Caterpillars, IDL Highlights, In the News, Invasive Species

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!