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Gall Flies on Knapweed
Feed Deer Mice

Seeley Swan Pathfinder
May 6, 1999
Game Range column

by Mike Thompson,
Wildlife Biologist
Fish, Wildlife & Parks


When Dean Pearson sat down to describe the behavior of deer mice, he had no way of knowing he was speaking with someone who can tell a mouse story or two.

When I used to work in the woods with my dad, we were happy to have whatever company presented itself. There was the chipmunk that would come into the warehouse for a handout at lunch time. There was the ruffed grouse that used to follow another guy wherever he went. Then came the deer mouse.

The deer mouse was particularly enamored with our trash can. He rustled around inside it while we ate our lunch, and came and went as he pleased as long as the can was at least half full of papers and food scraps. After about a month of lunches, someone finally emptied the trash can.

Well, our deer mouse learned the hard way that he was no Michael Jordan in the category of vertical leap. We got a big chuckle out of his rather noisy predicament while we ate our lunch. Then Pa told me to stick an axe handle in the can for our little pal.

I don't think I believed the mouse could make use of a nearly vertical axe handle, and if he could, I just assumed he would scamper up it like a squirrel. But, almost before the handle touched the bottom of the can, Mr. Mouse was on his way up it at a pace that would have brought him up my arm if I hadn't jumped back, noticeably enough to attract Pa's joyful cackle and commentary. But, to this day, I think the greatest measure of my surprise came with the sight of that mouse shinnying up the axe-handle like a kid up a flagpole.

Boys who play with mice sometimes grow up to lead productive lives, and some like Dean Pearson become productive in advanced studies of mice. Dean is a wildlife biologist with the Northern Rockies Research Station in Missoula. He came by last Friday to discuss his upcoming research that he has agreed to conduct on Calf Creek Wildlife Management Area, near Hamilton. While you may not be terribly interested in the goings on at Calf Creek, I'm quite sure you will be fascinated by Dean's recent and developing line of research.

You see, Dean is interested in knowing why deer mice shinny up knapweed stalks.

If you haven't noticed many deer mice swinging from knapweed stems, it may be because you've been looking in the right place at the wrong time. Deer mice are active only at night. So, the next time a warden stops to chat while you're spotlighting, tell him you're surveying for knapweed by counting the eyes of deer mice. (Remember to divide by two.)

Why do deer mice climb? According to Dean's research on Mount Sentinel, in Missoula, they're picking apart knapweed seed heads to eat the gall fly larvae. You can tell whether your knapweed patch has gall fly larvae and savvy deer mice by looking closely at the ground. If the answer is yes, you'll find small piles of dissected seed head parts.

The gall fly is a nonnative insect that was collected from Eurasia and purposefully introduced all across western Montana as a biological control agent for spotted knapweed. Although gall flies have had little apparent impact by themselves on knapweed populations, they have established and spread over the past 20-30 years so they now occupy virtually every location where knapweed occurs. Gall fly larvae live within knapweed seed heads and suppress seed production, but not enough to prevent knapweed plants from replacing themselves and spreading.

Deer mice adapted to gall fly larvae about as fast as humans adapted to computers. Only two or three decades ago, no deer mouse had ever seen a gall fly. Today, gall fly larvae are staples in the diets of many deer mice.

According to Dean, the mice have become very efficient in their searches for gall fly larvae. Most of the time they only attack seed heads with larvae inside. And, deer mice have voracious appetites for gall fly larvae. Under field conditions, Dean has documented consumption rates of 250 larvae per mouse per day. Under controlled conditions in a laboratory, champions have belted down over 1,000 in a day.

What effect do deer mice have on gall fly populations? Well, they certainly haven't helped. And, what about the effects of deer mouse predation on knapweed seed production? Aside from consuming the most abundant biological control agent on knapweed, the destruction of knapweed seed heads by deer mice has little impact on seed production because the seeds have already been released from the seed head by the time the mice begin their assault. Peak consumption rates occur in March.

One wonders if this evolving ecological relationship between knapweed, gall flies and deer mice is a sign that knapweed is finally being "naturalized" into the western world. But, upon closer inspection, it seems more likely that it is evidence for the serious consequences facing our native plant and animal systems with the continued spread of knapweed monocultures. Left out of the knapweed loop are voles, which are mouse sized creatures that depend upon native grasses and forbs. Voles are more useful prey for a wider variety of predatory birds and mammals than deer mice because they are active in daylight as well as at night, and they are dumb. They tend to "freeze" when a predator swoops down, while a deer mouse is much more elusive. As species such as voles become displaced from knapweed dominated communities, an entire chain of diverse predators and prey is lost as well.

Dean is interested in the way herbicide treatments affect these ecological relationships, and so are we. I'm sorry we couldn't bring his work to the Blackfoot-Clearwater Game Range, but, hey, you folks in Seeley Lake need to learn how to share!

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