Biocontrol Insect Exacerbates Invasive Weed -- Introduced flies create complex interactions that increase impact of invasive plant

Wow -

This is quite the mind-twister in terms of how humans are just one small cog in the wheel...

Humans introduce and spread damaging invasive knapweeds through contaminated agricultural seed supplies as well as through all the soil disturbances we create (via overgrazing, off road vehicles, etc.). Then we introduce an insect to control them - a natural predator of the weed from Europe. But unexpectedly, the 'biocontrol' agent insect ends up becoming a food source for native rodents, resulting in an explosion of their numbers. Of course, the main food source of these rodents is NATIVE plant seeds. The result is lower numbers of native plants, which creates more space for more knapweed to invade... Remember, this is a weed that causes ranchers tens if not hundreds of millions of dollars per year - not only in control costs, but also in lost forage, reduced livestock carrying capacity, and thus reduced land/real estate values.

Again and again, our unsustainable behavior ends up coming back to bite us in the ass. This is why the best solution to environmental problems like weed invasions is to prevent the human impacts that cause them from happening in the first place. The details of the study are below...

- Jon Gelbard

For immediate release: Wednesday, Sept. 3, 2008

Contact: Christine Buckley (202) 833-8773 x 211; christine@esa.org

or Nadine Lymn (202) 833-8773 x 205; nadine@esa.org

Listen to the ESA podcast with Dean Pearson at

http://www.esa.org/podcast/

Biocontrol agents, such as insects, are often released outside of their

native ranges to control invasive plants. But scientists in Montana have

found that through complex community interactions among deer mice,

native plants and seeds, the presence of an introduced fly may

exacerbate the effects of the invasive plant it was meant to control.

The authors report their results in the September issue of the journal

Ecological Applications.

Spotted knapweed, a flowering plant native to Eurasia, was first

discovered in the United States in the late 1800s. This broad-leaved

plant has an advantage over native plants because its natural enemies,

including insects such as European gallflies, do not naturally exist in

North America. Thought to have hitched a ride with hauls of alfalfa,

knapweed is now widespread in western North America and has become a

serious problem in the U.S. across Washington, Idaho, Wyoming and

Montana and in Canada across Alberta and British Columbia.

As early as 1971, U.S. scientists began releasing gallflies in an effort

to reduce populations of the invasive weed. Like all biocontrol agents,

the gallflies were selected because of their specificity to their host

plant, leaving little risk of direct harm to other plants.

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 woody galls wall off the fly larvae from within flower head," says

Dean Pearson, lead author on the study and a research ecologist with the

U.S.D.A. Forest Service's Rocky Mountain Research station. "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."

Scientists and managers expected that this seed deficiency would lead to

limited knapweed population growth. An unanticipated side effect,

however, involves the flies' furry neighbors. At the foot of the

Sapphire Mountains in western Montana, omnivorous deer mice, whose diet

usually consists of native seeds and insects, have also begun to prey on

the introduced gallflies.

"These mice are generalists and very effective at exploiting a new

resource," says Pearson. "They can tell which seed heads have the most

larvae inside them, and that makes them very efficient." Pearson says

that an average mouse can process 1200 larvae in one night. "A super

mouse could go through a whole lot more than that," he adds.

At Pearson's grassland 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 his coauthor, Ragan Callaway of the University of Montana, found

that this extra nourishment bolsters mouse population size, increasing

the numbers of hungry mice feeding on their original source of food: the

seeds of native plants. As mouse consumption of native plant seeds

increases, fewer native plants survive past the seed stage.

Pearson says that this exacerbation of the invasive species' impact has

a lot to do with the effectiveness of the fly at controlling the

knapweed.

"If the biocontrol agent is really effective, then it will eventually

eat itself out of house and home, and the community interactions become

less of an issue," Pearson says. He points 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 make 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. Pearson hopes that a better

understanding of food web ecology will lead to more effective tools for

invasion control.

"Everything's interconnected," says Pearson. "We need to understand the

ecology. If we can understand these complexities, we can attempt to

minimize the side effects and maximize the effectiveness of our tools."

To listen to a podcast of Pearson speaking about this paper in ESA's

Field Talk podcast series, please visit www.esa.org/podcast. Pearson's

podcast is titled "Biocontrol Insects and the Mammals Who Love Them."

The Ecological Society of America is the world's largest professional

organization of ecologists, representing 10,000 scientists in the United

States and around the globe. Since its founding in 1915, ESA has

promoted the responsible application of ecological principles to the

solution of environmental problems through ESA reports, journals,

research, and expert testimony to Congress. ESA publishes four journals

and convenes an annual scientific conference. Visit the ESA website at

http://www.esa.org.