Biocontrol insect spurs growth of invasive weed

September 4th, 2008 - 2:13 pm ICT by IANS  

Washington, Sep 4 (IANS) Biocontrol agents, usually insects, can unwittingly produce the opposite of intended results in weed control. For instance, spotted knapweed, a flowering plant native first discovered in US in 1800s, is now widespread in north-western America.

It has become a serious problem across Washington, Idaho, Wyoming and Montana and in Canada across Alberta and British Columbia.

As early as 1971, US scientists began releasing gallflies in an effort to reduce populations of the invasive weed. Adult flies lay their eggs in the weed’s flowers, and after the larvae hatch they induce the plant to grow tissue around the insect, encasing it and isolating it from the rest of the plant.

“The larvae then overwinter in the seed heads for about nine months. When the plant devotes all that extra energy to producing these galls, it has less energy to produce seeds,” said Dean Pearson, co-author of the study. He is an ecologist with the USDA Forest Service’s Rocky Mountain Research station.

These results were reported in the current issue of Ecological Applications.

Scientists and managers expected that this seed deficiency would lead to limited knapweed population growth. However, in an unanticipated outcome, deer mice, whose diet usually consists of native seeds and insects, have also begun to prey on the introduced gallflies.

Pearson said that an average mouse can eat 1,200 larvae in one night. “A super mouse could go through a whole lot more than that,” he added.

At the study site, spotted knapweed makes up more than half of the plant ground cover. The abundance of knapweed leads to lots of gallfly larvae, which serve as a food subsidy for the mice.

Pearson and co-author Ragan Callaway of University of Montana found that this extra nourishment bolstered mouse population size, increasing the numbers of hungry mice feeding on their original source of food: the seeds of native plants. Hence, fewer native plants survive past the seed stage.

He pointed out that even if the fly decimates 80 percent of the knapweed population, the 20 percent of seeds that are left to germinate are often enough to outcompete native plants.

The authors made out the case that although biocontrol agents are carefully selected for specificity to their host plants, these restrictions do not prevent them from drastically altering the community food web, which can have far-reaching repercussions.

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