Garlic mustard has become an invasive species in temperate forests across the United States, choking out native plants on forest floors and threatening ecosystem diversity. University of Illinois ecologist Adam Davis has created a computer model that in combination with quarantined research tests he believes will be able to predict the perfect predator -- a pest that can be introduced into a forested area that will help reduce the garlic mustard population.

鈥淭he traditional method has been to release multiple agents into the environment and overwhelm the pest,鈥� said Davis. 鈥淏ut with multiple introductions comes an increased likelihood that one of the agents will become invasive as well. So, what we鈥檙e trying to do is to figure out which one is the most likely to actually have an impact on garlic mustard (Alliaria petiolata) and release as few agents as possible.鈥�

Davis is a member of the research team for USDA鈥檚 Invasive Weed Management Unit at the U of I. He has been working since 2004 on a project along with scientists at Michigan State, Cornell, and the Commonwealth Institute of Biological Control in Switzerland.

Davis鈥� computer model creates a simulation of the population dynamics of the pest species 鈥� its seed and growth cycle, stressors, etc. 鈥淭hen you can introduce the biological control agents that you鈥檙e considering one at a time and see whether you can get away with just introducing one agent,鈥� said Davis. 鈥淚 collected data from field experiments and entered them into the computer model which then projects forward which agent will be most effective.鈥�

The computer simulation was used to select a tiny weevil, about the size of an 鈥渙鈥� in 12 point type. 鈥淭here are actually several weevils that feed on garlic mustard back home in Europe, where it comes from,鈥� said Davis. 鈥淭his particular weevil that we鈥檙e looking at (Ceutorhynchus scrobicollis) feeds on the plant at several stages in its life cycle so it鈥檚 a much more effective agent than some of the other ones.鈥�

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

鈥淎 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鈥檛 want to happen. But, this particular weevil has passed that test for a wide variety of plants.鈥�

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鈥檛 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. 鈥淭he idea is that you reunite plants with a natural enemy from back home 鈥� which in garlic mustard鈥檚 case is Europe. In Switzerland garlic mustard and the weevil coexist and neither one is invasive.鈥�

Garlic mustard was brought to the United States from Europe innocently in the 1870鈥檚 as a culinary herb but its natural enemy didn鈥檛 accompany it.

Davis says there are some concerns among ecologists about biological control because of the risk of negative effects on nontarget species. 鈥淏ut when you have invasive plant species covering very large acreages it鈥檚 almost impossible to manage by hand,鈥� he said. 鈥淭here are herbicides that will work on garlic mustard, but it infests millions of acres of forest and there鈥檚 no way you can get out there and spray all of that. And because garlic mustard has a really long-lived seed bank, in order to eradicate it, you鈥檇 have to hit it for about eight to ten years in a row.鈥�

Pending approval from APHIS-PPQ (the species evaluation and quarantine arm of the USDA) the weevil is scheduled to be released into an infested forest later this year. Davis said that the beauty of using simulation models to guide biocontrol is that this approach can be adapted for use in controlling other invasive species, not just garlic mustard.

Source: University of Illinois at Urbana-Champaign