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Dogwood, Rhododendron & Viburnum Clearwing Borers

Rhododendron borer damage to azalea. Note frass pellets and pupa in middle of crotch.


Dogwood borer male.

Viburnum borer adult and pupal skins protruding from burrows.

Species:

Dogwood borer - Synanthedon scitula (Harris); rhododendron borer - S. rhododendri (Beutenmuller); viburnum clearwing borer - S. viburni Engelhardt (& possibly the lesser viburnum clearwing - S. fatifera Hodges) [Lepidoptera: Sesiidae]

Distribution:

These clearwing borers are generally distributed in North America east of the Mississippi River. Most damage is reported from the northeastern states and southeastern Canadian provinces.

Hosts:

The dogwood borer attacks a wide range of hosts - apple (especially grafted stock), birch, beech, elm, oak, chestnut, hickory, pecan and dogwood. It is most famous as a pest of flowering dogwood. The rhododendron borer attacks rhododendron, mountain-laurel and azalea. It may occasionally attack deciduous azalea where these are grown near standard rhododendrons. The viburnum clearwings attack viburnum, especially cranberrybush.

Damage:

The larvae tend to feed within the sapwood of host plants, usually in a zone at or just above the soil level. The dogwood borer commonly infests the callous tissues formed at graft junctions of grafted tree stock. Small irregular patches of bark break off, and the larvae commonly push out sawdust like pellets of frass. If the larvae completely burrow around the sapwood of a host plant before the vascular tissues can be reconnected, sudden death from girdling occurs. Repeated attacks commonly cause shoots to die and break out of the center of the plant.

Description and Life Cycle:

The adult moths are generally 1/2 to 5/8-inch long with 3/4 to 1-inch wingspans. They are generally black with a bluish iridescence and the abdomens may have narrow yellow bands, depending on the species and sex. The wings are completely clear except for a band of bluish-black scales on the front margin, tip and along some of the major veins. The larvae are creamy white with reddish-brown head capsules. Mature larvae are 3/4 to 1-inch long. The pupae are formed in a burrow just under the bark, usually near a hole in the bark. Upon emergence, the pupal exoskeleton often remains protruding from the hole in the bark.

Adults of these clearwings generally begin emergence in late May and their peak flight is usually June into early July. However, adults may continue to emerge through August. Dogwood borer adults live for a week to 10 days while the rhododendron and viburnum clearwing appear to live only two to four days. The females prefer to attach their eggs to sites of previous damage or in the folds of branch crotches. The larvae burrow into the bark, and over a few weeks end up in the top layers of sapwood. Here, the larvae continue to excavate an irregular burrow followed by a trail of pellet-like frass. The larvae remain dormant during the winter months and finish feeding the next spring. The pupae are formed within the burrow and are usually covered with frass pellets.

Control Hints: As with most borers, these clearwings tend to infest plants that are under stress (dogwoods or rhodendorns in full sun), have damage to the bark or have callous tissue growth (caused by graft junctions or low pruning scars).

Option 1: Cultural Control - Plant Health Care - Plant dogwoods, rhododendrons and viburnum in appropriate sites recommended by horticulturalists. Try to avoid damaging the lower stems (weed whacker damage, scraping of the bark during application of mulch, etc.). Fertilize the plants lightly, not excessively. For viburnum, remove old stems and encourage new, young growth.

Option 2: Chemical Control - Protectant Insecticide Sprays - Application of a long residual insecticide to the bark of susceptible plants has been the traditional strategy for control of these clearwing borers. Such sprays should be applied when the first adults are trapped in pheromone traps. Most of the residual materials remain active in the bark of the plants for 30 to 50 days. Reapplications may be needed if adults are still active 30 days after the initial application. Thoroughly wet the lower stems and trunk, being sure to concentrate the spray around previous damage and branch crotches.

Option 3: Chemical Control - Systemic Insecticides - Few root systemic insecticides remain on the market for control of clearwing borers. However, if such products are found, they can be applied in mid-May in order to be present in the plant when the new brood of larvae begin to burrow into the plant. Applications of systemics to control larger larvae have not been overly successful.