Little, Fuzzy, But Not So Cute: The Hemlock Woolly Adelgid
To the casual observer, the tufts of small white fuzz on a hemlock branch might not look like insects. They’re minuscule, they’re covered in a waxy wool-like substance, they don’t move much once they pick their spot on a tree, and most of them can’t fly, either. And yet there are so many of them, spread up and down eastern North America, an existential threat to the region’s vast hemlock forests. Meet the hemlock woolly adelgid.
Adelges tsugae, known as the hemlock woolly adelgid, is a sap-sucking insect originally from Asia but also long-established in western North America, where it is an otherwise unremarkable member of local ecosystems. In eastern North America, however, it is a major pest of eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana), neither of which are evolved to resist the pest.
Mark Whitmore, extension associate at Cornell University, does not mince words in describing the hemlock woolly adelgid’s potential impact. “Eastern hemlock is a foundation species and one of the most common trees in northeastern forests,” he says. “I consider the potential loss of this species to be one of the most important ecological disasters our forests face, perhaps second only to climate change.”
Whitmore has teamed up with Samita Limbu, Ph.D., a postdoctoral research associate at Cornell, and Melody Keena, Ph.D., a research entomologist at the U.S. Forest Service Northern Research Station in Hamden, Connecticut, to shine a light on the pest in a new profile of the hemlock woolly adelgid in the open-access Journal of Integrated Pest Management. Published December 27, 2018, the article summarizes the hemlock woolly adelgid’s life cycle and behavior and outlines the combination of methods available for managing it. “There are many forest managers and the public that need something like this to use when planning what to do to deal with hemlock woolly adelgid on their hemlock trees,” says Keena.
The hemlock woolly adelgid (HWA) does its damage by sucking nutrients from its host trees. They hatch in a larval form dubbed “crawlers,” which move along the tree to find a feeding site. Then they proceed through four nymphal stages as they feed in place on the tree. The population in eastern North America reproduces through parthenogenesis—i.e., reproduction without fertilization, in which females produce offspring that are (roughly) clones of herself—and proceeds through two generations in a year’s time. The spring generation produces some winged females that cannot produce, while the rest remain wingless adults that do lay eggs (as is the case for all of the winter generation). A single tree with a heavy infestation can have millions of HWAs on it.
Meanwhile, “unlike most insects, this pest is active in the winter and aestivates in summer,” says Limbu, lead author on the JIPM article. (Aestivation is a period of dormancy akin to hibernation, but in response to high temperatures.) Limbu conducted her Ph.D. research at Penn State University studying an HWA predator insect. “This characteristic allows the pest to avoid the generalist predators that generally are active during the summer. The insect’s ability to adapt to cold winter conditions and capacity to rebound quickly after adverse conditions may also possibly allow the pest to expand its range in the eastern North America.”
Pesticide applications can be effective against HWA but must be administered one tree at a time, so they’re mostly limited to horticultural settings. For the region’s hemlock forests, biological control via entomopathogenic fungi and predator insects are the more promising methods, though they remain to be thoroughly established. And HWA’s two-generation-per-year life cycle further complicates efforts, Limbu says.
“There is ongoing research that seeks to locate predators that will attack the second, spring generation of hemlock woolly adelgid,” she says. “Finding suitable predators that are effective to control the spring generation of HWA are needed to maintain the predator pressure throughout the pest’s life cycle.”
HWA’s small size also makes it difficult to detect until present in large numbers. No traps work to catch it, and so visual inspection is necessary. One sampling technique, particularly for HWA in high branches, is to launch a Velcro-covered ball into a tree with a slingshot, retrieve it when it falls, and examine it for adelgids or their woolly covering. The technique was developed by Jeffrey Fidgen of the Canadian Forest Service.
“I was jokingly skeptical at first,” says Whitmore, “but, when it came down to it, I lost a six-pack of beer when I declared a stand clear of HWA and, after the fourth sample tree, Jeff had found it. It’s a sensitive technique that’s good for sampling stands with high canopies but can be difficult on steep slopes and near waterways.”
The effectiveness of Velcro for picking up hemlock woolly adelgids speaks to its ability to spread despite being relatively immobile. Birds, deer, and other wildlife that interact with hemlock are perfect rides on which HWA can hitchhike. And human-aided transport is suspected for its original arrival in eastern North America, as well.
“HWA is a good example of a species that came in on plants for planting,” says Keena. “The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service is working on closing this pathway for invasive insects, but it is important that people appreciate that whenever a living plant is introduced to a new region, there is a risk of introducing a non-native insect or disease. Our desire to have plants from other areas, especially abroad, creates the market for plants for planting and the increased risk of new invasive species hitchhiking on them.”
Journal of Integrated Pest Management