Viburnum borers are wood-boring insects that feed on the lower part of viburnum trunks, branches, and roots. There are two species of viburnum crown borers which have a similar look and biology. The adult viburnum clearwing borer (Synanthedon viburni) has white scales on its face while the lesser viburnum borer (S. fatifera) does not have the white scales. The adults are day-flying moths that look similar to wasps. They have 1/2 inch long, bluish-black bodies with yellow markings and clear wings, with a 3/4 inch wing span. Larvae are pinkish white caterpillars with reddish brown heads.
Adults emerge in June and July to lay eggs near wound sites. Larvae tunnel under the bark and feed on the cambium and inner bark of the shrubs. Damage may occur as high as 18 inches above the soil and may extend a few inches below the soil surface. The insects overwinter as larvae. Pupation occurs for about 30 days.
The first symptoms noticed are branch dieback and plant decline. Ultimately, the plant may die. Since they usually attack near the base of canes, swellings, cracks, and emergence holes may be seen at the base of the plants.
Viburnum borers are attracted to stressed plants. Plants should be sited properly. Keep them healthy by watering in dry periods and keeping them properly mulched. Prevent injuries and wounds such as those caused from weed whips, lawn mowers, and bad pruning. Plant resistant cultivars. Wayfaring tree (Viburnum lantana), European cranberry-bush (V. opulus), American cranberry-bush (V. opulus var. americanum), Korean spice viburnum (V. carlesii), nannyberry (V. lentago), hybrid leatherleaf viburnum (V. x rhytidophylloides), and Sargent’s cranberry-bush (V. sargentii) are all susceptible, but arrow-wood viburnum (V. dentatum) is reported to be resistant.
Beneficial nematodes (Heterorhabditis bacteriophora) can be applied as a soil drench in late August. To ensure that the nematodes stay alive, the soil should be kept moist.
Susceptible viburnums can be protected with an insecticide sprayed on the bark of the plant from the soil line to about 18 inches high. Timing is critical. Spray while adult moths are active beginning in June. To be sure adults are active, purchase pheromone traps to catch the adult males. Insecticide applications should be applied about 10–14 days after the first moth is captured. A second application may be required if adults are still flying 30 days after the first spray. Contact the Plant Clinic (630-719-2424 or firstname.lastname@example.org) for current recommendations. Use pesticides safely and wisely; read and follow label directions.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.