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The unmitigated gall fly

Deer mice win in effort to biocontrol knapweed

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Three decades after foreign insects were introduced in North America to prey on exotic invasive plant species like knapweed, biocontrol has become an important and established tool in mankind’s never-ending war on weeds.

Now, one researcher may have discovered an unintended and troubling consequence of the release of one of these foreign bugs.

Dean Pearson is a wildlife biologist and community ecologist with the research branch of the U.S. Forest Service on the UM campus. Pearson has conducted research into gall flies, a European insect that is a natural predator of Western Montana’s number one enemy.

Knapweed found its way to North America via immigrants and travelers from the Old World in the early 20th century, and has established itself here so tenaciously that ridding it from one’s property can be a lifelong project. Since the early 1970s, researchers have imported, bred and released no fewer than 13 insect species—gall flies among them—to go after knapweed. Gall flies were released in the Missoula and Bitterroot valleys, and are now widely established in the knapweed-infested West.

Gall flies prey on knapweed by eating the plant’s seeds. Pearson and his colleagues have also shown that eggs laid by adults become larvae able to overwinter in the plant’s seed heads from September to June. Native predators like squirrels, chipmunks, pine siskins and chickadees have discovered this winter feast at a time when food supplies are naturally scarce, but the native species most successful at making nutritional use of the bounty is the deer mouse.

On Mount Sentinel, where some of his research has taken place, Pearson found that gall fly larvae make up 85 percent of the deer mouse’s winter diet. During the rest of the year, except for summer, larvae comprise fully half of the rodent’s diet. Or, as Pearson puts it, deer mice “eat gall flies like popcorn,” consuming as many as 1,200 larvae per mouse each night. “That’s just an average,” he says. “That’s not super-mouse.”

Deer mice breed in summer, hit their peak numbers in fall and typically begin to die off in winter due to harsh conditions and a decline in food sources. But Pearson has found that in his study areas, deer mouse populations have increased by two-and-a-half to three times. “All the evidence is saying the deer mouse population is increasing because of this [gall fly larvae food source].”

It’s a substantial increase, and the most important consequence may lie in a corresponding increase in the number of mice carrying hanta virus. Deer mice are the reservoir and the primary transmission vector of the deadly respiratory disease, which was first identified in the United States in 1993. Deer mice transmit hanta virus to humans via spores inhaled from mouse feces. The mortality rate in humans is about 38 percent in the less than 300 cases so far documented in this country.

In a study he finished last summer, Pearson drew blood from mice captured in the knapweed-infested study area and tested it for hanta virus antibodies. He found that the number of mice testing positive for the disease was proportional to the increase in the mouse population—two-and-a-half to three times higher than normal.

Pearson cautions that his conclusions are hypothetical at this point, and have not been peer-reviewed. It will be written up and submitted for publication this summer. The correlation between gall flies and the increase in the number of deer mice found to have hanta virus antibodies, for instance, is not demonstrably causal. “Could other things be affecting this relationship?” he asks. More studies are needed and are in the planning stage, says Pearson.

Jim Story, a research professor at the Western Montana Agricultural Research Station in Corvallis, has bred and released gall flies—and other imported insects—for many years. He’s aware of Pearson’s research, but says some disturbances resulting from the importation of foreign species are inevitable. “Those kinds of things should be expected. Nature will evolve to bring things into balance. You’d like to think there wouldn’t be any unintended consequences. If indeed [hanta virus] comes out, then obviously that would be unfortunate and not to our liking.”

But it’s a big step from gall flies to hanta virus, Story says, and then there’s also much that remains unknown about a disease identified only a little more than a decade ago.

The main concern about importing foreign bugs to prey on foreign weeds is that they might prey on desirable plants as well. Insects are rigorously tested on a wide array of plant species before they ever leave their home turf, says Story.

No one can predict—or test for—negative consequences like that which Pearson may have found, and so Pearson urges caution.

“We need to be careful with biocontrol and be more ecological about it without using the shotgun approach. We just do what we know, and we don’t know very much. I am actually an advocate of biocontrol. I’m just trying to point out that we need to be smarter.”

The science of biocontrol is still in its infancy, and though there have been success stories, they’re mostly the result of plain old good luck. In California in the early 1900s, for instance, St. John’s wort was as big an ecological problem as knapweed is today in Western Montana. In 1945 three predatory insects were introduced, and within five or six years St. John’s wort had declined to 1 percent of its peak population. In Australia, prickly pear has been largely defeated by biocontrol methods.

“But we don’t understand what makes a success a success,” says Pearson. “That’s the key. It’s powerful. It can do powerful good or powerful bad. The invasive problem is a huge problem, and we haven’t wielded the tool with good effect. So far, biocontrol has not fixed the knapweed problem.”

Regardless, the gall fly is out of the bag now. It’s a permanent and widely distributed member of the Western Montana environment, and Story has moved on to other bugs.

Pearson will present his findings to his colleagues in Denver later this month.

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