By Summer Roberts, Community Forester
The WCCD has been receiving reports of spongy moth (Lymantria dispar L., formerly gypsy moth) caterpillar infestations from multiple landowners across Washtenaw County who have experienced the caterpillars and their frass (excrement) rain down on belongings, pets, and themselves. Spongy moths are an invasive species, meaning they are not native and cause economic or environmental harm or harm to human health. Discovered in Michigan in the 1950s, the caterpillars feed on leaves of more than 300 tree species, although they prefer deciduous trees. Large infestations can completely defoliate trees leaving them bare. However, the introduction of a fungus that only targets spongy moths, as well as a naturally occurring caterpillar-specific virus, has restrained outbreaks, creating a boom and bust population cycle that typically lasts only a few years without human intervention. An article by the Ann Arbor Forestry department describes how land owners can spread the fungus in their yard, search for and destroy egg masses attached to tree bark, and place sticky bands around tree trunks to help control outbreaks on their property.
Ryan Hodnett, CC BY-SA 4.0, via Wikimedia Commons
The Michigan Department of Natural Resources (MDNR) Forestry Division recognized a widespread spongy moth outbreak occurring in the lower peninsula in 2020 and the department, while taking precautions against COVID-19, attempted to perform some of their annual aerial surveys. They estimated almost 1 million acres in the northern Lower Peninsula had been defoliated! Thankfully, most deciduous trees, such as oaks, a L. dispar favorite, can recover from several years of heavy infestation although they will be more stressed and susceptible to other pathogens and negative environmental stressors, such as drought. Conifers, such as spruce, fir, and pine, cannot produce a second set of buds during the summer and will therefore die if all their needles are consumed. With respect to forests, the MDNR is not spraying insecticides, but allowing the fungal and viral pathogens to limit the size and length of outbreaks. The targeted fungus is ideal for containing L. dispar populations since it does not kill pollinators or other beneficial insects; however, the drawback is waiting, sometimes several years, for the fungus to make a significant impact on the population. Large-scale spray programs are not currently being operated by either the State or Washtenaw County. Check out this article from MDNR for additional information on L. dispar, the current outbreak in Michigan, and links to other resources.
For property owners dealing with a significant outbreak, the MDNR Forestry Division recommends that high-value trees be watered during times of drought and lightly fertilized in the fall to help trees rebound from multiple stressors. If a landowner cannot tolerate the aesthetic damage or is worried about the health of their trees, they should first confirm that they have L. dispar by reviewing the identification info on the Michigan.gov Gypsy Moth Fact Sheet and review the Michigan State University Integrated Pest Management FAQs about managing populations on their property. Property owners can also reach out to Dr. Shikha Singh, the Jackson, Lenawee and Washtenaw Cooperative Invasive Species Management Areas (JLW CISMA) coordinator, to ask questions or get recommendations for a specific situation. If pesticides are determined to be necessary, property owners can find qualified pesticide applicators on the Michigan Department of Agriculture and Rural Development or consult our list of contractors on the WCCD website. (WCCD continually updates our contractor lists and always appreciates hearing residents’ suggestions for contractors to include as well as experiences with contractors on the existing list.)
Unfortunately, L. dispar are well-established in Michigan and eradication is extremely unlikely. However, everyone can help stop their spread within Michigan and to other states by not transporting firewood and checking camping equipment, recreational vehicles, trailers, and wheel wells of cars for egg masses or caterpillars. Avoid touching the caterpillars with bare hands since their fuzzy hairs can irritate skin, but don’t just leave them on the ground! Both egg masses and caterpillars can be killed by placing them in a bucket of soapy water and the caterpillars can be squished with a rock or shoe.