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Connecticut Foresters Protect Threatened Hemlocks Using Biocontrols

HWA on a branch of a hemlock tree (photo credit: Milne)
Hemlock Woolly Adelgid on branch

Foresters play many roles as stewards of the land. Sometimes they help protect trees threatened by exotic invasive insects, such as the hemlock woolly adelgid (HWA).

Hemlock woolly adelgid (Adelges tsugae) is a tiny aphid-like insect that damages hemlocks by inserting its mouthparts (called stylets) into the base of the needles, essentially sucking out the tree’s nutrients. This feeding drains the tree’s storage reserves, causing the needles to drop off, slowing overall growth, and often leading to the death of the tree. The insect also secretes a white, waxy covering that resembles “wool” which repels water and protects it from predators. It is the easiest way to identify an infestation.

HWA is native to Japan and was first reported in the eastern United States near Richmond, Virginia in the early 1950s. It was discovered in Connecticut in 1985, and now has spread northward throughout New England and into Nova Scotia. When the insect was first identified in Connecticut, it was predicted that hemlocks would be eliminated from the state, as there were no natural controls of HWA.

Hemlocks are very important ecologically. They are found in most of our New England forests. Hemlocks often grow in valleys and north facing slopes, where their shade keeps streams cool for fish and other aquatic life. They also provide winter cover for wildlife and nesting sites for birds. While not one of the more valuable species for timber, the lumber is used to build barns, paper is made from the pulpwood, and the bark is used for landscape mulch.

Soon after HWA was found in Connecticut, scientists from the Connecticut Agricultural Experiment Station (CAES) traveled to Japan and found a tiny ladybeetle (Sasajiscymnus tsugae) which keeps HWA in check over there. The beetle underwent extensive testing and quarantining in the United States. Since 1995, hundreds of thousands of these ladybeetles have been released in Connecticut.

Foresters with the Connecticut Division of Forestry identify State Forests where HWA is prevalent. They work with Carole Cheah, the CAES entomologist, to release the ladybeetles and monitor the health of the hemlocks.

The program has been a success, because after almost 30 years of being infested by HWA, there are still hemlocks in our forests. Private landowners can buy these ladybeetles from the only commercial producer, Tree-Savers, at

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