Woolly Adelgid



Common Name: Woolly Adelgid, Hemlock Woolly Aphid, Hemlock Chermid – The aphid-like insect is covered with a protective waxy substance that looks like wool. Adelgid is from the generic name Adelges.

Scientific Name: Adelges tsugae – The genus is derived from the Greek adel meaning hidden or not apparent; they are about 1.5 millimeters long and difficult to spot without their woolly wax coating. The species name is the genus of hemlock trees, the principal host that they parasitize.

Potpourri: Just as great oaks from tiny acorns grow, tall trees can be felled by insects barely visible in the grandeur of lofty branches. Such is the case with the majestic hemlock and its nemesis, the miniscule woolly adelgid. The desecration of the hemlock guardians of the Appalachian uplands is just one of the more insidious aspects of burgeoning human populations – the introduction of alien species. Like the fungal blight that strangled the American chestnut and the gypsy moth that ate the oak canopy, the woolly adelgid was imported unintentionally, probably to Richmond, Virginia in the early 1950’s; they were first identified in 1951 at an arboretum in that general area. [1] The result was tragic but predictable – trees evolve with chemical defenses to ward off predation by their coeval animals and fungi over millennia with trial and error mutations. This is the ecology of what is. When the equilibrium is distorted by a new interlocutor, all bets are off. The new bug has no predators and the new host has no defenses. The result is the dystopia of what happens as the forest primeval perishes.

Adelgids are related to their better known aphid cousins; both are sustained by sucking vascular fluids from plants. The most obvious difference is in the choice of hosts – adelgids are constrained to conifer trees whereas aphids are the consummate garden pests subsisting on just about everything else. The profound success of these unobtrusive sap-suckers is evident in their omnipresence; they can and do reach population densities of over one million per acre (200 per square meter), a testimonial to the fecundity of the aphid reproductive process. Three evolutionary shortcuts have come about to achieve this end: parthenogenesis, viviparousness, and lack of wings. The first is the ability of adult female aphids to produce new offspring without the need for male insemination, bypassing the mating ritual altogether; parthenos is Greek for virgin or maiden and was an epithet for the goddess Athena – the Parthenon overlooks her eponymous city from the Acropolis. Viviparity is bearing live young which is standard practice for mammals (except the egg-laying monotremes) but unusual for everything else in the Kingdom Animalia; it saves incubation time. Wings are complex contraptions that require substantial nutritive resources to erect and maintain; doing away with them allows for faster growth and sexual maturity. While aphids will on occasion produce sexual forms to promote genetic diversity and winged forms to promote geographic dispersion according to parameters known only to them, they evolved to produce as many offspring as possible in their brief lifespan; it has been estimated that one aphid could theoretically produce over 500 billion offspring in a single year. Fortunately, they are also at the bottom of the food chain for a whole range of predators that keep them in check, most notably the lady bird beetles or lady bugs. One other note of some interest concerns the ecological niche occupied by aphids resulting from the consumption of large quantities of sugar-rich sap needed to extract proteins for growth. The excess sap passes through their digestive systems and is excreted as frass called honeydew with perhaps scatological humor; aphids are protected by some ants and shielded by some fungi as quid pro quo for the nutritive plant sugars they thus provide. [2]

Adelgids share many traits with aphids and proliferate according to the same Malthusian geometric progression of doubling and redoubling. The woolly adelgid has two distinct forms that hatch at different times and have unique behavioral quirks, a fact that may have arisen as a consequence of mutations incident to their expansion in the new environment to which they were introduced. The unisex female sistentes hatch out in late spring, estivate in a state of dormancy through the summer (the hot equivalent to cold winter hibernation), begin feeding on hemlock needles as the cool weather of fall prevails, and ultimately fatten to adults through the winter. In early spring, each survivor will each lay up to 300 eggs to continue the cycle and seal the fate of their arboreal host. Potentially bisexual progredientes hatch as both wingless and winged variants in the early spring and only survive for several months. The winged subspecies are called sexuparae and fly off in search of a place to lay their eggs. As their name implies, they would produce both male and female offspring if a suitable site for oviposition were available. In what is surely a vestigial genetic component of their Asiatic heritage, the spruce trees they seek by instinctual mandate are not to be found in North America, their flight a forlorn journey to nowhere. The late fall and early spring windows of adelgid opportunity are not random; these are the best times to feed on the flowing sap of hemlock trees absent an abundance of midsummer beetle predation to which natural selection would direct genetic predilection. [3]

