Issue 4, May 14, 2012

Hemlock Woolly Adelgid Found in Nearby State

The Indiana Department of Natural Resources recently announced that the hemlock woolly Adelgid was identified in the state for the first time. This invasive insect was found on a landscape tree in LaPorte County in mid-April.

What does this mean for Illinois?
Although the natural growing range of hemlock trees is just to the east of Illinois, these plants are quite adapted to landscape use and are popular in many areas within the state. Hemlocks make interesting trees in the cultivated landscape. These insects have spread rapidly through the Appalachians, mostly by wind or carried by migratory birds, mammals, and humans. However, infested nursery stock has also carried the insect into some areas. This insect is a concern within the state of Illinois, especially now since it has been confirmed in NE Indiana.

Why is the hemlock woolly adelgid important?
The adelgid feeds on the sap of these trees, which can deplete its essential fluid and nutrient supplies, and produces saliva which contains toxins harmful to the trees. The result of an infestation is defoliation and eventually death of Hemlock trees within 4-6 years. In instances where infestations are severe, hemlock species can be completely removed from a forest. Because the hemlock woolly adelgid has no real natural predators in the eastern United States, its populations have grown rapidly, allowing it to cause large amounts of damage to hemlock stands. The impact is already clearly visible in the Great Smokey Mountains National Park.

The natural autumn landscape of the Smokey Mountains is broken up by gray patches of dead Hemlock trees (image courtesy of the National Park Service).

How is the hemlock woolly adelgid identified?
Adelgids are mostly associated with coniferous trees. Hemlock is the primary host, with spruce being a possible secondary (alternative) host. These small aphid-like insects are less than 1/16 in long, black in color, and usually covered in a white puffy wax which progressively becomes thicker during their one year life cycle. The wax serves as a barrier to protect them from predators as well as to keep them from drying out during low humidity. They are usually found in great numbers almost covering entire sections of tree branches.

Left: Adult Hemlock Wooly Adelgids in their waxy coating on a hemlock branch. Right: Adult Hemlock Wooly Adelgid with waxy coating removed. (images courtesy of

What is the lifecycle of the hemlock woolly adelgid?
This particular species often undergoes a brief period of dormancy during the heat of the summer but can still be found on the stems of the plant. Adelgids, like many aphids, reproduce parthenogenetically, that is, the entire population is female and undergoes asexual reproduction. It can reproduce in large numbers very quickly. A typical adelgid can produce up to 300 eggs at a time and can reproduce twice in one year. While their populations increase on a tree, growth becomes stunted and needles begin to fade and fall off the tree. Within a few years a highly infested tree will perish.

Is there a way to manage the hemlock woolly adelgid?
Researchers are investigating the benefits and efficiency of introducing predatory insects into infested areas to serve as a control measure in large forested areas. Other treatments successfully used for individual trees include systemic insecticides, horticultural oils, and insecticidal soaps.

What should be done if you think you have hemlock woolly adelgid?
The hemlock woolly adelgid is not currently known to exist in Illinois. It is very important to identify any potential populations in the state early to eradicate and prevent the spread of this insect. If you believe you have hemlock woolly adelgid please contact Kelly Estes, State Survey Coordinator of the Cooperative Agricultural Pest Survey Program (217-333-1005, (Kelly Estes)

Kelly Estes

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