Field Notes: Fighting the Hemlock Woolly Adelgid on State Forests

By Lori Chamberlin, VDOF Forest Health Manager

Hemlock trees have been under attack since the introduction of the hemlock woolly adelgid, an invasive insect that was first discovered in eastern North America in the 1950s. These small insects settle at the base of hemlock needles, feed on plant sap, and surround themselves in soft, white ovisacs that resemble cotton balls. They may look harmless, but the hemlock woolly adelgid has caused widespread hemlock mortality in eastern North America. To fight back, the VDOF forest health program has implemented a variety of control techniques to protect remaining hemlocks on State Forests in Virginia.

Hemlock woolly agelgid on hemlock branch

When applied correctly, chemical control is very effective and can protect a hemlock tree for many years. In April, VDOF forest health staff and Shenandoah work area staff treated over ninety hemlock trees at First Mountain State Forest and over thirty trees at Paul State Forest. The hemlocks at both forests are infested with the hemlock woolly adelgid, but are in relatively good condition. A systemic insecticide was applied as a soil drench around the base of each tree; it will be taken up by the roots and distributed throughout the tree to provide protection for up to seven years!

Katlin DeWitt, VDOF forest health specialist, applying insecticide as a soil drench at First Mountain State Forest

Biological control is another method with the potential to provide long-term protection. All biological control agents are studied at quarantine facilities before they are approved for release, to ensure that they will not affect non-target hosts. Predator beetles (Laricobius spp.) have been released on public lands for many years, but this year VDOF participated in the release of a new biocontrol agent- silver flies. The larvae of Leucopis argenticollis silver flies prey on the eggs of hemlock woolly adelgid. In March, 435 adult Leucopis argenticollis flies from the NYS Hemlock Initiative’s HWA Bio-Control research lab at Cornell University were released at Sandy Point State Forest. We hope that these flies will establish a population and help control the hemlock woolly adelgid at Sandy Point for many years.

Forest Health: A Winter Pest Survey

Each month, Field Notes will bring you news from our forest health team. We kick off 2019 with a focus on winter activities and the hemlock wooly adelgid.

What do forest entomologists do in the winter? We look for hemlock woolly adelgid! The Forest Health program staff at VDOF surveys for many forest pests throughout the year, but the hemlock woolly adelgid is unique in that it is most active and visible during late fall and winter months. This invasive insect has been killing hemlock trees in eastern North America since its introduction into the United States in the 1950s. The tiny adelgid insects crawl along hemlock branches in search of a suitable feeding site, and settle at the base of needles where they insert their piercing sucking mouthparts into the plant tissue. They feed on the tree’s nutrients and produce a white ovisac, or covering, around their body for protection. If you ever see small white “cotton balls” on your hemlock tree, you probably have hemlock woolly adelgid.

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Hemlock woolly adelgid ovisacs on eastern hemlock

Most hemlock trees in Virginia are found in the western region, but there is an isolated population of hemlock situated east of the species’ major range at James River State Park in Buckingham County. In December 2018, VDOF visited this stand to assess the health of the hemlock trees and monitor the hemlock woolly adelgid population. While many hemlocks near the river still look healthy, there has been significant decline of hemlock trees further upland. Overall stand health was rated “fair” in comparison to the “good” rating just two years ago. Surprisingly, however, hemlock woolly adeglid populations appear quite low. The winter of 2017/2018 saw some extremely low temperatures which may have reduced adelgid populations. Bad news for adelgids, but good news for hemlocks.

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Katlin Mooneyham, VDOF Forest Health Specialist, beat sheet sampling for L. nigrinus beetles

What kills adelgids beside cold weather? Beetles! Small predator beetles have been released as biological control of the hemlock woolly adelgid on federal and state lands throughout Virginia by various universities and federal agencies over the past decade. In 2005, Laricobius nigrinus beetles were released at James River State Park to control the hemlock woolly adelgid and hopefully preserve the population of hemlocks within the park. L. nigrinus is native to pacific northwestern North America and has been thoroughly screened and tested to ensure it is not a threat to non-target species. Since the release, the stand has been monitored to assess the population change of the beetles and the adelgid over time. The presence of L. nigrinus is determined by beat sheet sampling- a fun technique in which you shake a branch over a large white sheet attached to a cross frame and then identify all the beetles than fall from the branch onto the sheet. This sampling is done on dozens of hemlock trees and typically lasts about an hour. L. nigrinus beetles are very small and difficult to detect, but we found 6 adult beetles during our visit in December. These six beetles indicate the population of L. nigrinus is surviving and feeding on hemlock woolly adelgid which gives us a glimmer of hope for our hemlock trees.

Field Notes: Hope for Hemlocks?

by VDOF Forest Health Specialist Katlin Mooneyham

Since its introduction to the United States in the 1950s, hemlock woolly adelgid (HWA) has been an unwanted resident in our hemlock forests. Feeding on eastern and Carolina hemlocks, this tiny sap sucking insect has established itself throughout most of the native range of both species. Unlike most insects, this tiny insect is active in the winter months, feeding on the stored nutrient reserves that help our hemlock trees produce new growth and foliage in the spring. You may not be familiar with the actual insect, but chances are you are very familiar with the “cotton ball” appearance of its protective covering found on the underside of hemlock branches.

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The white “cotton balls” indicative of HWA on hemlock branches

 

This pest is originally from Asia, and most likely arrived on imported nursery stock. Since it has made itself comfortable in our eastern forests for more than half a century, its impact on hemlocks is far and wide, leaving skeletons of what were once hemlock giants.

While this is a depressing tale, there is hope in the form of an unlikely hero- a small predatory beetle that feeds exclusively on HWA. The beetle, Laricobius osakensis, is also active in the winter and feeds on all life stages of HWA.

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Laricobius spp. adult under a microscope

It has been shown to only complete its life cycle by feeding on HWA and won’t feed on any other adelgid species. Many people are hopeful that this beetle will be able to establish and maintain HWA populations below damaging levels in our forests. Chemical options are available to protect trees, but are not feasible in large scale, forested settings. Natural predators, like our friend L. osakensis, are our best bet at naturally combatting the invasive HWA in the native range of our hemlock species.

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L. osakensis larva eating a HWA egg

These beetles have been mass reared to the tune of 51,000, released at 52 sites in eight states. Two of these sites are in Shenandoah National Park (SNP), and a number of interested parties helped get them there. Virginia Tech Department of Entomology has been mass rearing these beetles for release purposes since 2011. In November of 2015, biologists from SNP made their first release of 500 beetles after obtaining the necessary paperwork and public review period. Another release was done last November at another site within the park. Because this release site was done on a park boundary, approval was also granted from the City of Charlottesville, whose property borders the release site. On April 2, Dale Meyerhoeffer (Biological Science Technician with SNP), Jeremiah Foley (graduate student in the Entomology Department at Virginia Tech), and I, Katlin Mooneyham (Forest Health Specialist here at VDOF) met at these release sites to study beetle establishment and overall stand health. We were very enthusiastic to find a fair number of beetles at both sites. Jeremiah also took soil samples to look at differences between the two sites and how it might affect beetle populations.

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Jeremiah Foley (left) and Dale Meyerhoeffer (right) take samples and look for beetles in SNP

These sites will be continuously monitored over the next few years to determine how the beetles move through the forest and their impact on HWA populations. We are hopeful that these predators will continue to feast on HWA throughout the park and that we will see our hemlocks improve!

Image at top: Adult L.osakensis on a branch with HWA