Biocontrol advocates claim that releasing nonnative insects to control nonnative plants is safe for native species — but the number of “exceptions” keeps growing. The latest is a weevil intended to control a nonnative thistle. New research shows that the weevil prefers a native thistle and can reduce its seed set by 98 percent.
“Ecological risk was severely underestimated,” say Svata Louda of the University of Nebraska in Lincoln and Charles O’Brien of Florida A & M University in Tallahassee in the June issue of Conservation Biology.
The weevil (Larinus planus) is from Eurasia and is being released in the western U.S. to control Canada thistle, which despite its name is also from Europe. Canada thistle is an aggressive weed and may threaten large areas of range and crop land. The weevil damages thistles in two ways: the adults eat the leaves and the larvae eat — and so destroy — developing flowers and seeds. Since a 1990 study suggested that the weevil preferred Canada thistle to native ones, the weevil has been widely released in the U.S., notably in western national parks, forests, and monuments. However, Louda and O’Brien reanalyzed the 1990 results and found that the weevil fed equally on Canada and native thistles in laboratory tests.
While doing another study, Louda and O’Brien unexpectedly found that the weevil also feeds on a native thistle in the wild: Tracy’s thistle, a relatively uncommon species found only in western Colorado and eastern Utah. In 1992 and 1993, the U.S. Forest Service released the weevil on the edge of Gunnison National Forest, which is near Almont, Colorado. In 1999, the researchers collected 30 Tracy’s thistle flower heads from a roadside stand near the weevil release site. In 2000, the researchers double-checked their surprising find by collecting 185 Tracy’s thistle flower heads from the same stand and 166 from another stand that was further away, as well as 375 Canada thistle flower heads from three nearby stands.
Louda and O’Brien found that the weevil fed extensively on Tracy’s thistle. More than 75 percent of the flower heads either contained weevil larvae or had signs of larval damage. Worse, the weevil reduced seed set in infested flower heads by 98 percent: infested flower heads produced about one viable seed each, whereas undamaged flower heads produced about 45 seeds each. Overall, the weevil reduced the seed production of the Tracy’s thistle stands by two-thirds.
Moreover, the researchers found that the weevil had little effect on the nonnative thistle it was supposed to control. In fact, there was no evidence of weevils feeding in any of the three stands of Canada thistle studied. This is striking because Tracy’s thistle is sparse, and Canada thistle is relatively common in the study area.
Louda and O’Brien call for reevaluating the release of nonnative insects to control nonnative weeds in natural areas. “Current practices involving such introductions of exotic insects into nature reserves and national parks rely on incomplete assessments of ecological risk,” they say.
Louda, S.M. and C.W. O’Brien. 2002. Unexpected ecological effects of distributing the exotic weevil Larinus planus (F.) for the biological control of Canada thistle. Conservation Biology 16:717-727.