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Viburnum Borer

June 13, 2001

Viburnum borer attacks the base of Viburnum opulus compacta and other viburnums, causing dieback of the stems. There are actually two major species in-volved, the viburnum clearwing borer, Synanthedon viburni, and the lesser viburnum borer, S. fatifera. The larvae tunnel in the cambium from the soil line to several inches below the soil. Severe attack results in girdling below ground. Attack may also occur as high as 18 inches above ground, resulting in gnarled and scarred stems.

Adult moths are clearwinged moths, bluish-black with yellow markings and a 3/4-inch wingspan. They fly during the day, mimicking the flight and appearance of wasps. Female moths lay their eggs at the base of the plant in June, particularly near wounds. The resulting larvae are white and legless with brown heads. They burrow just under the bark in the cambium area. Full-grown larvae are about 3/4 inch long. They pupate the following May to emerge as moths in June.

Severely attacked bushes typically have sparse foliage with just a few leaves at the tips of some stems and other stems totally bare. Damage seems to be most severe in the first 2 to 3 years after transplant into the landscape. The entire bush may die. Surviving bushes appear to grow out of the damage because mature plants appear to be free of the borer, but close examination frequently reveals damage that is several years old.

Male moths are attracted to pheromone traps. Spray the base of affected shrubs with chlorpyrifos (Dursban) 2 weeks after peak moth catch. Treatment during the second or third week of June is usually successful. Once the larvae get larger, they are susceptible to insecticidal nematodes. Drench the soil with Heterorhabditis bacteriophora or Steinernema feltiae in late August and keep the soil moist after treatment.

Author: Phil Nixon


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