(CAROL STOCKER will be line live this Friday from 1-2 p.m. to answer you gardening and landscaping questions.)
WHITE ASH seems nearly invincible. It rises in our forests straight and sturdy, with dense wood and a hearty symmetrical crown. Ash trees become tool handles, baseball bats and, back in the day, cross-country skis. An ash is formidable and delightful to see. Yet a tiny invasive insect is now killing ashes by the millions. And our best hope for slowing the carnage may be an amiable wasp called Cerceris fumipennis.
The villain in this woodland drama is a beetle, the emerald ash borer. Shaped like a bullet, and indeed glittering like an emerald, it probably arrived in the United States hitchhiking on wood packing material carried on cargo ships or airplanes originating from its home turf in Asia. By the time the beetle was discovered here, first in Michigan in 2002, it was already too late.
The emerald ash borer has killed tens of millions of ash trees in southeastern Michigan alone. Tens of millions more have been lost elsewhere across the Midwest, New York, Pennsylvania, and now the beetles have appeared just 30 miles north of Vermont in the Province of Quebec. The adult beetle nibbles on ash leaves. But in its immature stage, its larval stage, it feeds on the inner bark and outer sapwood of an ash, impairing the tree’s ability to transport water and nutrients to leaves and roots. It’s the most effective way to kill a tree short of chopping it down. And the beetle, half an inch of terror, is now closing in on Vermont and New Hampshire. All we can do is wait … and watch. If you find any beetles that you suspect, report them to your state department of natural resources.
Finding emerald ash borers is almost like searching for, well, little green bullets shot into the woods. It is an expedition of high expense and low returns. But Cerceris fumipennis, our wasp (it has no common name yet), is now being enlisted in the hunt for the beetles. It turns out that Cerceris fumipennis can catch emerald ash borers and haul them back to a waiting researcher.
Here’s how it works. Cerceris fumipennis is a ground-nesting wasp. It digs burrows for nests in clearings that have packed, sandy soils. A colony can range from just a few to several hundred of these nests. The opening to each looks as if someone jabbed a pencil into a small ant hill. Researchers and volunteers have been finding colonies at baseball diamonds, parking areas, sand pits and open trails near woodlands.
Each nest in a colony is tended by a female Cerceris wasp. But she doesn’t just crawl in there and lay eggs. First, she flies off into the woods to search for, catch and stun wood-boring beetles in the family Buprestidae. She will then haul the paralyzed beetle back into her burrow, stash it in one of its many underground chambers and then lay on it a single egg. The disabled beetle becomes food for the larval wasp, which emerges the following year as an adult.
Sometimes a Cerceris female will drop her catch near the nest where it can be picked up and identified. Other times we can snatch up the returning wasp in a net in order to determine which beetle species she’s carrying. It turns out that watching Cerceris haul in beetles is the most effective way to monitor for emerald ash borer. Even if the female Cerceris is reluctant to give up her catch, we can net the wasp and steal a look at her prey. It turns out this wasp species is fairly docile in a human hand.