Viburnum Leaf Beetles at Work
Note: The larvae of viburnum leaf beetle were seen actively defoliating entire plants at a location in Amherst, MA on 5/29/19.
The viburnum leaf beetle (Pyrrhalta viburni) is a beetle in the family Chrysomelidae (the leaf beetles) that is native to Europe, but was found in Massachusetts in 2004. This insect was initially reported in North America in 1947 in Ontario, Canada. Since then, it has been reported in the United States in parts of Maine, New Hampshire, New York, Ohio, Pennsylvania, Vermont, and Washington. This beetle feeds exclusively on many different species of viburnum, which includes, but is not limited to, susceptible plants such as Viburnum dentatum (arrowwood viburnum), V. nudum (smooth withered), V. opulus (European cranberry bush), V. propinquum (Chinese evergreen viburnum), and V. rafinesquianum (downy arrowwood). Some viburnum have been observed to have varying levels of resistance to this insect, including but not limited to V. x bodnantense (Bodnant viburnum), V. carlesii (Koreanspice viburnum), V. davidii (David viburnum), V. plicatum (doublefile viburnum), V. rhytidophyllum (leatherleaf virbunum), V. setigerum (tea viburnum), and V. sieboldii (Siebold viburnum).
One generation of viburnum leaf beetles occurs per year. The adult female beetle lays up to 500 eggs in pits they chew in viburnum twigs in the summer and early fall (beginning roughly late-June to mid-July through as late as October or until the first killing frost). These egg laying sites are approximately 1 mm. in diameter, and usually (but not always) made on small, current season’s branches or twigs. That said, it is best to look at the tips or ends of branches for viburnum leaf beetle egg sites. Approximately 8 eggs are laid in each pit, which is then backfilled by the female beetle with chewed bark, excrement, and reportedly, a special “cement” they make. This makes the egg pits look brownish-black when compared to the greenish/brown bark of the twig. Capped egg pits are typically created in rows. These eggs are the life stage that overwinters, hatching in the spring (approximately late April to early May).
Larvae (immature insects) that hatch from these eggs conduct their feeding through early summer. They begin feeding by skeletonizing the underside of the leaf, typically leaving the midrib and large leaf veins behind. Newly hatched larvae may be about 1 mm. long and greenish-yellow or off-white in color, lacking spots. Viburnum leaf beetle larvae undergo 3 instars (3 stages between which they shed their “skin”). Second and third instar larvae are yellowish-brown with black spots and may be ½ inch in length upon maturity. If the infestation is large enough, as the larvae grow in size, they may completely defoliate the shrub. They then are reported to crawl down the shrub to the soil where they pupate. Pupae are very difficult to find, as this stage occurs in the soil. Moist soils are considered a favorable environment for this insect; if it is too wet or too dry, pupae may not survive as well.
Adults emerge from the soil by midsummer (typically early July, but sometimes by late June), feed again on viburnum foliage, mate, and females lay the eggs that will overwinter to provide us with a population of viburnum leaf beetles the next year. These adults are capable of migrating to new, previously not infested plants. The transition from egg hatch to adulthood can take 8-10 weeks. Adult beetles are brownish-yellow in color and approximately ¼ inch in length; females are larger than males. Unlike the skeletonized damage the immature larvae cause with their feeding, adult beetles create irregular, circular and sometimes elliptical holes in the leaves while feeding.
How to Manage Viburnum Leaf Beetle
If you have this pest on susceptible viburnum species or are looking to plant viburnum in your landscape, it is best to start with the resistant species of viburnum listed above, or remove and replace susceptible species with those that are resistant in areas where this insect is problematic.
If you have a small viburnum leaf beetle population, where entire shrubs are not yet being defoliated, mechanical management options exist. Prune and destroy infested twigs and branches (those with capped egg sites) over the winter. These egg laying locations are especially easy to see when the viburnum have shed their leaves. Prune and destroy these twigs. In small scale infestations, adult beetles can be hand collected and dropped into a can of soapy water. (Beware, they will drop to the ground when startled, so use this behavior to your advantage. Place your can of soapy water beneath them and gently tap the branch to get them to drop into the water below.) It has recently been discovered that larvae do not drop from the leaves to pupate in the soil, but rather crawl down the shrub to do so. This may make them vulnerable to sticky barriers placed around the base of individual shrubs.
Larvae, particularly newly hatched ones, may be treated with a product containing spinosad or insecticidal soap. Both of these reduced risk options require direct contact with the larvae to be effective. Other insecticidal options are available to manage this pest; however, some (depending upon the active ingredient) may pose greater risk to beneficial insects. For assistance managing viburnum leaf beetle, contact a Massachusetts licensed Pesticide Applicator.
For more information about this insect, go to http://www.hort.cornell.edu/vlb .
Tawny Simisky, UMass Extension Entomologist