Invasive Pine-Killing Wasps Are Even Worse Than They Sound

Photo Courtesy Ann Hajek

A new insect invader is spreading in eastern North America’s pine forests, and it’s causing trouble wherever its wings can carry it.

The invasive Eurasian woodwasp (Sirex noctilio) is a pest of pine trees, capable of inflicting huge ecological and economical losses. Now, scientists are reporting that the spread of the Eurasian woodwasp may also make casualties out of its native cousins. The newcomers are outcompeting the North American species in a one-sided war that threatens the native species’ survival and could have ripple effects on its ecosystem.


Despite their name, woodwasps aren’t close relatives of the cranky, soda can-spelunking, living pain dispensers of summertime familiarity. They’re sawflies, a faction of insects only loosely allied with bees, wasps, and ants. Woodwasps don’t do the hive thing, instead going solo and injecting their eggs beneath the bark of typically dead or ailing pine trees using a long, stinger-like egg chute. Greasing the payload is a venom toxic to the tree. The woodwasp also infects the injection site with the spores of a symbiotic fungus that rots the wood from within as the tree’s defenses wane. When the eggs hatch, the larvae feed on the rotten wood and fungus.

Normally, this reproductive method is an integral part of the cycle of decomposition in the Eurasian woodwasp’s home forests. But in places where the woodwasp has been inadvertently introduced, it can be a menace. The woodwasps can actually overwhelm and kill naive pines.

Unfortunately, this tree slayer has found its way to the North American continent. It first showed up in 2004, and has since spread throughout several Northeastern U.S. states and into two Canadian provinces. Considering that Eurasian woodwasp infestations have killed off more than three-quarters of all North American pine species grown in certain plantation settings, there’s plenty of concern about the insect spreading into the West and Southeastern U.S., the latter of which contains endangered targets like the longleaf pine (Pinus palustris).

But the impacts of the woodwasp scourge don’t appear to be limited to trees, with new research published recently in the journal NeoBiota suggesting that North America’s own woodwasp (Sirex nigricornis) is outmatched by this bold, brash competitor.

Longleaf pines—like these in Ocala National Forest—are a potential (and already vulnerable) target for invasive woodwasps. Photo Courtesy Bill Lea

The research team—made up of scientists at Cornell University and SUNY’s College of Environmental Science and Foresty in Syracuse—investigated how these two woodwasps were faring in places where they both infest pines. Researchers found pines in Upstate New York and Pennsylvania that had signs of an invasive woodwasp infestation, felled the trees, and cut apart sections housing the larvae.


These were moved to the lab, where months later, the emerging adults could be counted, measured, and identified. The team tracked how the relative abundances of each woodwasp species changed in the early 2010s by sampling trees every season.

“We would often observe both species emerging from the same infested pine trees, but the ratios changed with time,” explained lead author Ann Hajek in a statement. “Shortly after the invasive colonizes an area, the native wasps emerging from the trees would equal the invasive. However, a few years later, the natives started to get fewer and fewer.”


Within only three years, native woodwasps simply stopped showing up in trees from central Pennsylvania.

A female native woodwasp (Sirex nigricornis). Photo Courtesy Erich G. Vallery

This apparent steamrolling by the invasive species may be due to some key advantages, including the fact that the Eurasian species grows substantially larger and carries more eggs, has a larger venom gland, and reaches adulthood faster. This may give the Eurasian species a head start on find suitable trees and laying eggs.

It’s possible native woodwasps could persist by shifting to rearing young in less prefered tree species. But, scientists don’t yet know if that’ll happen, or how North American pine forests will respond to this bigger, badder bug pushing out its meeker competition. The native woodwasp doesn’t have such a lethal impact on native pines, meaning North American pine forests will face new sources of mortality in areas where the invasive species has taken over. 


By first identifying how and why the native wasps are getting whipped in this war, the hope is to formulate a strategy for protecting the embattled insects and, potentially, their habitats.

Jake Buehler is a Seattle area science writer with an adoration for the Tree of Life’s weird, wild, and unsung—follow him on Twitter or at his blog.


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Jake Buehler

Jake Buehler is a science writer living on Washington’s Olympic Peninsula with an adoration for the Tree of Life’s weird, wild, and unsung.