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Undaunted. The spotted knapweed pumps out so many seeds that a biocontrol insect can't thwart it.

Insects No Match for Invasive Weeds

MADISON, WISCONSIN--When plants such as kudzu creep across the landscape, resource managers often release exotic insects to stem the invasion. But despite consuming the plants, most biocontrol insects fail to stop their spread. Now field studies suggest that the insects are simply overwhelmed. The results, presented here on 8 August at the annual meeting of the Ecological Society of America, could help researchers come up with better tests for picking biocontrol agents.

Before release, biocontrol insects are usually tested to make sure they eat the invader and are likely to establish themselves in the invaded habitat. But few are tested in the field to see if they'll beat back plant populations, says ecologist Amanda Stanley of the University of Washington, Seattle. For instance, two species of seedhead gallflies were imported about 30 years ago from Russia to control spotted knapweed, another Russian import that is taking over grasslands and savannas across the Western United States and Canada. Although the gallflies breed easily in the region and eat the seeds of spotted knapweed, the plant continues to spread unchecked.

To find out why, Stanley looked at how gallflies and spotted knapweed interact in the field. They caged off some plants to keep gallflies out and compared their fates to those in plots where gallflies had free access. Gallflies ate most of the spotted knapweed seed, cutting seedling density by 50% in areas exposed to gallflies. But the same number of seedlings survive to adulthood with or without the gallflies. Apparently, gallflies can't keep spotted knapweed populations down because the plants produce so many seeds that plenty escape the ravenous insects.

"I thought this was a fascinating study," says ecologist Tim Craig of the University of Minnesota, Duluth. The work also shows that more studies should be conducted prior to release that address whether biocontrol agents actually cut plant populations, he says.

Related sites

Images of the spotted knapweed life cycle
Background on the use of biocontrol insects to fight weeds