Eastern Montana moth populations seeing a boom this year

While large swarms of grasshoppers or mosquitoes have been typical summer fair for the region over the past few years, they are being joined by another group this year: miller moths. In recent weeks, area residents may have noticed that there has been a significant increase in the moth population, and while they may not be the most harmful insect around, their presence has certainly been noticed.

According to Montana State University entomologist Dr. Robert Peterson, it has been a combination of factors that have allowed the population of moths to explode this season. Namely among them, a large migration of moths last year, good snow coverage over the winter and now plenty of moisture around the time of their emergence at the end of May. As he explained, miller moths annually migrate to the Rocky Mountains to breed, then back to the plains to lay their eggs. Those eggs then hatch into the army cutworm, which feed on plants before hibernating for the winter, emerging as moths in the late spring/early summer.

“Last year we saw large numbers of moths emerging throughout the Great Plains and migrating to the Rockies. We also saw a big re-migration back to the Great Plains. So, we started last fall with a large population of moths. The females then laid their eggs, the caterpillars hatched, and partially fed on plants until hibernating a couple of inches below the soil surface. Good snow cover (which provides insulation) for the caterpillars, warmth and spring rain (but not too much) are ideal for eventual emergence of the moths, which is what we’re seeing now in Eastern Montana,” Peterson explained.

He added that aside from the moths spawning locally, there are also additional flocks of moths continuing to move in from further east, stopping in Glendive along their migration route.

While mosquitoes can be bothersome to people, grasshoppers can shred crops and the army cutworm itself is an “agricultural pest,” according to information Peterson provided, miller moths are actually a bit more beneficial, as they serve as pollinators for many different plants and a food source for many other animals, even grizzly bears.

As the moths continue to emerge, Peterson said it will likely be another couple of weeks until local populations thin out as they make their migration back to the Rockies to begin the cycle again.

“The moths in Glendive will be around for another couple of weeks because they are emerging as adults locally... After a week or two, they will be gone because they all will migrate to the Rockies above the treeline to spend the rest of the summer feeding on nectar from alpine flowers (and providing fat-dense food for grizzly bears),” Peterson said.

Peterson was part of a study on the migration patterns of miller moths between 2019 and 2021. The study noted that moth migration patterns had not been adequately recorded since roughly the mid-1900’s and sought to update that data. According to the reports findings, moths aren’t just migrating east to west, but are also moving north to south, which is a new finding for moth migrations.