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Synanthedon rhododendri

Adult rhododendron borer moth. Photo: John W. Neal Jr., USDA - ARS.
Scientific Name: 
Synanthedon rhododendri
Common Name: 
Rhododendron Borer
Growing Degree Days (GDD's): 
192–298 GDD's; 533–707 GDD's, Base 50F, March 1st Start Date (Source: Cornell Cooperative Extension and Robert Childs, UMass Extension.)
Host Plant(s) Common Name (Scientific Name): 
Azalea (Rhododendron spp.)
Mountain laurel (Kalmia latifolia)
Rhododendron (Rhododendron spp.) *Preferred host.
Insect Description: 

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 similiar 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, wasplike moth emerges. A single generation occurs per year. 

Damage to Host: 

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.

Cultural Management: 

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.

Natural Enemies & Biological Control: 

Ichneumonid parasitoid wasps are noted to impact rhododendron borer populations, along with woodpeckers acting as predators (Engelhardt,1946).

Chemical Management: 

Abamectin (NL)

Acephate (NL)

Acetamiprid (L)

Azadirachtin (NL)

Beauveria bassiana (NL)

Bifenthrin (NL)

Chlorantraniliprole (NL)

Chlorpyrifos (larvae) (N)

Cyantraniliprole (NL)

Flonicamid+cyclailiprole (N)

Horticultural oil (L)

Indoxacarb (L)

Insecticidal soap (NL)

Isaria (paecilomyces) fumosoroseus (NL)

Malathion (L)

Neem oil (NL)

Permethrin (L)

Pyrethrins+sulfur (NL)

Spinosad (NL)

Tebufenozide (NL)


Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirachtin (injection, soil drench), chlorantraniliprole (soil drench), cyantraniliprole (soil drench, soil injection), and neem oil (soil drench).

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.

Read and follow all label instructions for safety and proper use. If this guide contradicts language on the label, follow the most up-to-date instructions on the product label. Always confirm that the site you wish to treat and the pest you wish to manage are on the label before using any pesticide. Read the full disclaimer. Active ingredients labeled "L" indicate some products containing the active ingredient are labeled for landscape uses on trees or shrubs. Active ingredients labeled "N" indicate some products containing the active ingredient are labeled for use in nurseries. Always confirm allowable uses on product labels. This active ingredient list is based on what was registered for use in Massachusetts at the time of publication. This information changes rapidly and may not be up to date. If you are viewing this information from another state, check with your local Extension Service and State Pesticide Program for local uses and regulations. Active ingredient lists were last updated: January 2024. To check current product registrations in Massachusetts, please visit: .