Illustration by Adelaide Tyrol
Move over emerald ash-borer and hemlock wooly adelgid. There’s a new invasive pest in town, and it may be coming to a viburnum near you.
The viburnum leaf beetle, (Pyrrhalta viburni) was first detected in Ontario in 1947 and has since spread to New Hampshire and Vermont. As the name suggests, the beetle eats viburnums – a type of shrub (there are around 150 species) that are prized by gardeners and landscapers. Unfortunately, the beetles have spread from the ornamental viburnums in the garden to the native viburnums in the forest.
Forest-dwelling viburnums are typically tall shrubs (some might consider them small trees) and are a common part of the understory of many northern forest types. The most instantly recognizable may be the hobblebush (a.k.a. witch hobble, moosebush) that grows in high-elevation deciduous forests. All species have umbrella-like or flat-topped clusters of white, spring flowers that give way to berries that provide food for birds and other wild animals in the fall and winter. The color of the berries varies from red in highbush cranberry to blue in arrowwood.
Some species of viburnum, including hobblebush, mapleleaf viburnum, and nannyberry, have been able to withstand beetle infestations. Other species, including arrowwood, possum-haw, and highbush cranberry (both the European and American varieties) are more vulnerable. The beetle larvae may completely skeletonize the leaves of a plant by June, and after two to three years of consecutive infestation by the beetle, the plant may die. The problem has become serious enough in recent years that it has led biologists to think that some native viburnum species may become extirpated in large areas, especially in northern states, since the beetle eggs need a period of cold weather to hatch in the spring.
The rice-sized larvae are yellowish-green with black dots; they emerge in May, and promptly start feeding on the underside of leaves, creating a lace-like appearance by feeding nearly exclusively between leaf veins. In June, they crawl down the shrubs and burrow underground to pupate and become adults. In July, the adult beetles emerge and start adding more oblong holes to the leaves, rather similar to the ones caused by the larvae. The beetles are coppery brown or greenish, covered with very thin fuzz. They are quick to drop to the ground when disturbed. The females lay their eggs in little holes that they chew in the terminal twigs, adding more damage to the already stressed plants. They lay as many as a dozen eggs, which they cover with a mixture of sawdust and their own feces. The eggs hatch the following spring when the plants are beginning to leaf out.
The beetle seems to have few enemies. Some of the non-native viburnum species have built up a defense: the twigs, stressed by the egg sacs, develop a tough and thick layer of scar tissue that crushes the eggs. Unfortunately, the viburnums of our forests lack that defense, and are helpless against this pest.
The Cornell University Viburnum Leaf Beetle page has a few suggestions on how to minimize the damage. The most effective measure is to destroy the eggs during the fall and winter by pruning the infested twigs. Insecticidal soap can kill larvae in spring and horticultural oil can smother the eggs in fall; however, these methods work best in gardens or small fields and are not practical in forests.
Dr. Paul Weston and others at Cornell University are studying the possibility of using biological controls, but that remains far in the future. For now, the best we can do is hope that our native viburnums don't go the way of the American elm.
Beatriz Moisset, a contributing editor to BugGuide.net, is a retired biologist who enjoys studying pollinators in her post-retirement second career.