revenue, management costs, and loss of ecosystem services in North America. A FWRC scientist sees these insects as a cautionary tale as she seeks to rewrite the story. Dr. Ashley Schulz, assistant forestry professor, is part of a team working to predict whether insect species will highly-impact North American conifers and hardwoods before they arrive here.
“We set out to predict the next high-impact insect invaders; the insects that are not in North America yet, but, if they do get introduced, can be the next emerald ash borer or redbay ambrosia beetle. Our overarching goal was to determine what factors drive impact of these introduced insects and then use this information to develop models to predict which insects will have significant effects on native tree species should they get imported and introduced into North American forest ecosystems,” Schulz said.
Schulz is part of the High-Impact Insect Invasions working group, a consortium of university, governmental agency, and industry experts aimed at predicting invasive insects. Over the last few years, the team has been laying the groundwork for possible models. This includes delving into the factors that make an invasive insect successful. One such factor is insect characteristics, which is well documented. However, the working group, who recognized that there was more to the story, focused on four factors related to trees and insects in North America, including insect traits, host tree traits, evolutionary history between the insect’s native range and local host trees, and whether there was a related residing insect on the same host tree as the non-native insect.
“We hypothesized that all insect invasions are driven by at least one of these four factors, but most are likely driven by more than one. Most other studies just focus on insect traits or sometimes host traits, so our study was one of the first to empirically demonstrate the importance of host evolutionary history as a major driver of impact for insects that feed on hardwoods,” Schulz said.
Schulz relates an invasive insect finding a suitable native host to a familiar childhood fairy tale. “We use the phrase ‘Goldilocks effect’ in science when something falls within certain margins as opposed to the extreme edges of a range (i.e., it’s ‘just right’). Here, we found a sweet spot for the insect and sour spot for the tree in the evolutionary history between an insect’s hosts in its native range and hosts in North America. The insect can identify the tree as a host, but the host is far enough removed from the insect’s native host that it likely does not have the co-evolved defense mechanisms necessary to defend itself against the insect,” she explained.
The work was published in Biological Invasions and the team is now incorporating their models into a new pest predictor tool for i-Tree, a peer-reviewed software suite from the USDA Forest Service and Davey Tree Expert Company that produces forestry analysis and benefits investment tools. Thus far, the team has collected data for generalist insects, which feed on a range of hosts, and developed new models for both generalist and specialist insects, which feed on a narrow range of hosts. The team has a dataset of 300 European insects that have not yet arrived in North America that they plan to run through their models to try and identify any insects that might have a high probability of impact should they arrive in North America.
“These results can have implications for prevention and early detection and can affect federal policy at places such as the USDA APHIS ports of entry in the United States and the Canadian 2022 ANNUAL REPORT 7 Food Inspection Agency,” Schulz said. While the team has yet to predict whether an insect will establish in North America should it arrive, they’re closer to understanding the factors at play that pave the way for an insect to become high-impact like spongy moth, beech scale, or hemlock woolly adelgid.
“We’ve built the models and are establishing a baseline that will determine which insects will be risky. Most invaders are not high-impact so there will only be a handful of insect species that might require immediate action. Although we’re expecting to only find a few, it’s finding out which few are going to be bad that is the big question,” said Schulz, who hopes one day the team’s work will be adopted and adapted by other countries to develop similar tools to predict impact on their native tree species.