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Chemical Attractants Draw Weevil to Invasive Aquatic Plant

April 18, 2006

Scientists at the University of Minnesota have discovered, for the first time, at least two chemical insect attractants released by an aquatic plant. Professors Ray Newman and Florence Gleason, and their graduate student, Michelle Marko, identified the chemicals as glycerol and uracil. These compounds are produced by Eurasian watermilfoil, an invasive plant, and lure a native species of weevil (Euhrychiopsis lecontei) to eat, mate, and lay their eggs on the plants.

Both glycerol and uracil are produced by other plants, but are released at higher concentrations by Eurasian watermilfoil. This is bad news for the milfoil because the weevils are a natural biocontrol measure for this exotic plant that has plagued North American waters since the late 1940s. Eurasian watermilfoil can form dense mats of vegetation and crowd out native aquatic plants, clog boat propellers, and make water recreation difficult. It has spread to 177 lakes, rivers and streams in Minnesota.

“If you look at terrestrial systems,” said Newman, a professor with the department of fisheries, wildlife, and conservation biology, “there are many insects that specialize by feeding on certain plants — cabbages, for instance, and the attractants are well-known. This is the first time a chemical attractant for an insect has been found for an aquatic plant.”

The researchers gathered milfoil from lakes across Minnesota and then grew the plants in lab tanks for several days before extracting chemicals the milfoil released into the water. They tested the extracts on the weevils to determine their preference.

To pinpoint the attractants, the researchers used techniques as complicated as mass spectrometry and nuclear magnetic resonance spectroscopy, and as simple as a salad spinner.

Glycerol is a sweet-tasting thick liquid used in many products humans eat, such as candy, cakes, food coloring, and flavorings like vanilla. Uracil is a more complicated chemical with derivatives that are important to cell metabolism, particularly carbohydrate metabolism.

The weevil’s attraction to these compounds increases as concentrations increase. “Although other aquatic plants also release glycerol and uracil, the higher concentrations released from Eurasian watermilfoil as it grows make it more obvious to the weevils,” said Gleason, a professor with the department of plant biology.

Weevils used in the experiment were collected from the same lakes as the milfoil used by the researchers. The tiny weevils were placed in a “Y-tube” that had attractant materials in one side of the top of the Y and control materials placed in the other side. Weevils usually made their preference for the attractant materials clear within five minutes. The research team’s results were published in the December 2005 issue of the Journal of Chemical Ecology.

“These findings would be useful for sampling or collecting adult weevils,” said Newman. “The attractants could be used to lure weevils living in a lake into a trap so that they could be released elsewhere or used for research purposes, or the attractants could be used to get an idea of how many weevils live in an area.”

What’s the other mystery attractant? “There’s a third compound we’re aware of, but can’t identify,” said Newman. “Results are clear that glycerol and uracil are attractive to the insects, but there seems to be at least one more.”

Reprints of the article, “Chemically Mediated Host-Plant Selection by the Milfoil Weevil: A Freshwater Insect-Plant Interaction,” are available free by contacting Minnesota Sea Grant at seagr@d.umn.edu or (218) 726-6191. Ask for JR 511.

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