Biocontrol to Save our Hemlock Trees
Eastern hemlock (Tsuga canadensis) trees are fairly common across Breaks Interstate Park, including some very large diameter trees. This is a rarity across the region and something worth trying to protect. Generally, eastern hemlock grows in moist shady hollows; within the Park, this is true, and there are many hemlocks growing on drier ridge sites as well. Why the scarcity in other locations? Hemlock woolly adelgid (Adelges tsugae), or HWA as it is commonly known, is the primary causal agent of decline and mortality. This insect is native in the Pacific Northwest of the United States as well as in Japan, but it is not native in our eastern forests. Our version was accidentally introduced from Japan on nursery stock over one hundred years ago. Yet, it took until the 1950s before it was widespread enough to be recognized in Virginia. This tiny little critter is a sap sucker (similar to an aphid) that feeds at the base of hemlock needles. It has two generations per year with a late summer lull. If you see white “cottony” balls at the base of hemlock needles, you have HWA.
HWA ovisacs evident on the underside of an eastern hemlock branch.
These ovisacs are created around the critter for protection. As HWA are tiny, it takes a LOT of them to damage a tree. Most infested trees experience a slow decline over a period of years rather than sudden death. This slow decline provides a hopeful opportunity for intervention. How can eastern hemlock trees be protected? For a yard, park, or other “urban” type tree, chemical control with an insecticide can be very effective and one application may protect a tree multiple years. Within the Park, 174 such trees were treated in 2023. Nevertheless, for the landscape as a whole, chemical control is neither desirable (e.g., other insects can be killed inadvertently) nor practical. Hence... biocontrol. The idea is to introduce a biological agent that kills the pest and only the pest (or at least not greatly affect other populations). For HWA, biocontrol efforts have focused on predatory insects. In 2023 and again in 2024, Laricobius nigrinus beetles were released in the Grassy Creek area of the Park.
Adult Laricobius nigrinus feeding on HWA.
A single Laricobius nigrinus beetle at the tip of the finger along with scattered hemlock needles.
These insects have been released across the East coast since 2003 after a period of intensive study to make sure that they would not unduly impact other species. This particular Laricobius species is native to the Pacific Northwest where it feeds on native HWA in western hemlock trees. The predatory beetles have one generation per year with no activity in the summer time (similar to HWA). They feed on HWA eggs, nymphs, and adults. Studies in the East have shown successful long-term establishment on numerous sites as well as an increase in the amount of disturbed HWA ovisacs (opened for eating) with beetle presence. Specific small scale study work has shown more new shoot growth on trees with fewer HWA due specifically to L. nigrinus predation. While it is well established that L. nigrinus reduces the number of adelgids, it will not eliminate HWA from the landscape. An additional insect that preys in the summer would be helpful to further reduce HWA (there have been some released in the past though not in the Park); however, even with multiple biologic control agents, elimination of HWA is unlikely. One reason is the high fecundity of HWA. Another reason: If the population of HWA gets very low, then the population of its predators will also decline due to lack of food availability. This predator reduction can occur when we have periodic very cold weather (like a polar vortex) which may kill up to 90% of the adelgids. Fortunately, eradication of HWA is not necessary for healthy eastern hemlocks – just a reduction in the population of this invasive pest. A few other notes about the released predatory beetle: L. nigrinus is very closely related to the native L. rubidus beetle as evidenced by hybridization between the two. It can fly to find new trees with a food source, but the beetle generally does not fly much or for long distances meaning natural spread is relatively slow. Additional biocontrol predators for HWA have been released or are undergoing laboratory testing before being approved for release. Some of these are from Asia (Japan or China) and others are from the Pacific Northwest in the United States. One of the issues with biocontrol agents, including L. nigrinus is the difficulty and expense in collecting and/or breeding them for dispersal. 500 beetles were released in the Park in 2023 and 650 released in 2024. More insects to release would be great to build a helpful population faster as well as in multiple locations across the park.
Submitted by Greg Estoll – Forester with Virginia Department of Forestry serving Buchanan, Dickenson and Tazewell County landowners with forest plans, recommendations, and implementation assistance including forest health assessments.