The woolly adelgid that has devastated the eastern forests in removing one of its most iconic tree species is a case study in invasive behavior and a cautionary tale for a future of ecological disruption. As counterpoint, there is a western variant of the woolly adelgid that is endemic to the Rocky Mountain uplands, having evolved within its surrounding forest so that there is a balance between feeding bugs and fed-on western hemlocks. Its carpetbagging eastern variant is succubus to the sessile Carolina and eastern hemlock trees on which it sucks. Since temperature plays a pivotal role in adelgid viability, the degree of infestation of hemlocks varies according to geography. Field studies have shown that in southern forests, the woolly adelgid advances southward at a rate of almost 16 kilometers per year, depletes the canopy by 80 percent in 4 years and kills about 85 percent of the trees in 7 years. Conversely, the northern advance is at half that rate and results in the same mortality rate only after 15 to 17 years; a warming climate will make this worse. The hemlocks of the southern Appalachians have essentially been extirpated; a fate reminiscent of the chestnut blight of the mid-20th century. The result has not been nearly as catastrophic, as hemlocks comprised only about 5 percent of the canopy by volume compared to roughly 50 percent for the chestnut. However, the hemlock’s importance as a foundational tree for the forest ecosystem amplifies its significance beyond mere tree count. [4] The evergreen hemlock canopy shaded woodland streams through all four seasons, dropping slowly decomposing needles to establish unique habitats for many species, notably salamanders. A study of eastern red-back salamanders and eastern red-spotted newts conducted in a New England forest revealed that the population of these two key indicator amphibians would eventually decline by 50 percent with the loss of the eastern hemlock canopy cover. [5] The ultimate impact is still unclear, but the tragedy is visible; the hemlocks of Limberlost were the only old growth forest in Shenandoah National Park when they were felled in the first decade of the 21st century.

Woolly Adelgid Hemlock Dead White Oak 2013
Dead Eastern Hemlock on White Oak Canyon Trail in Shenandoah National Park, Virginia

The hemlock holocaust that has spread over most of the Appalachian Mountains incited a full court press about twenty years ago among foresters to control the adelgid contagion with an imported biological agent. Like the five stages of grief, dealing with invasive species usually follows a predictable pattern that starts with mechanical methods such as infested tree removal and even to the futility of insect removal with high pressure water spray. Chemical warfare follows but pesticides eventually become too expensive, too environmentally damaging to other species, or both when used expansively. Eventually, the only possible remedy is to do what nature does over time – biological controls with established predators. The basic procedure is to go to the site of invasive origination and seek out local candidates from among the local predators for introduction as anti-invasives. In the fighting fire with fire category, the introduction of a second alien species to control the first is not without the inherent danger of unintentionally creating a new and perhaps worse problem. The classic example is the introduction of Indian mongooses to control the Hawaiian rat population by the sugar cane growers in 1883; they have since nearly eliminated the ground-nesting bird population and cause about $50 million in damages annually. [6] The lesson learned methodology is to quarantine candidate foreign predators in a dedicated research facility where they can be evaluated for biological and ecological compatibility. A second somewhat less onerous method is to find a native predator species that might be encouraged to relocate to adelgid infested areas and become established in a sustainable ecological balance. Researchers at Virginia Tech, Clemson, and the University of Tennessee have been involved in the quest to save the hemlocks since the early 2000’s; their efforts are just now starting to show some progress but success is anything but assured. [7]

