Hemlock woolly adelgid

Departments - Under the Microscope

Previously confined to the coasts, this pest has made an appearance in the Midwest.

October 13, 2015

Hemlock woolly adelgid, Adelges tsugae, is a native of Asia and was first detected in the Eastern U.S. in 1951. The insect was introduced to the Western U.S. in the 1920s, but is not considered a pest there. However, it is decimating hemlock stands across much of the east, where it has spread from Georgia to Maine and into the Midwest. DNA evidence suggests that the invasive eastern U.S. population came from Japan and not the western U.S.

In the Northwest, the insect is confined to Oregon and Washington, and it is confined to Georgia to Maine in the East. Or at least it had been until it appeared in Michigan this summer.

John Bedford, pest response program specialist, Michigan Department of Agriculture and Rural Development, says his state has had hemlock woolly adelgid incursions before, often related to infested nursery stock, despite the external quarantine that has been in place since 2002.

“That smoking gun is hard to find, but we had some hard evidence in a couple cases that it was infested nursery stock being moved into Michigan either prior to the implementation of our quarantine or in violation of our quarantine,” Bedford says.

The quarantine restricts and regulates the movement of hemlock nursery stock entering Michigan and absolutely prohibits the importation of nursery stock into Michigan from areas of the country known to be infested.

The Guide

Follow these tips to prevent hemlock woolly adelgid from wreaking havoc on your nursery.

Know what to look for.
The telltale sign of hemlock woody adelgid is the white, waxy material at the base of the needles. The tiny insects secrete that wax as they feed on sap.

It’s hard to find in small numbers.
HWA is a difficult insect to survey, because its tiny size makes it difficult to detect at low levels. Bedford says the Ottawa County infestation may have been there since 2001, hovering out there for 14 years before it reached a level where it was noticeable.

Symptoms aren’t always apparent.
Sometimes the trees can look fairly healthy and still be infested. Bedford says it just depends on the number of insects on any given tree or branch.

Control is possible.
All chemical control timing will vary due to your specific location and what is allowed in your state. Your local extension service can help you choose the products that will work best for your particular situation.

If a grower gets approval from MDARP for insecticide treatment, Dudek says imidicloprin is effective as a soil drench in the fall or the spring, or as a foliar spray. Dormant oil is effective when the plants are dormant in late April/early May.

If you find it: To report a possible HWA detection in Michigan, contact MDARD at 800-292-3939 or MDA-info@michigan.gov. To read the quarantine details, visit: www.michigan.gov/exoticpests.


The facts about hemlock woolly adelgid

In the eastern U.S., Adelges tsugae is killing eastern hemlocks and Carolina hemlock in large numbers from Connecticut south along the Appalachian Mountains. From Massachusetts north, or at high elevations (like the Pacific Northwest), tree mortality has been restrained by higher rates of mortality of adelgids in winter due to low temperatures.

Eggs are initially brownish-orange, but will darken as the eggs mature. Eggs and very young adelgids can be carried by birds and can be moved on hemlock nursery trees, logs or firewood.

Hemlock woolly adelgids are small, and to the naked eye only their woolly coverings are easily visible. Crawler stage nymphs produce white cottony/waxy tufts which cover their bodies and remain in place throughout their lifetime. Generally, they are brownish-reddish, oval, and about 0.8 mm in length. The adults are small (1/32 inch), oval and reddish purple, although covered with white, waxy tufts. The white masses are 3 mm or more in diameter.

Life cycle
These small insects display several different forms during their life history, including winged and wingless forms. The insect has two generations per year and growth occurs from fall through late spring. Insects in summer are inactive and scarcely visible at the bases of needles as black dots. Woolly masses (the sign allowing the species to be recognized) develop in October and are present thereafter through June of the following year.

The presence of woolly masses on the bark, foliage, and twigs of hemlock is a sure sign of hemlock woolly adelgid. These tiny insects secrete white wax as they feed on sap from hemlock shoots and branches. Over time, growth slows as trees become less vigorous and trees may take on a grayish-green appearance. Infested hemlocks, especially large, old trees, are often killed when other stress factors, such as drought, affect trees.

Insecticidal soap and horticultural oil sprays can provide effective control of HWA even when the waxy covering is present. Relative to most other insecticides, insecticidal soaps and horticultural oils have fewer potential adverse effects to the user, with minimal harm to beneficial predators, parasites and the environment. Complete coverage is needed for effective control, so a high-pressure spray is necessary. A high level of control is possible with just one spray. Evaluate a week after spraying to see if a second spray is needed.

Horticultural oil may cause some phytotoxicity when applied during the growing season, especially during hot, dry weather. For this reason, a 1 percent solution of horticultural oil is recommended from May through September, while a 2 percent solution can be used from October to April. Insecticidal soap sprays may occasionally cause some phytotoxicity on tender new foliage. It is best to not apply horticultural oil or insecticidal soap if the temperature exceeds 90°F. or drops below 45°F. Spraying trees with horticultural oil or insecticidal soap before trees are infected does not act as a deterrent to HWA infestation.

Imidacloprid can be used as a foliar spray, applied in the root zone as either a soil drench or soil probe injection, or injected directly into the tree trunk. Foliar imidacloprid sprays can be made on trees away from sources of water. Timing of imidacloprid sprays is best between mid-May and mid-June, and again between late July and October. The root zone or trunk injection methods are much longer lasting than the foliar application, and the level of control is generally better. One application to the root zone or the trunk can potentially provide a year or more of control.


Sources: USDA Forest Service, Michigan Department of Agriculture and Rural Development, Bugwood.org