Moths of North Carolina
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6 NC Records

Synanthedon rhododendri (Beutenmüller, 1909) - Rhododendron Borer Moth

Synanthedon rhododendriSynanthedon rhododendriSynanthedon rhododendriSynanthedon rhododendri
Taxonomy
Superfamily:
Sesioidea		 Family:
Sesiidae		Subfamily:
Sesiinae		Tribe:
Synanthedonini		P3 Number:
55a0092		 MONA Number:
2551.00
Comments: North America has 136 or more species in the family Sesiidae, and the large genus Synanthedon constitutes around half of the 37 species found in North Carolina, many being similar in appearance to one another. Some sesiids, known broadly as clearwing borers, are significant pests of commercial crops. Almost all are mimics of wasps and hornets.
Identification
Field Guide Descriptions: Online Resources: MPG, BugGuide, iNaturalist, Google, BAMONA, GBIF, BOLDTechnical Description, Adults: Eichlin and Duckworth (1988)Technical Description, Immature Stages: Neal (1982)                                                                                 
Adult Markings: This species is easy to distinguish from our other Synanthedon species based on its small size (forewing length = 5-7 mm) and dark abdomen with yellow bands on segments 2, 4 and 5. The following detailed description is based on that of Engelhardt (1946).

The antenna is black-violaceous and the labial palp black above and yellow beneath. The head is predominantly black, except for a lustrous green frons and a face that is well-marked with white. The collar is black on top and white along the sides and beneath. The thorax is black with steel-blue reflections and has a broad patch of shiny white or pale yellow on each side beneath. The abdomen is lustrous steel blue or coppery black, and with segments 2, 4, and 5 narrowly edged with yellow. Each side has a narrow yellow stripe from the base to the band on segment 2, while on the underside, segments 3, 4, 5, and 6 are whitish yellow. The fan-shaped anal tuft is lustrous black and touched with yellow to yellowish-white along the sides. The underside is black with yellow in the middle. The coxa of the foreleg is shiny white or pale yellow, with the femora steel blue or purplish-blue. The tibia of the hindleg is pale yellow at the spurs and violaceous-black between the spurs, while the tarsi are pale yellow and shaded with black above. The forewing is transparent, with the costa, margins, discal mark and inner veins violaceous-black. The outer margin between the veins is golden yellow, and the fringes rusty black. On the underside, the costa is heavily scaled with yellow, but otherwise as above. The hindwing is transparent and narrowly margined with shiny black, while the fringes are paler, with a mixture of black and yellow. The females resemble the males, except that abdominal segments 2, 4, and 5 are more broadly banded with yellow, and the short, rounded anal tuft Is heavily mixed with pale yellow inwardly from the sides.
Wingspan: 10-15 mm (Engelhardt, 1946).
Forewing Length: 5-7 mm (Eichlin and Duckworth, 1988).
Adult ID Requirements: Identifiable from good quality photos of unworn specimens.
Immatures and Development: The larvae are borers in both native and ornamental rhododendrons, and to a much lesser extent in Mountain Laurel. The summary of the life history that follows is based mostly on studies by Neal (1982) at two commercial rhododendron nurseries in Maryland where the larvae attack ornamental varieties of Rhododendron. The larvae pupate within stems or small branches, and the adults emerge during the morning hours while the humidity remains high. Both sexes are sexually mature at emergence and the females mate on the first day. After mating, the female moves into the inner canopy of the plant and seeks sites for egg laying. The eggs are tucked deeply into cracks and bark crevices, and frequently where there are old pruning scars, narrow V-crotches, scars where the inflorescence was broken off while still green, and near old larval feeding galleries and protruding pupal skins. Eggs that were incubated at 25°C hatched in 11 to 13 days. Upon hatching, the larva immediately chews into and enters the bark, and typically where there are roughened bark areas near old wounds, or old borings with new callous tissue.