Laricobius nigrinus is a black beetle native to the Pacific northwest that was first discovered by scientists in the 1990’s when the eastern hemlock crisis became manifest looking for anything with a demonstrable appetite for adelgids. There was good reason to believe that it could be brought east with minimal environmental repercussion and this proved to be the case. They were introduced to North Carolina for assessment and release to the wild started in 2003. Based on successful propagation and some preliminary success in curbing the adelgid blight, almost half a million beetles have since been released throughout the eastern Appalachians and the upper Midwest. Unfortunately for all but the adelgids, the success of L. nigrinus predation was limited by their life-cycle; they become inactive in late spring just when woolly adelgid populations surge. The search for biological controls continues apace – another beetle, silver fly larvae and at least one insect killing fungus, Lecanicillium muscarium, are being considered for programmed release. [8] To date, the best foreign candidate for adelgid archvillain was found in its home range of Japan in the early 1990’s. A ladybird beetle variant named Sasajiscymnus (formerly Pseudoscymnus) tsugae killed and ate about 90 percent of the adelgids in its habitat based a 24 site evaluation. Approved for release in Connecticut in 1995, initial results were promising – it reduced adelgid infestations by over 80 percent in 5 months. This is in no small part because its life cycle matches the woolly adelgid in seasonal fluctuation. About two million ladybugs have since been released in 16 states from Georgia to Maine. [9]

Woolly Adelgid Hemlock Regrowth White Oak 2012
Regrowth of Eastern Hemlock on Limberlost Trail in Shenandoah National Park Virginia

While it may seem that science has come to the rescue, the adelgid is a wily antagonist and nature is an unwitting accomplice. In the 1900’s over 30 predators were introduced to control the closely related invasive European balsam woolly adelgid (that feeds on subalpine fir trees); it is still a serious problem. While the current epidemic of adelgids is up against a much more formidable phalanx of scientific expertise and experience, the outcome remains in doubt. The Japanese lady bird beetles introduced at great fanfare and expense disappeared mysteriously without any noted effect on the adelgid population. While the jury is still out on the native predator variants – one researcher noted that “biological controls are not for the faint of heart” – there is some thought being given to finding hemlocks with natural resistance to adelgids. [10] There are hemlocks arising from the detritus of their progeny in Shenandoah National Park’s Limberlost. The work goes on – bagged hemlock branches monitored from 2014 to 2018 revealed that L. nigrinus, the Pacific black beetle, had eaten about 40 percent of the eggs. It was also determined that they had interbred with native beetles such that 2 percent were hybrids. [11] All of this is really just a natural reaction to human time relative to geologic time. Invasives move by truck, plane and boat in weeks or even hours and nature reacts with a long, drawn out sigh that spans generations. We may be able to speed it up. But maybe not.

1. Stokstad, E. “Double Trouble for Hemlock Forests” Science 19 December 2008
2. Marshall, S. Insects, Their Natural History and Diversity, Firefly Books, Buffalo, New York, 2006, pp 105-107.
3. https://www.inspection.gc.ca/plant-health/plant-pests-invasive-species/insects/hemlock-woolly-adelgid/fact-sheet/eng/1325616708296/1325618964954  – A fact sheet from the Canadian government’s Food Inspection Agency.
4. Vose, J. et al. “Hemlock woolly adelgid in the southern Appalachians: Control strategies, ecological impacts, and potential management responses” Forest Ecology and Management. Number 291, 2013 pp 209–219.
5. Siddig, A. et al. “Assessing the impacts of the decline of Tsuga canadensis stands on two amphibian species in a New England forest”. Ecosphere Volume 7 Number 11 November 2016
6. https://dlnr.hawaii.gov/hisc/info/invasive-species-profiles/mongoose/
7. Kok, L. et al. “Biological Control of the Hemlock Woolly Adelgid”. Virginia Tech College of Agriculture and Life Sciences, Department of Entomology August 2006 available at https://web.archive.org/web/20060828192331/http://web.ento.vt.edu/ento/project.jsp?project=Biological%20Control%20of%20the%20hemlock%20woolly%20adelgid
8. Vose, J. op cit.
9. Carole, A. et al “Sasajiscymnus (formerly Pseudoscymnus) tsugae (Coleoptera: Coccinellidae)” Valley Laboratory, The Connecticut Agricultural Experiment Station, Windsor, Connecticut. Available at https://biocontrol.entomology.cornell.edu/predators/sasajiscymnus.php
10. Popkin, G. “Battling a Giant Killer” Science Volume 349, Issue 6250 August 2015, pp 803-805.
11. Popkin, G. “In effort to save hemlocks, a rare glimpse of hope” Science, Volume 367 Issue 6475, January 2020, p 238.