Caterpillars Defoliating Glossy Buckthorn in the Bog
Jim Reinartz
I led a walk on the UWM Field Station boardwalk in the Bog on July 24th this summer. In the conifer-hardwood swamp just 100 ft. from the beginning of the boardwalk we noticed a large area where the glossy buckthorn had been almost completely defoliated by a large population of caterpillars.
Orbed Narrow-wing caterpillar in the Bog.
Orbed Narrow-wing caterpillar in the Bog.
Only the petiole and midrib of the leaves remained attached to the stems, the leaf blade had been completely consumed by the ravenous caterpillars. I estimated that at least a half acre of buckthorn had been defoliated in this area, and do not know if there were similar outbreaks in other parts of the Bog.
Defoliated glossy buckthorn in the Bog
After 35 years of working on projects to control buckthorn on 600 acres of the Bog, I was extremely excited to find an insect doing this work for us and wanted to identify our co-worker. We took photos of the caterpillars, and I collected one to rear to an adult. With photographs of the caterpillars taken by me and members of the Native Niche Inc. team, the very hungry caterpillars were identified as Orbed, or Variable, Narrow-wing moth (Magusa divaricate).
Orbed Narrow-wing moth adult. (Minnesota Seasons: https://www.minnesotaseasons.com/Insects/orbed_narrow-wing.html)
Orbed Narrow-wing is an uncommon, migratory, moth. It is found from central South America to southern Canada.
Orbed Narrow-wing range (iNaturalist)
Orbed narrow-wing adults occur year-round as far north as Texas. They migrate northward every year, bypassing the middle latitudes of North America and arriving in Wisconsin and Minnesota usually in late-July to September (Minnesota Seasons: https://www.minnesotaseasons.com/Insects/orbed_narrow-wing.html).
Orbed Narrow-wing in the United States (iNaturalist)
Orbed Narrow-wing adults do not migrate south again in the fall, and I was disappointed to learn that they are not known to survive our northern winters. Their long-distance migration results in two distinct, widely separated ranges in the U.S. In the northeast they occur from Maine to New Jersey, west to Minnesota and northern Illinois. In the southwest they occur from Texas to eastern Arizona. Between these ranges there is a sprinkling of widely scattered records.
Orbed Narrow-wing observations in Wisconsin (iNaturalist)
Orbed narrow-wing caterpillars feed on Carolina buckthorn, glossy buckthorn, and possibly other plants in the Rhamnaceae (buckthorn) family, and their preferred northern host is glossy buckthorn. Their northern range appears to be expanding, possibly due to the expanding range of glossy buckthorn (Minnesota Seasons). In our area, the adults are active where there is an abundance of buckthorn from July to October.
It is curious that this migratory behavior would evolve in a species that cannot survive to reproduce in the north where it appears those migrants reach an evolutionary dead-end. There is one observation in southeastern Kenosha County recorded in iNaturalist that notes “It appears that these caterpillars were in a small area [in 2013], but the population has exploded [in 2014]”. So perhaps there is some hope that we will see more of them in the Bog in 2027; they are certainly welcome any time they want to come dine on our buckthorn.
