How Tree Diversity Affects Invasive Forest Pests

“Invasive insects and diseases pose both ecological and economic threats to our forest ecosystems,” says Qinfeng Guo, USDA Forest Service research ecologist.

Recent research has shown that trees grown under sunnier conditions may be more likely to survive hemlock woolly adelgid infestations. Photo by Chris Evans, courtesy of

Guo is the lead author of a broad-scale study of U.S. forest data that examines the relationship between the number of native tree species and the number of nonnative forest pests.

Across their native range, hemlocks are being lost to hemlock woolly adelgid, an invasive insect from Asia. Hemlocks are often found along stream banks, where they provide shade and keep water temperatures cool for fish species like brook trout.

The nonnative emerald ash borer beetle has killed hundreds of millions of ash trees across the eastern U.S. The cost to treat, remove, and replace trees infected with the beetle is estimated at more than $10 billion.

The research team includes Songlin Fei of Purdue University, Kevin Potter of North Carolina State University, Andrew Liebhold of the Northern Research Station, and Jun Wen of Duke University.

Their results were published in Proceedings of the National Academy of Sciences.

Emerald ash borer has been found in 35 U.S. states and can cause damage in all 16 species of native ash trees. Photo by Elizabeth Benton, courtesy of

The scientists examined Forest Inventory and Analysis records of tree species collected across public and private lands. They combined these data with the Alien Forest Pest Explorer, a web tool developed by co-author Liebhold. Its database includes county-level records of nonnative forest pests.

The researchers focused on 66 of the top nonnative invasive species – 51 insects and 15 pathogens. Together, these datasets describe where and how many tree species and invasive pest species co-exist.

The study found a significant – but not simple, linear – relationship between tree diversity and pest diversity.

“There’s a long standing hypothesis in invasion biology that high biodiversity can help to resist biotic invasions,” says Guo. “Past experiments are mostly limited to single species at fine scales. They have shown evidence of dilution or facilitation but rarely both. Our study expands this inquiry and examines 66 important invasive pests found in natural forest ecosystems across the conterminous U.S.”

Listen to a brief audio clip by author Qinfeng Guo describing this publication. • Text Transcript

“When tree diversity is relatively low, the number of pest species increases with increasing tree diversity. This supports the facilitation hypothesis: greater tree diversity means more ecological niches for pests to exploit,” says Guo. “However, when tree diversity continues to increase, we saw a threshold where pest species begin to decrease. This is dilution: high tree diversity usually means fewer individuals of each host tree species in the forest. This can suppress invasion, as certain pest species may no longer have enough resources, either food or habitat, to support their populations.”

The study suggests that facilitation and dilution are both occurring in a forest community. The importance of each, however, shifts with overall tree diversity.

The researchers looked at other factors, including temperature, precipitation, elevation, and human population density, as a proxy for pest arrival. Although these factors also seem important, tree diversity remains a strong predictor of nonnative pest invasions.

Further research is needed on how climate change may affect the geographic ranges of native tree species and thus potential habitats for future pest invasions. Photo by Lance Cheung, USDA.

“There is a lag between a pest’s arrival, its impact, and our detection of it. We know which native tree species are hosts for major invasives in the potential pest species pool. We know that the major pathways for pest introduction to new areas include trade or travel that involves moving infested wood materials. One way that we can slow or stop the spread of nonnative pests is by taking early action in prevention and detection. Our work emphasizes the critical importance of conserving native biodiversity in reducing the damages from pest invasions,” adds Guo.

“Diverse plant communities may contain non-hosts as well as species that are natural enemies for potential nonnatives. Both of these can help land managers prevent the establishment of invasive pests – by making them work harder to find resources,” says Guo.

The scientists hope that their results will inform and prioritize monitoring efforts for areas at greater risk for future pest invasions.

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For more information, email Qinfeng Guo at

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