The invasive hemlock woolly adelgid (Adelges tsugae) is a serious pest of hemlock trees in North America, and early detection is critical for adequate response. Researchers in Canada have developed a wide funnel trap that can detect infestations up to three years before they become visually evident. (Image originally published in Fidgen et al 2025, Journal of Economic Entomology)

By Paige Embry

Paige Embry

When a devastating forest pest infiltrates a new area, finding the infestation in its infancy can greatly enhance control efforts but requires a deep understanding of the pest. Where does it live at its different life stages? When do they occur? How, when, and where does it reproduce? Is it mobile? How is it mobile—e.g., does it fly, crawl, or create tiny parachutes to catch the wind? The need-to-know list goes on and on and is critical in selecting and using the right detection tools.

In eastern North America, scientists have been working on ways to better detect hemlock woolly adelgids (Adelges tsugae), an invasive species on eastern hemlock (Tsuga canadensis). A study published in June in the Journal of Economic Entomology explores the efficacy of a new type of trap.

Jeff Fidgen

Eastern hemlock forests, with their deep, cool shade and acidic, nutrient-poor soils, provide a unique habitat both on land and in the rivers that flow through them.⁠ Most hemlock species have some resistance to this adelgid, but T. canadensis (and another eastern North America native, Tsuga caroliniana) do not. Most trees die within three to 15 years of becoming infested. When enough die, their unique habitat goes with them.

The hemlock woolly adelgid is a small insect with piercing-sucking mouthparts and a complex life cycle.⁠ It is native to Asia and was first found in eastern North America, in Virginia, in 1951. It has since spread as far as northern Georgia, the Great Lakes area, and southern Canada. In its native range, the hemlock woolly adelgid (HWA) reproduces on both hemlocks and spruces, sexually on spruce and asexually (two generations per year) on hemlock. North America has no spruce species appropriate for HWA reproduction so the adelgids get by—indeed flourish—via asexual reproduction alone.

To try and find HWAs, scientists have shot velcro-covered balls into the tree canopy hoping adelgids will stick, pruned out branches as high as they can reach and inspected them, and deployed existing types of passive traps. All have their limits. “Early detection tools are needed so that we can buy time to plan and execute rapid responses to infestation,” says Jeff Fidgen, a researcher at Natural Resources Canada and lead author on the study.

The invasive hemlock woolly adelgid (Adelges tsugae) is a serious pest of hemlock trees in North America, and early detection is critical for adequate response. Most hemlock species have some resistance to these tiny, white, fuzzy insects (shown here infesting a hemlock branch), but hemlock species in North America do not. Most trees die within three to 15 years of becoming infested. Researchers in Canada have developed a wide funnel trap that can detect infestations up to three years before they become visually evident. (Photo by USDA Forest Service – Southern Research Station , USDA Forest Service, SRS, Bugwood.org)

Consequently, he and others designed a new trap—a souped-up funnel with an expanded collection area of 1,431 square centimeters (222 square inches), upward-facing, with a collection jar at the bottom. They stretched hardware cloth over the top to keep out large debris. The goal is to catch the mobile first instar of both asexual generations, called crawlers, which can be dislodged from trees to fall into traps below.

The scientists evaluated their trap over a range of infestation levels at eight sites in southwestern Nova Scotia. They assessed how long the traps should be deployed, how many were needed, and the best way to sift the crawlers from the other debris and count them. They compared the funnel results to pruning and sticky traps. The funnel traps detected crawlers at all eight sites, sticky traps and pruning at only six. Each site had three funnel traps with trapping periods of 0.5, 1.5, two, or three weeks. Leaving traps out for two or three weeks versus half a week doubled the proportion of positive detections.

In eight of the 40 trapping events, only one of the three traps contained HWAs, so using fewer traps could have yielded a false negative result. However, the scientists used the statistical method of bootstrapping to simulate hundreds of field collection events. The results indicate that deploying one or two of their traps for two weeks per generation and moving them between generations should achieve, as the authors write, “satisfactory detection rates.”

The researchers investigated the best way to find crawlers among all the debris in the trap. They dumped everything in one trap through sieves and then poured the material from the 100 micrometer sieve onto gridded filter paper, yielding 219 piles of crawlers and similar-sized debris to sort through. It’s a “tedious” job, so they tried sampling only 20% of the piles, but they found false negatives could result when crawler numbers are low. The authors suggest examining piles until the first crawler is found or all have been inspected.

Fidgen says he was surprised that, “the sensitivity of the traps far exceeds the other methods we tried.” And high sensitivity matters—it can buy time.

“In Nova Scotia, we see impacts from HWA in three to five years after infestations are detected,” Fidgen says. “We have some evidence to suggest that use of the traps in an early detection capacity buys three years before infestations are seen visually.”

And a three-year head start could be a big boon in at-risk forests.

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.