The larvae commonly bore in the central pith of small branches and twigs where they dig long tunnels that are filled with small, reddish frass. The tunnels also serve for overwintering and for pupation in spring (Engelhardt, 1946). They larvae will also tunnel under the bark on older and larger parts of the plant. Feeding continues during the warmer months of the year until the onset of severe cold weather. The larvae then overwinter when nearly mature in a light, silken chamber. Feeding resumes in early to mid-March with the spring warm-up, as indicated by large amounts of fresh frass and wood particles at the base of infested plants. In Maryland, the larvae continue to feed until late April or early May, at which time they chew their way to a site under the bark that is suitable for emergence. The sides of the gallery at the pupation site are enlarged and the walls are made very smooth. The mature larva, which is milky white with a brown head, leaves a thin outer layer of bark intact that serves as an escape window, then pupates in a thick cocoon that is composed on silk and wood particles. The pupa averages 6-10 mm in length, is yellow-tan, but darkens to black prior to adult emergence. Just before emergence, the pupa cuts through the cocoon and window and protrudes through the escape hole. The adult emerges shortly thereafter by rupturing a thin suture along a median line on the dorsum of the head of the pupal skin.
Larvae ID Requirements: Identifiable from good quality photos, especially where associated with known host plants.
Distribution in North Carolina
Distribution: Synanthedon rhododendri is endemic to the eastern US where it occurs from New Hampshire and Massachusetts southward through Pennsylvania, Maryland, West Virginia, and Virginia to eastern Tennessee, western North Carolina and northern Georgia. The range extends westward in the northern part of the range to northeastern Ohio. As of 2024, all of our records are from the Blue Ridge, with elevations ranging from the lower valleys to Mt. Mitchell.
County Map: Clicking on a county returns the records for the species in that county.
Synanthedon rhododendri
Flight Dates:
 High Mountains (HM) ≥ 4,000 ft.
 Low Mountains (LM) < 4,000 ft.
 Piedmont (Pd)
 Coastal Plain (CP)

Click on graph to enlarge
Flight Comments: The adults have been observed from May through July in different areas of the range. Neal (1982) found that two local populations that he tracked with pheromone traps in Maryland had a flight season of around 5-6 weeks, while Jackson (2002) trapped specimens from mid-May to late-July in eastern Tennessee. As of 2024, our records range from early-June through mid-July.
Habitats and Life History
Habitats: Local populations rely heavily on native rhododendrons for successful reproduction and can be found in mesic habitats in the Blue Ridge from lower elevation sites to the highest peaks. They also will use ornamental varieties of rhododendrons in nurseries and residential settings.
Larval Host Plants: Native and ornamental rhododendrons (Rhododendron spp. and their hybrids) appear to be the primary hosts (Forbes, 1923; Craighead et al., 1950; Baker, 1972; Eichlin & Duckworth, 1988). Neal (1982) noted that the larvae will also use Mountain Laurel (Kalmia latifolia) and deciduous azaleas when grown with heavily infested rhododendrons in nurseries, but it is uncertain if these hosts are used in natural settings. - View
Observation Methods: The adults are diurnally active and do not come to lights or bait. They are occasionally seen visiting flowers or resting on vegetation, but are most easily obtained by using pheromone traps.
Wikipedia
See also Habitat Account for Montane Rhododendron Thickets and Balds
Status in North Carolina
Natural Heritage Program Status:
Natural Heritage Program Ranks: [GNR] S1S3
State Protection: Has no legal protection, although permits are required to collect it on state parks and other public lands.
Comments: Synanthedon rhododendri appears to be rare or uncommon in North Carolina where it approaches the southern limit of its range and is restricted to the Blue Ridge. Populations in the southern Appalachians may be disjunct from the main range that occurs farther north. More information is needed on its distribution, abundance and host use before we can accurately assess its conservation status within the state.

 Photo Gallery for Synanthedon rhododendri - Rhododendron Borer Moth

Photos: 6

Recorded by: Jim Petranka on 2022-06-23
Madison Co.
Comment: Seven individuals collected with wing length ca. 7-8 mm.		Recorded by: Jim Petranka on 2022-06-23
Madison Co.
Comment:
Recorded by: Jim Petranka on 2020-06-08
Madison Co.
Comment: 		Recorded by: Jim Petranka on 2020-06-08
Madison Co.
Comment:
Recorded by: Jim Petranka and Becky Elkin on 2019-06-30
Madison Co.
Comment: 		Recorded by: Jim Petranka and Becky Elkin on 2019-06-30
Madison Co.
Comment: