Viburnum leaf beetle has been known in some eastern states since the early 1990’s. It is a relatively new pest to the Chicago region. A few possible sightings of this pest were reported in 2013 an 2014. In 2015, the beetle was reported across the Chicago region with some regularity. This insect feeds as both larvae and adults and can do extensive damage. If left unchecked it can lead to the death of the shrub.
Adult beetles are fairly non-descript and are easily over-looked. They are brown and about ¼ inch in length, with the females being slightly larger than the males. Close inspection reveals that the insect is covered with fine golden hairs.
The young (larvae) are tiny upon hatching and will only reach about 1/3 of an inch in length at maturity. Feeding damage may be noted before the actual insect is seen. The larvae vary in color from pale green to pale yellow. The body is marked with black dots along the sides and a row of black dashes along the back. As the larvae grow, they will molt and shed their skin, so cast off skins may be noted on the leaves of the host plant.
Egg-laying sites may be seen on twigs from fall until spring. The actual eggs are not visible. The eggs are laid in small holes on the twigs and then the holes are capped with a mixture or chewed wood and excrement. The caps are dark and stand out against the bark of the twig, making them easy to see. They are often in rows.
Viburnum leaf beetle feeds only on viburnums. Some species of viburnum are more susceptible to the beetle than others. A preliminary list of susceptible and resistant cultivars has been developed by Dr. Paul Weston of Cornell University (see http://www.hort.cornell.edu/vlb/suscept.html for a full list.)
Highly susceptible species are the first to be attacked, and are generally destroyed in the first 2-3 years following infestation. Common viburnums that are considered highly susceptible include: Viburnum dentatum (arrowwood viburnum), V. nudum, (possum-haw, smooth witherod viburnum), V. opulus (European cranberry-bush viburnum), and V. opulus var. americana, formerly V. trilobum (American cranberry-bush viburnum).
Susceptible species are eventually destroyed, but usually are not heavily fed upon until the most susceptible species are eliminated. Common viburnums that are considered susceptible include: V. acerifolium (mapleleaf viburnum), V. lantana (wayfaringtree viburnum), V. rufidulum (rusty blackhaw, southern black-haw), and V. sargentii (Sargent viburnum).
Moderately susceptible species show varying degrees of susceptibility, but usually are not destroyed by the beetle. Common viburnums that are considered moderately susceptible include: V. burkwoodii (Burkwood viburnum), V. x carlcephalum (Carlcephalum viburnum), V. cassinoides (witherod viburnum), V. dilatatum (linden viburnum), V. farreri (fragrant viburnum) (except 'Nanum', which is highly susceptible), V. lentago (nannyberry viburnum), V. prunifolium (blackhaw viburnum), V. x rhytidophylloides (lantanaphyllum viburnum).
Resistant species show little or no feeding damage, and survive infestations rather well. Common viburnums that are considered resistant include: V. carlesii (Koreanspice viburnum), V. x juddii (Judd viburnum), V. plicatum (doublefile viburnum), V. plicatum var. tomentosum (doublefile viburnum), V. rhytidophyllum (leatherleaf viburnum), V. sieboldii (Siebold viburnum).
Most species in all susceptibility groups exhibit more feeding damage when grown in the shade.
Life Cycle and Damage
Viburnum leaf beetle has only one generation per year. This insect overwinters as eggs in the tips of twigs of the host plant. New larvae will start to hatch out around mid-May in most years and will move to the new leaves to feed. The larvae will go through three instars, growing as they molt. They will skeletonize the leaves, eating the tissue between the veins. The amount of feeding on susceptible species can be very damaging.
From early to mid-June, the larvae will crawl down the stems to pupate in the soil. Pupation takes about ten days. After that time, the adult beetles will emerge and they will also feed on the leaves, continuing to skeletonize them. Adults will generally be present from early July until frost.
The female beetles will lay their eggs in the cavities they chew into the stems. Several eggs are laid in each cavity. Egg-laying will occur from late summer into autumn, with each female able to lay as many as 500 eggs.
Since both the larvae and the adults can feed, damage occurs over the majority of the growing season and it can be quite extensive on highly susceptible species. Heavy feeding for 2 to 3 years can lead to the death of a viburnum.
Consider planting the more resistant species of viburnums. Susceptible species do not have to be abandoned, but it may be wise to plant them in moderation. Excessive use of any species always has the potential to magnify pest problems.
One of the easiest ways to reduce the population of beetles is to remove and destroy the twigs that bear the eggs. The egg-infested twigs can easily be seen once the leaves have fallen. They can be removed from October through April. Once the twigs have been removed, destroy them by burning (where permitted), chipping , composting or burying.
There are some beneficial insects that will attack the viburnum leaf beetle. Encourage beneficials to come to your yard by planting a diverse group of flowers and by limiting the use of broad spectrum insecticides that may kill them.
A sticky barrier like Tanglefoot® applied to stems may keep some larvae from crawling to the ground to pupate.
If insecticides are used, it is best to treat young larvae as they are the easiest to kill. If larvae can be killed early in the season, the amount of feeding damage can be lessened. Insecticides can be applied to adults, but by that time there may be a lot of damage already done.
When selecting an insecticide to treat larvae early in the season, look for products that are less likely to kill beneficial insects; products like insecticidal soap or spinosad. Insecticidal soap is a soap prepared specifically for insect management. Do not use dish soap or other household soaps as they may be damaging to plants. Spinosad is a natural substance made by a soil bacterium. It can be toxic to a number of insects.
Studies at Cornell University have shown some success in using horticultural oil at a high concentration (4%) on egg-infested twigs to suffocate the eggs. This must be done before the leaves come out as the oil may damage leaves at this high concentration.
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