NC State Extension Publications

Description and Biology

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The adult rhododendron borer moth, Synanthedon rhododendri, is a small, black, slender moth with three narrow yellow bands across the abdomen. They are active in May and June. These moths lay up to 40 eggs each on the bark of rhododendrons, azaleas, and mountain laurels from which hatch tiny caterpillars that bore into the bark. These small caterpillars then bore into the sapwood and may cause the infested stem to wilt although larger stems may not. As the borers mature they grow to almost ½ inch long and are yellowish white with brown heads. The following spring, rhododendron borers cut exit holes that they plug with frass. They then spin a silk cocoon in the tunnel that incorporates some frass and bits of wood and pupate. Pupae are brown and slender. Two weeks or so later the next year's generation of moths emerges. We have one generation of rhododendron borers in North Carolina.

Moths of rhododendron borers

Moths of rhododendron borers are small, wasp-like insects.

Male rhododendron borer moths (left) are slightly smaller than f

Male rhododendron borer moths (left) are slightly smaller than females (right).

rhododendron borer

The rhododendron borer is a small, whitish caterpillar.

Host Plants

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Rhododendron borers infest rhododendrons, azaleas (especially deciduous azaleas), and mountain laurels. Sometimes the first indication that a plant is infested is that infested stems break off when brushed against. Fast growing cultivars are especially vulnerable to the borers.

Stems infested with rhododendron borers often break off easily.

Stems infested with rhododendron borers often break off easily.

Residential Recommendations

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Prune out and destroy heavily infested parts of the plants whenever wilting stems are noticed. Rhododendrons can be protected from further infestation by rhododendron borers by spraying the stems with one of the pyrethroid insecticides labeled for home landscape use. These are available in most big box stores, plant centers and nurseries. The first application should be made in mid April, May, or the first weeks in June. The most accurate way to time an application would be to use a pheromone-baited trap to lure males. When the first males are noticed, wait a week and then spray. Pheromone baits and traps are available through Gemplers (http://www.gemplers.com), Arbico Organics (http://www.arbico-organics.com) and other vendors.

References

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For assistance with a specific problem, contact your local Cooperative Extension Center.

This Factsheet has not been peer reviewed.

Author

Professor Emeritus
Entomology and Plant Pathology

Publication date: Jan. 23, 2017
Revised: Oct. 11, 2019

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