URI researchers release new biological agent to fight invasive weed

University of Rhode Island entomologists reached a milestone in their efforts to control the invasive weed swallow-wort this month with the first release of a biological agent to fight the pest.

Last week, the URI scientists, led by Professor Richard Casagrande and Research Associate Lisa Tewksbury, sent 500 larvae of the moth Hypena opulenta to partners in Canada for release in patches of swallow-wort near Ottawa.

“Swallow-wort is an aggressive invasive perennial weed that forms dense patches in a wide variety of habitats and may have negative impacts on monarch butterfly populations,” said Casagrande. “But we believe that this moth has potential for keeping the weed in check.”

In 2006, URI doctoral student Aaron Weed discovered the moth larvae feeding on swallow-worts in southern Ukraine. He brought the larvae to partners at the Commonwealth Agricultural Bureau International (CABI) in Switzerland for rearing and initial testing. Research on the biology, impact, and host range of these insects was conducted at CABI and in the URI Insect Quarantine Laboratory over the next six years by Weed, now a postdoctoral researcher at Dartmouth College, and a second URI graduate student, Alex Hazlehurst.

After finding that the moth larvae will only attack and survive on swallow-worts, the URI scientists and colleagues in Canada and Switzerland petitioned the U.S. Department of Agriculture in 2012 to allow field release of this biological agent in North America. The review panel recommended it for USDA approval on September 4, 2013. The USDA has additional steps in its approval process before the agent can be released in the United States next spring, but the Canadian government granted permission for immediate release.

The URI team sent larvae to partners at Canada Agriculture late last week for the first release. According to Naomi Cappuccino of Carleton University, the release appeared successful and larvae were already pupating in preparation for the Canadian winter.

Pale and black swallow-wort were accidentally introduced into the United States from Europe over a century ago and have since spread throughout the Northeast and well into Canada and the Midwest. These toxic vining plants are major pasture pests and serious weeds in many agricultural, ornamental, and forest environments. In addition to their invasiveness, swallow-worts are closely related to milkweeds and threaten monarch butterfly populations. Monarchs readily lay eggs on swallow-worts, but all larvae that hatch on the plant perish.

“We believe that swallow-worts are particularly problematic in North America because they left all their natural enemies behind in Europe, and indeed, several European insects attack these plants in their native range,” said Casagrande. “In addition to the moth just released, a second moth with populations found from Finland to Ukraine is under evaluation in our quarantine laboratory.”

These agents will be compared with a related species under study by USDA scientists to select the next best species if there is need for further releases.

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