It sounds unlikely. But scientist Dean Pearson - a wildlife biologist who works in community ecology at the U.S. Forest Service's Rocky Mountain Research Station in Missoula - is exploring the complex relationship among spotted knapweed, gall flies, deer mice and people, and finding implications for human exposure to hantavirus.
It's too early to say there is a direct relationship between the abundance of knapweed and the occurrence of hantavirus pulmonary syndrome in people, Pearson said in an interview. But research is pointing in that direction.
One thing we know: "Knapweed, through the gall fly, is elevating deer mouse populations," Pearson said. "You don't want to have deer mice around your house. What we don't know is if that's increasing hantavirus."
Montana has seen 21 cases of hantavirus pulmonary syndrome since state officials began keeping records in 1993, said Brant Goode, infectious disease intervention specialist at the Missoula City-County Health Department. In May, the state had four cases, and two of those sick people died. All four cases were east of the Continental Divide.
Often, but not always, victims of HPS have had contact with rodents or have mouse infestations in their homes, Goode said. Deer mice are known carriers of hantavirus. Goode would rather be conservative than sorry when it comes to infectious disease. He convinced Pearson to talk about his research before it is fully developed because of the public's high interest in HPS prevention this spring.
"More mice increases your risk," Goode said. "If you can reduce the amount of knapweed around your home, you can reduce the deer mouse population."
Here's how it works, Pearson has found: Spotted knapweed is a plant species native to Europe and Russia that has invaded the American West at the expense of native plants and animals. When a plant comes from elsewhere, it leaves behind its natural enemies.
In an effort to control knapweed, plant scientists went back to knapweed's home ground and looked for its predators. They found the gall fly, brought it back and introduced it as an ally in the war against the weeds.
In Montana, insect control of knapweed is one of the oldest biocontrol programs. Since 1972, 13 species of exotic insects have been introduced to prey on knapweed.
Gall flies lay eggs in the immature seed heads of knapweed, Pearson said. When the larvae hatch, the plant forms woody galls around the larvae to isolate them and minimize the damage to the plant.
"It happens at the time that the knapweed would be putting its energy into producing seed heads," Pearson said. "So you produce a metabolic energy sink, and seed production is reduced."
The gall fly has been good at its job and is now dispersed across most of western North America, he said.
However, knapweed plants live six to eight years and produce hundreds of thousands of seeds. So even if the gall fly reduced the production by
80 percent, it couldn't eliminate knapweed. That's why we need other insects, too.
"Knapweed is obviously still growing and expanding and invading," Pearson said.
The arrival of the gall fly has been serendipitous for deer mice, who are adaptable opportunists. They climb the knapweed, harvest the seed heads, crack them open and eat the larvae.
In Pearson's study areas on Mount Sentinel, a single deer mouse can eat 600 larvae a night in winter. In the lab, each can harvest 800 seed heads and eat 1,200 larvae in a night.
"These guys are just raging," Pearson said.
Now, gall fly larvae have become up to 85 percent of deer mice's diet in winter, he said, and a mainstay most of the year.
The normal population declines of deer mice in winter appear to be reduced by this available food. Deer mice are two to three times more abundant in winter in knapweed-invaded areas, Pearson said.
Right now is the time of year that gall flies are pupated and hatching. Soon, deer mice will move on to other foods for the summer, mostly native plants and seeds. Deer mice are breeding and moving around, Pearson said, and if a barn or house or outbuilding has
readily available food, they will move there.
"Now, if you think about a knapweed monoculture that's next to your barn..." Pearson said.
Most cases of hantavirus illness occur in spring because the virus is at its highest saturation in the deer mouse population then, Pearson said.
The mice transmit it to each other as adults, probably through fighting and biting, not mother to infant. In spring, before breeding, most deer mice are adults who could be infected. As the summer goes on, the percentage of infection is diluted by the birth of young mice who are not carrying the virus.
This is also the time when people begin hiking and cleaning out their cabins and outbuildings.
The relationships among deer mice and the plants and animals in their environment are complex, Pearson said, and much of what he's working on is still speculative. Pearson and University of Montana plant ecology professor Ray Callaway will publish a paper about one aspect of the research later this summer in the journal Trends in Ecology and Evolution.
The research is funded by the National Science Foundation and has also been funded by the U.S. Forest Service and the University of Montana.
Reporter Ginny Merriam can be reached at 523-5251 or at gmerriam@missoulian.com
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