NPS photo.

This biocontrol beetle, Sasajiscymnus tsugae, ("ST" for short) feeds on invasive hemlock woolly adelgids.

Using biocontrol to stop the spread of hemlock woolly adelgid

The hemlock woolly adelgid (HWA), an invasive insect from Asia, has wreaked havoc on many of the hemlock forests in the eastern United States. Swarms of adelgids can kill a tree or weaken it so secondary pests such as the hemlock borer eventually do.

What are we doing to treat the hemlocks? Scientists first discovered hemlock woolly adelgids in Great Smoky Mountains National Park in 2002. Since then, Park Foresters have worked to save hemlocks using these four methods:

• soil drenching (pouring insecticide around a tree base)
• injection (pumping insecticide into the tree)
• spraying the canopy and branches, and
• releasing biocontrol beetles.

They have released over 350,000 biocontrol beetles to treat 100,000 trees in 44 hemlock conservation areas. Watch the PODCAST of a beetle release in Cataloochee.

You can also read more about the history of the adelgids and chemical treatments in the Park, then use the back arrow to return to this page.

NPS photo.

A jar of "Larry" biocontrol beetles are ready for release.

Beetle backgrounds: Beetles to be released are born and raised at the Lindsay Young Beneficial Insects Lab at the University of Tennessee. Laricobius nigrinus (affectionately dubbed “Larry”), is featured in the upcoming podcast. This species is native to the Northwestern United States, where it feeds on adelgid species from that area. The other species the lab raises is Sasajiscymnus tsugae (or "ST") from Asia: it’s in the lady beetle family, although it resembles a black sesame seed more than the red and black spotted lady bug.

Researchers drive the beetles from the lab in Knoxville to local protected lands: Great Smoky Mountains National Park, Frozen Head State Park, and the Cherokee National Forest. Each week, one of these locations receives a small army of 300 beetles, which Park foresters and biologists place on understory hemlocks in the hopes that the insects will climb the “ladder” of plants to feed on HWA in the highest hemlock branches.

NPS photo.

Foliar treatment.

Success? In 2005, managers began monitoring the health of treated versus untreated hemlocks.

Here’s what they found:

• Beetles do help the trees, as long as the insects are used with other forms of adelgid control such as soil drenching.
• Trees do best when the weather cooperates—the soil has to be moist enough for the tree to draw up the chemical that foresters pour around its base. A drought in 2007 and 2008 made it harder for treated trees to do that.
• Foliar treatments (spraying the needles) are effective but short lived (around six months) and are limited to smaller trees.

NPS photo.

Soil drenching works best for homeowners treating HWA.

Should homeowners use biocontrol beetles? According to Forestry Technician Jesse Webster, no. “The best, most effective [and] economical control of hemlock woolly adelgids,” he said, is “systemic treatment [such as soil drenching], not biological control with beetles.” Beetles work best in a huge forested landscape, because they work over several years as the beetle population grows and establishes itself. The most cost-efficient treatment for most homeowners is soil-drenching with chemicals available at most hardware or garden supply stores, or from a private tree-treatment company. 

NPS photo.

NPS Foresters in Hemlock Conservation Area.

One place you can see the stark comparison between healthy and dying hemlocks is at Cataloochee, in the North Carolina section of the Park. This area is featured in the podcast that you can access above. Although 80% of this area was once logged to make way for apple orchards and hog farms, it is now lush green, with towering hemlocks, thick twisting arms of rhododendron, and tall patches of dog hobble. Vegetation managers began treating hemlocks here preventatively, and just a few years later, untreated stands are brittle grey. This emphasizes the importance of treatments. "If we can save just 10 percent of the hemlocks," Jesse Webster said, "our efforts are worth it."

Return to Dispatches from the Field: Issue 2.