Yikes! Euonymus leaf notching caterpillars are on the loose. Credit: Jeremy S.
Over two decades followers of Bug of the Week have witnessed tales of exotic, invasive pests like emerald ash borer, brown marmorated stink bug, and spotted lanternfly racing across our country killing trees, pillaging crops, and sometimes invading our homes. Take spotted lanternfly, which, since its discovery in Pennsylvania a mere decade ago has traveled more than 600 miles, or brown marmorated stink bug which spread more than three thousand miles in roughly two decades. As of March 2024, EAB has killed tens of millions of ash trees in 36 US states and the District of Columbia as well as five Canadian provinces, since its discovery in Michigan in 2002. This week we visit another non-native pest. This slow mover has remained more or less bottled-up in Maryland and Virginia for more than 20 years.
During March and April, hordes of caterpillars strip leaves and create frass fouled foliage (repeat three times fast).
Back in 2002 a new pest was discovered in Fairfax, VA, when a homeowner noticed a voracious caterpillar munching her ornamental euonymus. The caterpillars were sent to Eric Day at the Insect Identification Laboratory in Blacksburg, VA. Eric reared the larvae and sent the unknown moths to specialist John Brown at the Systematic Entomology Laboratory, USDA. Dr. Brown identified the moth as one not known to occur in the US – a new, exotic, invader. The scientific name of this alien is Pryeria sinica. Prior to its discovery in Fairfax, this pest was only known from eastern Russia and China through Korea, Japan, and Taiwan. In 2003 more moths were collected in Northern Virginia and on May 28, 2003, Gaye Williams at the Maryland Department of Agriculture identified specimens of Pryeria sinica from Anne Arundel County, Maryland. Somewhere along the way the new pest was dubbed the euonymus leaf notcher due to the distinctive pattern of feeding caused by the caterpillar. As large caterpillars eat, sections of leaf along the margin disappear down their gullets, hence the name leaf notcher.
The leaf notcher passes winter as taupe-colored eggs deposited in clusters or 150 or more on pencil-sized twigs near terminals of branches. Eggs hatch in mid-March and early April and tiny caterpillars first feed in tight silken webs spun around unfolded leaves at terminals. As larvae grow, they move to expanded leaves to feed and are often found in large groups. Their presence is easily recognized by marginal notches and coarsely shredded leaves. When abundant, these caterpillars can entirely strip shrubs. After completing development, larvae wander from the plant seeking protected locations to pupate. Large numbers of wandering caterpillars may alarm homeowners, but citizens should remain calm as caterpillars are not known to eat humans or pets.
In early spring, euonymus leaf notchers hatch from eggs and scores of caterpillars begin to strip the leaves of euonymus. After completing development in spring, caterpillars spin silken cocoons in protected locations. In late October and November, adult moths emerge from very cute pupae, mate, and deposit overwintering eggs on twigs of euonymus.
Adult euonymus leaf-notcher moths fly in autumn and mimic wasps. This one was found less than 10 miles from the Pennsylvania boarder in Hampstead, Maryland. Credit: Charles Krause
In autumn, caterpillars spin cocoons amidst fallen leaves and adult moths appear to fly, mate, and lay eggs on the terminals of euonymus branches. Unlike many moths, euonymus leaf notchers are day fliers. They have unique patterns and colors on their body and wings that make them closely resemble wasps. The fact that they mimic wasps may help them avoid being eaten by day feeding predators such as birds. In North America the leaf notcher has been reported on Euonymus japonicus and E. kiautschovicus ‘Manhattan’. In its native range in Asia, the pest has been reported feeding on E. sieboldianus, E. japonicus, and E. alatus. Moreover, other members of the Celastraceae family such as Celastrus punctatus and C. orbiculatus are recorded as hosts for this pest.
The pest has two obvious weak points that provide excellent opportunities for management. From the time that egg laying ends in December until eggs hatch in spring, eggs can be crushed on the plant or simply removed by pruning off the terminal and disposing of it. If larvae are small or in restricted areas on a plant, then they can also be removed by a gloved hand or pruner. If larvae are widely distributed, abundant, or otherwise difficult to control manually, then several insecticides should perform well. Some of the most “environmentally friendly” insecticides for killing caterpillars contain Bacillus thuringiensis kurstaki (Btk) or the active ingredient called spinosad. Btk destroys cells in the gut of the caterpillar, a slow and painful death to be sure. Spinosad acts on the nervous system of the caterpillar, inducing a more rapid, twitchy form of death. Both insecticides can be purchased as brands that are listed by the Organic Material Review Institute (OMRI) for use in organic food production, which makes them safe enough to be applied to those vegetables and fruits labeled “organic” in your supermarket.
Euonymus leaf notcher, where are you now? Two excellent sources provide clues to locations where euonymus leaf notcher can be found in the DMV. The Maryland Biodiversity project lists four locations in Maryland with this rascal including Baltimore County, Baltimore City, Prince Georges County, and Anne Arundel County. Carroll County should likely be added to this list as Bug of the Week reported a finding of the leaf-notcher in Carroll County back in November of 2020. A second source of information on locations of the leaf-notcher is iNaturalist which tallies observations in Fairfax and Prince William counties.
Acknowledgements
Many thanks to Jeremy S. and Charles Krause for sharing their wonderful images of euonymus leaf notcher and providing the inspiration for this episode. Thanks also to Gaye Williams of the Maryland Department of Agriculture for confirming the identity of the adult moth.