Mining for Moths


As we crested a windswept 11,500’ ridge in the southeast Absarokas, one of my hiking companions Don, exclaimed, “Guys, there’s a bear!”  Sure enough, a careful look with good binoculars revealed two adult grizzly bears on a steep talus slope about a half mile away.  We tried to slip back over the ridge, but in spite of the large distance, the bears spooked and headed for the timber below their slope. As we got out onto the east side of the peak we hoped to summit that morning, we realized there were three more bears working a talus slope at 12,000’.  Again, even though we spotted the bears at great distance, the sow and two yearling cubs sensed us and fled their feast.  As we climbed further up the peak, we traversed high steep talus slopes that were covered with “moth mines” – the 5’ across, 2’deep holes that were everywhere in the talus.  This was not terrain you would choose to travel across unless you were trying to scramble up the peak.  The diggings, as well as the abundant bear dung, were evidence that we had invaded a very heavily used late summer moth site for grizzly bears. 

Since those bear sightings, I was fortunate enough to hear Dr. David Mattson talk about bears and their food sources.  Bears have lost three of their four top food sources, with moths being the only one of those top foods left. Our wonderful bear sightings demonstrate the bear’s new dependence on cutworm moths for food, and how exposed to observation this behavior makes the bears.

Dr. Mattson’s research has clearly demonstrated the importance of late summer diet to the ability of breeding sow bears to be able to produce 2 or 3 cubs, which are the numbers required for the tenuous Yellowstone bear population to maintain present numbers.  Because of the disappearance of other desirable food sources, many bears in this population now are dependent on small moths that migrate from great distances each summer to the alpine zones of the Rocky Mountains. Further, the great bears must spend all day excavating these insects on totally exposed rock slopes.

I’ve also begun to understand that current grizzly bear population modeling is likely leading to inflated estimates. One big change in the way bears are counted is that the number of reconnaissance flights has increased substantially, up to three times more flights per year!  I’ve hunted quite a little, and have always noticed the amount of wildlife one sees is directly proportional to the time spent in the field.  I would think the same would apply to aerial surveys.  When you add the fact that bears are increasingly spending more and more time on totally exposed alpine slopes to the fact that the time spent looking has tripled, it seems obvious that bear counts will rise.  Just as we saw a lot of grizzly bears on our hike this summer, more frequent flights in late summer are going to see more bears because the bears have to be out on those open slopes to find the food that will get them ready for winter. 

The other lesson from our Absaroka hike this summer is that, in the future, I will be prepared to change trip routes and plans if I spot bears trying to eke a living out of the alpine zone.  We caused at least six bears to interrupt and flee their chosen food sources by hiking and climbing on the same mountain at the same time that they were using those sites.  When I saw the moth mining zone above 12,000’, I realized that grizzly bears are presently at the extreme fringes of available food sources.  I wish we had not disturbed their industrious food gathering on that mountain side. I have said many times in the past that the ideal bear sighting is at large distance above timber line.  After our experiences this summer, I realize those sightings are not necessarily ideal for the bears.

The importance of establishing every level of protection possible for lands like where we hiked last summer is paramount.  It is at the edges of the GYE where populations of threatened species like grizzly bears need security to expand their ranges.   I will alter my recreational goals in the future to be less intrusive in these special places.

Kim Wilbert – Sierra Club member, Riverton, WY



One thought on “Mining for Moths

  1. Thanks Kim, sounds to me like the availability of abundant moths is going to be key in the near term for griz’s ability to deal with the diminishing white bark pine and other sources. I am hopeful that successful control measures of the lake trout give chance for recovery of Yellowstone’s cutthroat and that the bears can take advantage of their resurgence on historic spawning beds. Something I hope the science community is evaluating?

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