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Spring 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

April 3, 2012

For Eastern hemlocks: Bad news and good news

A mild winter isn't much help to the East Coast's oldest evergreen trees, the Eastern hemlock, which are beleaguered by the hemlock woolly adelgid. The notorious insect pest from Japan, which latches onto the tree's needles and relentlessly sucks its sap, began spreading from the Richmond area in the 1950s and has since killed thousands of acres of hemlocks from southern Maine to Georgia. In particular, it has devastated hemlocks in the southern Appalachians.

Brad Onken of the U.S. Forest Service expects a significant increase in and spread of the insect when the region has a warmer-than-normal season. However, several consecutive years of good moisture conditions have strengthened hemlocks, he says, "so the immediate impact on tree health will hopefully be minimized."

Overwintering adelgids cover themselves in a white, woolly blanket of wax into which each can lay hundreds of eggs. The bugs are exclusively female.

Can unseasonably warm weather trick the insect into a vulnerable stage at which it could be whacked by freezing temperatures? No, says Onken. Only near-zero temperatures or sustained freezing temperatures seem to knock back the adelgid.

Eggs hatch into crawlers, which settle either on the same hemlock or travel to new hemlocks by sailing off on breezes or hitching rides on the legs of birds — a good reason to place bird feeders far from hemlocks.

Urban hemlocks can be treated with insecticides, but that's not always practical for forest trees, so biologists have been using biological control agents. Among the most promising are a black lady beetle from Japan — the size of the Q on a quarter — which appears to be aiding a recovery of hemlocks in Connecticut, and a beetle from the Pacific Northwest that is increasing in numbers and "is likely to be a major player in the long-term solution," says Onken.

SOURCES: Carole Cheah, Connecticut Agricultural Experiment Station; Fifth Symposium on Hemlock Woolly Adelgid in the Eastern United States

Hemlock woolly adelgid