Last year, I wrote a column for the Casper Citizen touting the annual migration of lowly miller moths (the army cutworm, Euxoa auxiliaris) through central Wyoming as something to be celebrated. I said it was a lot like other great migrations made over hundreds of miles by creatures such as African wildebeests or monarch butterflies.
Reaction from most readers ranged from surprise to annoyed acceptance to “what a crackpot idea.” But now the messy little flying nuisances are back, and I want to make the case for millers because it’s such a good case.
What’s happening with millers these days is what’s happening with so many insects in temperate regions -- climate change means that our winters are no longer all that cold. In particular, we don’t have the two or three blood-crystallizing events that used to kill off many overwintering insects. It’s generally warmer, too, so insects now successfully complete more generations than they used to.
That brings me to the crux of my miller advocacy. It’s well known that the Rocky Mountains’ high-altitude whitebark pines are being decimated by the onslaught of bark beetles, a pest that is new to them. They were once protected from insect attack by intense winter cold at their preferred elevation, but no longer. That means that whitebark pine nuts are no longer available to provide needed energy for grizzly bears fattening up for their winter sleep. And while native cutthroat trout used to provide good food for bears, the fish face competition from introduced trout species with different spawning habits, like the lake trout in Yellowstone Lake.
For bears, that means there’s less to fatten up on -- except for the transient miller moths. Though the moths were born and raised on various grasses mostly on the High Plains east of here, as adults they “get the urge for going” and fly off for a cool summer in the mountains. There they mate, feed on nectar from wildflowers and shelter in huge numbers among the rocks on talus slopes.
That’s where hungry bears find them and take advantage of their high fat content -- up to 70 percent of their body weight. Just crush one in your fingers and feel the oil. After summering in style in the mountains (yes, it’s the miller high-life), the moths wing back through this area, returning to the plains, where the females lay their eggs in the soil. That migration is much smaller than their June one, partly because of the work of hungry bears.
During their migration, millers are like vampires, fearing daylight. So just before dawn, they stop their flight and find a nice tight cranny for daytime shelter. It might be a spot under bark, or maybe under a rock or fallen log. Or they’ll creep deep into foliage and enjoy an occasional sip of nectar from a blooming Russian olive or lilac flower.
At our ranch, they seek out every single narrow dark spot on every building. They wedge their way into and through the shingles, the siding, the walls and trim, the edges of screens. They shimmy their way through windows and doorframes. Unfortunately, many of them end up inside the buildings and can’t figure out how to get back out.
Come nightfall, all they want is to be on their way. They bounce off the windows and screens. But if you turn on your lights, they'll be drawn to them and be with you all night.
Sure, you could do what the extension guys recommend: You can vacuum them up. Or get out the fly swatter, smack ‘em and watch the dust fly. (And boy, will it ever. These moths get their common name “miller” from the abundance and looseness of those floury, dusty scales on their wings.) Or you can set up a bucket of soapy water under a light and watch the madly circling insects fall into it and drown.
But the compassionate thing, for both the moths and the bears, is to encourage your houseguests to be on their way. In the evenings at our house, we keep the house lights to a minimum and wait until large numbers of moths are fluttering and beating against the screens. Then we remove the screens and wish them happy trails. Bats, nighthawks and late-flying swallows immediately appear and pluck many of them from the sky.
But many more moths make it through the flying gantlet. They’re off to high ground where the bears are waiting -- and hungry.
W. S. Robinson is a contributor to Writers on the Range, a service of High Country News. He teaches biology at Casper College in Wyoming and also researches the behavior of Asian honeybees and Ecuadoran parasitoid wasps.