In mountain ranges of the southwestern U.S., Nepytia janetae was an innocuous, little-studied species of moth until 1996. Since then, four devastating N. janetae outbreaks have defoliated and killed tens of thousands of acres of conifer trees. A new study details the life history of the species and examines contributing factors to the outbreaks in hopes of predicting future ones—though much remains a mystery. (Public domain photo via New Mexico State Collection of Arthropods)

By Paige Embry

Paige Embry

In the high country of four widely separated mountain ranges in Arizona and New Mexico, some very hungry moth larvae have become an unexpected problem. Prior to 1996, the larvae of Nepytia janetae were innocuous. Since 1996, there have been four devastating N. janetae outbreaks that have led to the defoliation and often death of trees in a number of conifer species (see map and table below). Why? How? Several scientists set out to learn more about the first three outbreaks and the moths themselves, and they report their findings in an article published in June in Environmental Entomology.

The study was wide-ranging. The researchers investigated the life history of N. janetae, a species of looper in the family Geometridae about which little was known. They collected what meteorological data were available for each area. They studied how the outbreaks ended—primarily from parasites, viruses, and starvation, although heavy rains during egg deposition played a role in one. They assessed the forest types affected, which varied between the two states: larvae fed in the mixed conifer zone in New Mexico and in the higher-elevation spruce-fir forest in Arizona. The researchers set up plots where the three earliest outbreaks occurred and collected piles of data on the trees themselves (tree species, dead or alive, diameter, relative size compared to other trees on the plot, bark beetle activity, etc.) and on plot-level variables like aspect and stand density. They assessed the damage done to the trees, estimating the percentage of defoliation and describing how the needles were eaten. In some places in each study area, tree mortality approached 100% and was associated with how badly the trees were defoliated, moisture availability just before or early in the outbreak, and bark beetle activity.

In mountain ranges of the southwestern U.S., Nepytia janetae was an innocuous, little-studied species of moth until 1996. Since then, four devastating N. janetae outbreaks (mapped here) have defoliated and killed tens of thousands of acres of conifer trees. A new study details the life history of the species and examines contributing factors to the outbreaks in hopes of predicting future ones—though much remains a mystery. (Image originally published in Lynch et al 2026, Environmental Entomology)

Ann Lynch, Ph.D., is a recently retired research entomologist with the U.S. Forest Service and the University of Arizona and lead author of the study. She says that the variables the researchers looked at “are the types of variables that you know through experience are going to point you in the right direction if there is a direction to be pointed at. In this case, hardly any of that helped.” She adds, “You really do expect to find patterns, and we did not.”

Ann Lynch, Ph.D., is a recently retired research entomologist with the U.S. Forest Service and the University of Arizona who led a study on life history of the moth Nepytia janetae and the factors contributing to four outbreaks of the species between 1996 and 2020. (Photo courtesy of Ann Lynch, Ph.D.)

But they still learned a lot. The researchers discovered that N. janetae larvae are winter feeders in places where minimum temperatures can hit close to or below 0 degrees Fahrenheit. Of the other Nepytia species whose life cycle has been studied, none are winter feeders. “The fact that it’s a winter-feeding insect at high elevations just amazes me—still,” Lynch says. “Do these bugs not have enough sense to come in out of the cold?”

They found that the larvae are “wasteful” feeders, destroying more needles than they consume. They may eat only part of a leaf but still kill it by eating the base or opening it up to dehydration, and tree mortality is linked to the extent of defoliation. The researchers suspect that two or more species or subspecies of Nepytia may be at work. The New Mexico moths (adults and larvae) have some differences in appearance from those in Arizona, and they feed on different tree species. In Arizona, the larvae largely fed on Englemann spruce and corkbark fir, whereas the New Mexico larvae were conifer generalists that leaned toward Douglas fir and white fir.

The only weather commonality the researchers found was low snow pack, which was either light or gone early in the season the year the outbreak started or the year before. “We expected to see trends with low temperatures, and we didn’t,” Lynch says.

They used existing weather stations and therefore got whatever data was available. It wasn’t always optimal. In one location the station was 700 meters below the outbreak zone in a different forest type. “The data that we really, really, really, wish we had is temperature data, weather data,” Lynch says. “We have all these forest pests that are responding to climate change and we still don’t have weather stations in the high-elevation forests.”

In mountain ranges of the southwestern U.S., Nepytia janetae was an innocuous, little-studied species of moth until 1996. Since then, four devastating N. janetae outbreaks have defoliated and killed tens of thousands of acres of conifer trees. The first outbreak spanned 1996 to 1999 in Arizona’s White Mountains (shown here), with more than 10,000 acres defoliated. A new study details the life history of the species and examines contributing factors to the outbreaks in hopes of predicting future ones—though much remains a mystery. (Image originally published in Lynch et al 2026, Environmental Entomology)

Despite the dearth of supporting data, Lynch and colleagues think that climate is probably responsible. “We think that a causal relationship between N. janetae outbreaks and climate is likely, but the relationship is indirect, perhaps temporally or spatially distant, or may require multiple climatological and phenological conditions to co-occur or succeed one another,” they write in their report, in which they take a few pages to lay out their rationale. Lynch says, “Your logic takes you to climate change because nothing else makes sense, but the mechanism for how climate affects the system? You barely have any hints.”

Lynch expects more N. janetae outbreaks. “I don’t know when and I don’t know where and I don’t know what type of climate conditions are needed for it to happen,” but, she says, she’d put all her eggs in the “there will be more outbreaks” basket.

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.