Insects pose major threat to N.E. hemlocks
The Boston Globe, March 19, 2002
They look like the tips of tiny cotton swabs, clinging delicately to the undersides of hemlock branches across Massachusetts. But inside are millions of woolly adelgids, a sap-sucking insect that threatens to devastate one of New England's signature evergreens.
After a mild winter, the sandgrain-sized Asian woolly adelgid, usually killed by the cold, is thriving and expected to burst into dozens more Massachusetts communities, some as soon as this summer. And hemlocks, weakened by one of the worst droughts on record, are not expected to withstand the infestation for long.
''This bug is slowed by very cold temperatures, and trees that are in very healthy and vigorous condition,'' said Peter Del Tredici, director of living collections at the Arnold Arboretum in Jamaica Plain. ''If you throw in the no insect mortalities and the drought ... we could see some serious damage.''
Nearly all the Arboretum's 2,200 hemlocks show signs of the bug, which is often, if not always, fatal to hemlock trees.
The woolly adelgid has attacked Massachusetts hemlocks since at least 1989, when it was first spotted in Springfield. Since then it has spread slowly across the state. The insect has attacked about 10 percent of hemlocks around the Quabbin Reservoir, and foresters there are attempting to limit damage in part by cutting down trees. Meanwhile, thousands of homeowners in the 120 towns hit by the adelgid are watching their trees die.
In Connecticut, the bug has already overrun hemlocks. State-owned walking trails in Guilford were closed indefinitely three months ago because of the danger of falling hemlocks.
Maine is so intent on protecting its vast commercial forest from the dreaded disease that state officials now have an all-points bulletin on the bug. When they spot the tiny pest, they send a team out to kill it. So far, they've been successful - although some imported trees have been infested, none of the bugs have made the jump to the state's millions of hemlocks. In New Hampshire, a small infestation has been reported in Portsmouth. Vermont, so far, is believed to be free of the bugs.
Although tree blights are nothing new in New England, scientists consider this infestation one of the state's most serious. The long-living hemlock trees - some in Massachusetts are 400 years old - represent vast amounts of the region's old-growth forests, the only landscapes that have not been worked over by farmers or loggers. The hemlock is one of the three most common trees in the region, although exact numbers are hard to come by because hemlocks are distributed among other growth.
It is also key to New England's ecosystem. Often growing on steep inclines near riverbeds, hemlocks have an insulating function: They keep streams from freezing over in winter, allowing brook trout and Atlantic salmon to swim. In the summer, they offer a cool, soothing shade. They also absorb and hold on to vast amounts of nutrients that would otherwise flow down watersheds into rivers. ''These are incredibly valuable trees,'' said David Orwig, a forest ecologist at the Harvard Experimental Forest in Petersham.
It can take three to 10 years for a hemlock tree to fall to the adelgid, which cuts a broad swath across the hemlock population. ''With other pathogens that have entered our forest, they don't eliminate all sizes and age classes,'' said Orwig. ''This does.''
The East Asian insect, which reproduces twice a year, appeared in America in 1924, when it was found in the Pacific Northwest. It injects a long feeding tube into young twigs to suck out sap. It appeared in Virginia in 1950 and began its slow movement north, hitching rides on wind, birds, and even nursery trucks.
Once a hemlock is stricken by the adelgid, its needles become discolored and change from deep green to gray green, eventually dropping off prematurely. The tree dies from the continued sapping of nutrients, as well as from toxins injected by the bug.
There is one bright spot: ladybugs, which feed on the adelgid. Scientists in recent years have released tens of thousands of ladybugs in Wilbraham, Manchester-by-the-Sea, Milton, and Newton. Although it's too early to declare the experiment a success, Massachusetts forester Charles Burnham says results look promising.
Meanwhile, homeowners can buy a nicotine-based adelgid poison that tree experts inject into the soil. The hemlock absorbs the poison without damage, and the woolly adelgid is killed.
Jim Ingram, vice president of Bartlett Tree Experts, a tree-care company, says large swaths of hemlocks can be treated with the injection. The company currently has an exhibit at the Boston Flower Show about hemlocks and their importance in the environment.
Still, foresters say they'll never fully get rid of the dreaded bug - even the ladybugs can't reproduce as fast as the bugs can, and it's impossible to treat a whole forest with any of the known techiniques for killing the bug. ''We're hoping to at least buy time,'' said Dave Struble,
entomologist for the state of Maine. ''We know it's coming, but we're doing everything we can to stave it off.''
This story ran on page A1 of the Boston Globe on 3/19/2002.