A Japanese Insect Has Killed Millions of Trees in the US

A recent study highlights the critical need for understanding and managing nonnative forest specialist and generalist pests. 

The hemlock woolly adelgid, a nonnative invasive insect and specialist, has killed millions of hemlock trees in the US, highlighting the potential threat and impact such pests can have on our forests.

“We classify pests for easier management; it’s just a matter of which we should focus on first, given limited labor and funding,” says Qinfeng Guo, a research ecologist with the USDA Forest Service and lead author of the study.

The hemlock woolly adelgid, introduced accidentally in the 1950s, remains a major tree-killer in the US. In Japan, its population is controlled by natural predators and host tree resistance, but in the eastern US, where it's invasive, these controls are absent, causing significant damage. The National Park Service reported a loss of 80-90% of eastern hemlocks at some sites. The hemlock woolly adelgid is one of 66 nonnative species analyzed by researchers in this study.

“Invasive insects are the biggest threat to forests. This study aims to shed light on the best methods for defining and confronting this vulnerability,” says Kevin Potter, second author of the study.

Specialist and generalist classifications are used frequently in the field of forest management. Specialist pests are those that consume one or very few host species, believed to take more of a toll on their hosts, while generalists usually infest more species and sometimes across multiple genera or even families.

The key finding of this study was that specialists caused more tree deaths than generalists. However, contrary to expectations, generalists inflicted more nonlethal damage due to their broader impact across multiple species. The study also found that newer nonnative pests caused more tree mortality than older ones, possibly because older pests have already killed the most vulnerable trees, spread to larger areas, and allowed host trees to adapt over time.

Guo and Potter stress the importance of better classification. As it is now, there is no standard approach to differentiating specialists from generalists, especially when it comes to assessing which is more damaging. Depending on the way the damage is measured, different conclusions can be drawn, leading to conflicting findings.

In their study, Guo and Potter analyze damage from 66 nonnative pest species, looking for differences between nonnative generalist and nonnative specialist pests. They do this using two approaches: the binary approach (each pest is either a generalist or a specialist) and the specialist-generalist continuum. 

Moving firewood can carry nonnative pests such as the emerald ash borer into uninfested areas. Emerging emerald ash borer larvae caused the damage seen on the trunk of this dead ash tree. 

Of the two approaches explored, the specialist-generalist continuum approach made for clearer findings given the limited data available.

“If you have enough species to form a gradient or continuum from specialists to generalists, you can examine host responses along the gradient in terms of infestation and impacts,” Guo comments, reiterating the importance of clear classifications and a need to fill the current gaps in data.

In order to understand current infestations and predict and prepare for future challenges, it’s important that we further efforts to understand the dynamics of invasive pest infestations and how to best manage them.

“Unfortunately, invasive pests are here to stay, and we need to understand how to manage them,” says Potter.

Reference: Guo Q, Potter K. Generalist pests cause high tree infestation, but specialist pests cause high mortality. Forests. doi:10.3390/f16010127

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