Problem type: Insect
Name of problem: Viburnum Borer
Plant name(s): American highbush cranberry and other viburnum species
Symptoms / Characteristics:
Borers attack plant stems usually at or up to 18" above the soil. Stems may appear gnarled and scarred and patches of bark may be broken off. Larvae leave behind sawdust as they bore in the host. If larvae bore tunnels encircling the stem before tissues can heal, sudden dieback of that stem will occur. Heavily infested plants appear unhealthy and can exhibit major branch dieback, swellings, cracks and emergence holes at the base of the plant or even plant death.
Larvae are white with reddish brown heads and are responsible for the damage by feeding within the sapwood, damaging vascular tissues that transport water and nutrients. Adult borers are moths that closely resemble wasps but do not sting. They have dark bodies with yellow markings, clear wings, measure about ½" long and are responsible for depositing eggs on the bark in the few days they are alive. Once larvae hatch, they burrow into the bark, where they excavate tunnels and overwinter. Feeding commences in the spring and adults emerge throughout June and sometimes later.
Plants that are already injured or stressed seem to be targeted over strong, healthy plants. Stress to plants may be due to mechanical injury, drought stress, over watering and recent transplanting, though healthy plants can still be attacked.
Control / Preventions:
Keep plants healthy with proper watering and fertilizing practices. Avoid injuries to plants, especially to lower stems while trimming and other mowing practices or during construction. Remove old, damage or infected stems and encourage new, succulent growth.
Chemical control can be applied to control adult moths, as larvae are protected under the bark. Pheromone traps can be used to monitor when adults are becoming active. Residual insecticides can be applied to the lower portions of the bark ensuring thorough coverage of lower stems or trunk, previously damaged areas and branch crotches. Most residual insecticides remain active for over a month, but repeat applications may be needed if adults are still active 30 days after the initial application.