Viburnum Leaf Beetle: Yet Another Invader
In 2002, Paul Weston of Cornell University told me about the viburnum leaf beetle and the damage it was inflicting in New York State. It is one terrible non-native invasive pest. Paul studied its life cycle and published several journal and popular literature articles, and most usefully, developed a list of susceptible and less susceptible viburnum cultivars published as a Cornell University fact sheet.We were recently informed by Gaye Williams at the Maryland Department of Agriculture that the viburnum leaf beetle had found last November in western Maryland in Grantville. It was found in Elk Ridge Nature Center by Liz McDowell, with damage to the plant, and eggs laid in the stems. Liz said she found eggs in twigs on Viburnum acerifolium, V. dentatum, V. lentago, and V. trilobum, as well as old feeding damage.
It will likely take a while (hopefully) to become a major pest in other states, but let’s start preparing for this pest now and understand how to deal with it.
The good part of this is that two of my personal favorite viburnums are pretty much resistant to this pest: V. plicatum var. tomentosum, doublefile viburnum and V. rhytidophyllum, leatherleaf viburnum. (Viburnum plicatum ‘popcorn’ pictured right)
Complete Defoliation is Common
Fortunately, it feeds only on viburnums. Paul established that it has one generation per year in New York State. I suspect it will be similar here in Maryland.
From Paul’s work we know that adult females lay up to 500 eggs on viburnum twigs in summer and early fall. The eggs overwinter and hatch in spring. Larvae feed on foliage until early summer, then crawl down the shrub and pupate in the soil. Adults emerge from the soil in midsummer, feed again on viburnum foliage, and mate. From egg hatch to adult takes just 8 to 10 weeks.
Females typically deposit 8-10 eggs into the hole, then seal it with a lid or “cap” made of chewed bark and excrement held together by a special cement. A female will continue up the twig excavating cavities and laying eggs, leaving a distinctive row of caps, usually along the underside of the branch or twig. This is something you use your IPM skills to monitor for in the nursery and landscape.
The early instar larvae feed together, beginning on the underside of tender, young, expanding viburnum leaves. They often start with lower leaves, skeletonizing them, leaving only the midrib and major veins intact. If the infestation is heavy enough, the larvae will completely defoliate the shrub.
Larvae go through three instars (stages). Paul found that in New York State in early to mid-June, when they reach about 10 to 11 mm long, the larvae crawl down the shrub, enter the soil, and pupate. Pupae are very hard to find. They need moist soil. If it’s too wet or too dry, the pupae don’t seem to survive as well. So hope we have a couple of droughty summers.
In early July, the adult beetles emerge from the soil and start feeding on viburnum foliage. They continue to feed, mate, and lay eggs until the first killing frost. This is the stage where they may migrate to other plants that haven’t yet been infested.
Paul Weston and Carolyn Jones developed this list of susceptibility to viburnum leaf beetle. Smart nursery owners and cut flower farmers will plant more of the less-susceptible species.
Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]