The rhododendron borer is a tiny, native clearwing moth. Adult moth emergence may begin in May and last through July, depending upon local temperatures. Adult males are metallic black in color with narrow yellow bands across the abdomen on the second, fourth, and fifth abdominal segments, and a yellow line on each side of the abdomen. Wings are transparent. Females are very similar in appearance, although the yellow bands are broader and cover the undersides of the abdomen as well (Beutenmueller, 1909). After mating, adult females lay their eggs (up to 40 each) near locations of the host plant with defects or injury - particularly, branch crotches, pruning cuts, scars, or old larval feeding galleries. Eggs are tucked into these areas by the females, who perish shortly thereafter. Larvae hatch and chew their way to the inner bark, where they tunnel and feed and fill these tunnels with small, reddish frass pellets. By late fall, the larvae move to the sapwood to overwinter. Feeding by rhododendron borer caterpillars begins again the following spring around mid-March as temperatures begin to warm. It is at this time the caterpillar chews more holes in the outer bark, pushing out more frass that may be noticeable to the observer. Oblong cells are created in the bark by early May, within which the mature caterpillars prepare to pupate. At maturity, caterpillars are up to 1/2 inch in length, whitish yellow in color, with brown heads. A woodchip and frass covered cocoon is formed at this time. Once ready to emerge, the pupa wiggles through a hole in the cocoon and the in the bark, leaving the pupal shed skin stuck halfway out of the host plant as the adult, wasp-like moth emerges. A single generation occurs per year.
The early signs of infestation may look similar to drought stress - leaves may become pale green, olive in color, and eventually chlorotic. Bark of trunk and branches of rhododendron and occasionally mountain laurel and deciduous azaleas, particularly those growing near heavily infested rhododendrons, is impacted. Larvae of this clear-winged moth bore into the stems, often near the base of the host plant. Infestation is usually not detected until branch or plant death occurs, and may be most visually apparent in the fall. Search dwarfed or smaller branches for holes, particularly near branch crotches, scars, or other old injuries. At certain points in the growing season, these holes may contain shed or cast pupal skins that extend halfway from the holes. Sawdust-like frass may accumulate in these areas or on the ground beneath infested plants. Loose bark covering length-wise scars may indicate past infestation. Heavily infested branches may break. Fast-growing rhododendrons may be more susceptible to damage done by this insect.
Pheromone traps can be used to help time chemical management sprays for adults, and these may be commercially available. After the first male rhododendron borer moth is detected in the trap, wait approximately 1 week and then plan to apply chemicals management options according to label instructions.
If wilting stems are seen, prune out and destroy heavily infested branches to reduce the population of this insect on a single plant or planting.
Ichneumonid parasitoid wasps are noted to impact rhododendron borer populations, along with woodpeckers acting as predators (Engelhardt,1946).
Acephate (NL)
Bifenthrin (NL)
Chlorantraniliprole (NL)
Chlorpyrifos (larvae) (N)
Cyantraniliprole (NL)
Permethrin (L)
Active ingredients that may be applied systemically include: acephate (injection), chlorantraniliprole (soil drench), and cyantraniliprole (soil drench, soil injection).
When used in a nursery setting, chlorpyrifos is for quarantine use only.
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.
