As with peachtree borer, now is the time of year to be on the lookout for viburnum borer. There are actually two species involved, 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 grade. Heavy infestations can result in girdling below ground. An attack may occur as high as 18 inches above ground, resulting in gnarled and roughened stems.|
Adults are clearwinged moths, bluish black, with yellow markings and a 3/4-inch wingspan. They fly during the day and resemble wasps in both flight pattern and appearance. Female moths lay eggs at the base of a plant around June. Wounded areas are primary locations where eggs are laid. The eggs hatch into larvae that are white and legless, with brown heads. The larvae burrow just underneath the bark in the cambium region. Full-grown larvae are about 3/4-inch long. They pupate the following May and then emerge as moths in June and July.
Severely attacked shrubs typically have sparse foliage with just a few leaves at the tips of some stems, whereas other stems are totally bare. Damage appears to be most severe in the first 2 to 3 years after shrubs have been transplanted into the landscape. The entire shrub may eventually die. Surviving shrubs appear to grow out of the damage because mature plants appear to be free of the borer; but under closer inspection, it is frequently revealed that damage is several years old. Viburnum opulus, particularly the variety ‘Compacta’, appears to be more susceptible than other viburnums to borer attack.
Similar to peachtree borer, male moths are attracted to pheromone traps. These can be used to help time applications of the insecticide permethrin (Astro). Spray the base of affected shrubs 2 weeks after peak moth catch, which typically occurs in early June. Treatment during the second and third week of June is usually necessary. Once the larvae get larger, they are susceptible to beneficial nematodes (Heterorhabditis bacteriophora). Beneficial nematodes may be applied as a drench to the soil, typically in late August. The soil must be moistened before and kept moist after treatment.