Tag Archives: 2023 Lab Highlights

2023 Spongy Moth Season Kicks Off in Wisconsin

Spongy moth (Lymantria dispar) season has officially begun here in Wisconsin. Earlier this week (May 3rd), the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) Pest Survey Program reported caterpillars emerging from an egg mass in far southwestern Wisconsin. Based on forecasting models from the USA Plant Phenology Network, we should expect to see caterpillar emergence begin across much of southern Wisconsin over the next week. As things continue to warm up, the pattern will push further north in the state.

Forecasting map from USA National Phenology Network for predicting hatching of Spongy Moth eggs. Source: USA-NPN (www.usanpn.org)

We may be facing a challenging year from this invasive insect. Dry conditions suppressed a beneficial fungal disease (Entomophaga maimaiga) the last two years and allowed spongy moth populations to build up. Along these lines, DATCP reported a 102% increase in male spongy moths caught in trapping surveys last year.  Likewise, the Wisconsin Department of Natural Resources (DNR) Forest Health Team reported a significant uptick in defoliation last year—from 294 acres defoliated in 2021 to over 85,000 acres defoliated in 2022.

At this point in the season, it’s hard to know how much damage will ultimately occur on yard and forest trees in 2023. However, you can get an estimate of potential defoliation in your own yard by counting egg masses.  Each overwintered spongy moth egg mass contains upwards of 1,000 eggs, so a yard with dozens of egg masses could soon face tens of thousands of hungry caterpillars in the near future.

Dozens of spongy moth egg masses on a tree trunk. Photo credit: Karla Salp, Washington State Dept. Agriculture via Bugwood.

Over the next few weeks, we can expect spongy moth caterpillar activity increase. These small caterpillars will cause a trivial amount of damage at first but will gradually become larger, hungrier, and more damaging over time. Keep an eye out for activity in your yard. If you are inundated with caterpillars, consider using sticky barrier bands and burlap barrier bands to trap them as described on the UW-Madison Extension Spongy Moth website.

Looking at the bigger picture, Mother Nature could hold a trump card for our spongy moth situation. If we end up having a rainy spring, damp conditions could encourage the fungal disease Entomophaga maimaiga to kick in and crash spongy moth populations (this halted outbreaks in SE Wisconsin in 2004 and 2010) . However, if we experience another dry season, it could allow spongy moth populations to build further—stay tuned and hope for rain!

Snow Falling on Cutworms

When most folks think of “cutworms” they probably envision big fat caterpillars munching on plants in their yards and gardens.  Sure enough, we have many species from the cutworm & armyworm family (Family Noctuidae)  that can be notable crop pests, such as black cutworm, bronzed cutworm, variegated cutworm, true armyworm, fall armyworm, and others.  Readers should keep in mind that the Family Noctuidae is surprisingly diverse with over 2,500 species known from the US and Canada.  Many of these species play important roles in the food web and can be as important as bees for pollination of certain crops!

While cutworms can be common during the summer months into the fall, we usually don’t expect to see them this time of the year.  Surprisingly, there’s one cold-hardy species that has been common in Wisconsin recently—the “winter cutworm” (Noctua pronuba).  This species gets its name due to the fact that the caterpillars are cold-tolerant and can be active when temperatures dip.  While they won’t be out-and-about during a polar vortex, I’ve had many recent reports at the UW Insect Diagnostic Lab of winter cutworm caterpillars wandering on the snow when temperatures have been in the 20’s.  Overall, there are very few caterpillars that venture onto the snow, period.

Winter cutworm caterpillar wandering on the snow. Photo submitted to UW Insect Diagnostic Lab in late 2022.

Similar to other cutworms, the winter cutworms are plump, earth-toned caterpillars—they are brownish with pairs of black dashes bordered by white running down the sides of their backside, which makes them easy to identify.  (Technically, a greenish form of the caterpillars also occurs). In addition, if you see any caterpillar that even looks “cutworm-like” out on the snow, it’s almost certainly this species.  They pass through the the winter as nearly-mature caterpillars before pupating in the spring.  The adult moths are active during the warmer months and display an amazing array of different color forms—ranging from very light beige to grey or brownish—although the hindwings (typically tucked under the body at rest) are a diagnostic yellow color.  Thus, adults are commonly known as “large yellow underwing” moths.

Larger yellow underwing moth showing the distinctive yellow color of the hindwings. Photo credit: Olaf Leillinger via Wikipedia CC.

The winter cutworm can be found from coast-to-coast in much of the US and Canada, although this wasn’t always the case.  This species is common in Europe and wasn’t known from North America until 1979 when it was first spotted in Nova Scotia.  While the caterpillars do feed on a wide range of plants, they’re rarely a notable pest.  Outbreaks and reports of damage aren’t common, although there was a notable outbreak in Michigan around 2007-2008.  In most cases, these caterpillars are simply a curiosity as they wander across the snow on mild winter days—sometimes by the thousands.  In addition, winter cutworms can occasionally sneak into structures as well, which can be another surprise to see caterpillars actiely wandering in garages or barns when the temperature is in the 40’s.

Winter cutworm caterpillar found in a garage last December. Photo submitted to UW Insect Diagnostic Lab.