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Basic Instinct: Plant Chemicals Found that Attract Insect

Eurasian Watermilfoil.

Eurasian Watermilfoil.

Like bees to nectar, a native species of weevil finds a non-native and invasive water plant irresistible. Why? Scientists at the University of Minnesota have identified glycerol, uracil, and at least one other mystery ingredient produced by Eurasian watermilfoil (Myriophyllum spicatum) as attractive to hungry Euhrychiopsis lecontei weevils.

These chemicals are also produced by other plants, but are released into the water at higher concentrations by Eurasian watermilfoil, luring the weevils to eat, mate, and lay their eggs on the plants. The weevils are used as a form of natural biocontrol for the milfoil, an 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.

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.

Former Sea Grant-funded graduate student Michelle Marko and her faculty advisors, professors Ray Newman and Florence Gleason pinpointed the attractants as glycerol and uracil. They used techniques as complicated as mass spectrometry and nuclear magnetic resonance spectroscopy, and as simple as a salad spinner.

Plants sometimes inadvertently release chemicals that plant-eaters can use to zero in on them. Distribution of the chemicals can happen through the air or water.

“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.”

Before Eurasian watermilfoil invaded area lakes, the weevils ate northern watermilfoil, a benign native species, Newman explained. “There’s something about Eurasian watermilfoil that makes it more desirable to the weevil than northern watermilfoil.”

Although the way insects interpret the world through chemicals is loosely similar to tastes or smells perceived by humans, insects rely on a higher degree of sensitivity and specificity. Their receptor systems screen out irrelevant chemical messages and sense attractive compounds at extremely low levels.

The weevil’s attraction to glycerol and uracil 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.

What are Glycerol and Uracil?

Glycerol is a sweet-tasting thick liquid used in many products that humans eat, such as:

  • flavorings like vanilla
  • food coloring
  • candy, cakes, and casings for meats and cheeses
  • shortenings and margarine
  • low-fat food products like cookies

Uracil is a more complicated chemical with derivatives that are important to cell metabolism, particularly carbohydrate metabolism. It is also used to transfer chemical energy and translate genetic information between cells.

Both glycerol and uracil are examples of kairomones: chemicals produced by one species and used to the advantage of another. (For example, carbon dioxide given off by humans and other animals is used as a kairomone by female mosquitoes seeking a blood meal).

“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.”

However, they will have to find another graduate student who is up to the required detective work. Michele Marko moved on to conduct postdoctoral work with the Connecticut Agricultural Experiment Station in New Haven, investigating ways to control nuisance aquatic plants.

For more information, read JR 511 offered on the MN Sea Grant.

By Marie Zhuikov
June 2006

Return to June 2006 Seiche

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