Fighting a Bug Invasion with ... Bugs

The Hemlock Woolly Adelgid leaves tell-tale evidence of an infestation in Eastern hemlocks, seen here as cottony balls on the under-side of branches.

Photo: U.S. Forest Service

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The graceful groves of eastern hemlock trees in sleepy ravines familiar to hikers and country dwellers are in danger of disappearing.

From Georgia to Maine, and as far west as Pennsylvania, West Virginia and Tennessee, hemlocks are besieged by a tiny Asian aphid, the hemlock woolly adelgid. Like other invasive species, it arrived most likely by accident in a shipment of wood product or other import, and like other invasive species, it has run rampant through an ecosystem that did not evolve any resistance, predators or other defenses.

Now, according to the News and Observer, scientists are looking to the western hemlocks, a related species that has survived aphid infestations. Part of that is natural resistance, but part may be the presence of predatory flies that feast on the tiny aphids.

State and federal forest managers have already tried releasing Asian bugs – a species of ladybird beetle, or "lady bug" – without great success.

The strategy of fighting invasive foreign species with more foreign species is risky. It can be less toxic and more effective than using pesticides, less time consuming and more effective than attacking pests by hand. But it risks unintended consequences which, as every invasion demonstrates, can come in tiny packages and without warning.

This video shows the damage done by the hemlock woolly adelgid.

And this one shows a presentation by scientist David Orwig, a prominent scientist studying the adelgid and its impacts on, among other things, water quality of America's drinking water reservoirs.

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