Wasps versus Borers

A square-headed wasp and its emerald ash borer prey.

The adult emerald ash borer (EAB) is a beautiful, metallic-green beetle, too pretty, it would seem, to be so deadly. And yet, in its larval form, the EAB has killed millions of ash trees since its inadvertent introduction to Michigan in 2002. This notorious insect, in the family of wood-boring beetles called the Buprestidae, is now spreading east from Pennsylvania, and south from Ontario, and has been identified in the town of Carignan, 30 miles east of Montreal. In June of 2009, EAB was discovered in New York for the first time, in the town of Randolph, in Cattaraugus County.

Residents of New York and New England are bracing for what appears to be the inevitable advance of this alien intruder. The challenge is finding these tiny insects (1/2 inch long and 1/8 inch wide) before they become established in a new area. To aid in their detection efforts, scientists have enlisted help from an unlikely source: a small native wasp.

The square-headed wasp (Cerceris fumipennis), a solitary wasp that hunts buprestids, is found throughout the continental U.S., ranging as far north as Maine and southern Ontario. The female wasp bears three distinctive, creamy- white marks on her face and a single, cream-colored band on her abdomen. She prefers to construct her underground nest on a site with hard-packed, sandy soil that is near a wooded area. An abandoned country road or unpaved parking area often provides good habitat – as does the ground around third base in many baseball diamonds.

Philip Careless and his associates from the Insect Systematics Lab at the University of Guelph in Ontario, Canada, have conscripted this wasp to help with EAB biosurveillance efforts. He and his associates have been carefully excavating around square-headed wasp nests, then transporting the nest and resident female wasp to suspected ash borer sites to determine if any of the wasp’s prey are in the vicinity. Once released, the wasp will selectively search out buprestid beetles and begin bringing them back to the nest. If there are no EAB beetles found in the wasp’s first 50 prey items, scientists believe there are probably none in the area.

This past summer, the state of Maine began a pilot program that used local populations of square-headed wasps for biosurveillance. According to forest entomologist Colleen Teerling, 37 colonies were identified throughout the state (square-headed wasps, while solitary, often nest en masse; thus, one site may hold from 3 to 500 nests). Fourteen of these colonies were observed by volunteers, including amateur entomological societies, girl scout troops, and members of town government. Thankfully, no EABs were found. Entomologists in New York, New Hampshire, and Vermont are also employing the wasp to look for colonies.

Since square-headed wasps are already present in much of the Northeast, it’s possible for amateur entomologists to find local nest sites and do their own EAB monitoring. The best time to watch square-headed wasps in action is between 9:30 a.m. and 6 p.m. on a warm, sunny day. To find a potential nest site, look for rejected buprestid carcasses and the presence of other scavengers, like tiger beetles, in the area. A small sand cone with a pencil-wide opening at the apex characteristically marks the nest.

If you happen to observe an incoming Cerceris wasp bearing her “jewel beetle” prey, you may also see her perform evasive maneuvers to avoid being ambushed. An opportunistic “satellite” or Miltogrammine fly (family Sarcophagidae) sometimes orbits the nest, trying to lay its own egg –
or live-born larva – on the paralyzed beetle while the prey-ladened wasp is coming in for a landing. If the wasp is forced to drop her quarry, she prefers to search for a new beetle instead of reclaiming the dropped one – possibly to avoid using parasitized prey.

Although square-headed wasps and other cleptoparasites help reduce the EAB population, their predation is insignificant compared to the number of beetle larvae that are currently damaging all species of North American ash trees. The wasps at this point are useful only in identifying new infestations.

To date, EAB has not been detected in Vermont, New Hampshire, or Maine. For now, entomologists, foresters, and landowners are watching and waiting. While September is the last month warm enough for active monitoring, it’s never too early to plan next summer’s activities. This tiny, square-headed wasp could be an important tool in the search.

For more information on EAB biosurveillance, go to www.cerceris.info


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