A Solution for Garlic Mustard?

Univoltine Root Mining Weevil (Ceutorhynchus scrobicollis)
Credit: Hariet Hinz & Ester Gerber, CABI Biosciences, Bugwood.org

Garlic mustard (Alliaria petiolata) is an invasive plant that is widespread in forests of the eastern United States. When it gains a foothold, it rapidly spreads to cover much of the forest floor and crowds out native plants and the animals that depend on them. It is especially common in disturbed areas or forests with a high deer population. Because the plant reproduces rapidly and its infestations are so widespread, effective controls are difficult to apply.

The tiny insect pictured above, the univoltine root mining weevil (Ceutorhynchus scrobicollis), may provide a solution. The USDA's Invasive Weed Management Unit has been investigating biological controls that could be applied to reverse the plant's spread. The team considered several parasites that attack garlic mustard in various stages of development. The trick was to find a parasite that could keep garlic mustard in check without becoming invasive itself.

The computer simulation was used to select a tiny weevil, about the size of an "o" in 12 point type. "There are actually several weevils that feed on garlic mustard back home in Europe, where it comes from," said Davis. "This particular weevil that we're looking at (Ceutorhynchus scrobicollis) feeds on the plant at several stages in its life cycle so it's a much more effective agent than some of the other ones."

What happens if the control agent also becomes an invasive species?

"A stringent battery of tests is performed on each biocontrol agent in quarantine before it is ever released. For example, garlic mustard is in the same family (Brassicaceae) as cabbage, so one test might be to only feed the weevil cabbage and see if it survives on it or can reproduce on it. If it does, then the possibility exists that it could move from the garlic mustard and threaten cabbage plants, which we don't want to happen. But, this particular weevil has passed that test for a wide variety of plants."

Davis said that there are different strategies for biological control. One strategy is inundative in which the control agent eats its way through the garlic mustard and then dies out itself because there isn't anything left to support it. The other strategy is to introduce a natural enemy that will just bring the population down to a lower level and the plants and pests just continue to coexist. "The idea is that you reunite plants with a natural enemy from back home -- which in garlic mustard's case is Europe. In Switzerland garlic mustard and the weevil coexist and neither one is invasive."

If this control method is approved by the USDA, the univoltine root mining weevils could be introduced into American forests later this year. They will have a lot of eating to do.