Synanthedon scitula

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Author: Eric Day, Virginia Tech

3066008
Taxonomy
Kingdom: Animalia
Phylum: Arthropoda
Class: Hexapoda (including Insecta)
Order: Lepidoptera
Family: Sesiidae
Genus: Synanthedon
Species: scitula
Scientific Name
Synanthedon scitula
(Harris)
Common Names

dogwood borer

Contents

Plants attacked

Dogwood, pecan, elm, hickory, and willow

Description of damage

Larvae feed in the inner bark of live, healthy dogwood trees. The damaged area of the trunk or branch swells and eventually the bark will fall off. Leaves turning red prematurely in mid-summer on a lone branch are an early sign of dogwood borers. Infested branches and limbs will die. Dogwood borers often will not kill the tree in the first year, but reinfestation in successive years will.

Identification

Dogwood borer larvae are white with pale brown heads and they bore in the cambium of dogwood, gaining entrance under rough bark or through wounds. Late-summer adults are clear wing moths and somewhat resemble wasps. The body is blue with yellow stripes and yellow-banded legs.

Life History

Dogwood borers overwinter as larvae under the bark. The larvae usually pupate in the spring and emerge in late May and June, but may pupate and continue to emerge throughout the summer. Adults are active from May until September. Adults mate and eggs are laid on wounds or old borer injuries. After hatching, the small, light-colored larvae wander over the branches until an entrance point is reached. Tunneling is confined to the cambium and bark. The larvae, in all stages, overwinter. There is only one generation per year.

Control

Insecticides applied to the trunk and damaged areas beginning in mid-May provide effective control. The treatment should be repeated at 6-week intervals 2-3 times. A fine wire inserted into the entry hole will sometimes kill the larvae.

In addition, some cultural practices will reduce borer injury. Regularly watered and fertilized trees are able to maintain vigor and are less susceptible to borer injury. Avoid pruning during the summer months when the moths are laying eggs and avoid wounding or injuring the tree. See the Virginia Pest Management Guides for Home Ornamental Plants or for Nursery Ornamentals for specific control recommendations. If pest management guides are not available, see your Extension agent for specific recommendations in your area.

Remarks

Special care should be taken to treat all wounds or injuries to dogwoods, etc., to prevent infestations with borers attracted by the resinous smell released from the wounds.

External Links


Hosts

Dogwood, pecan, hickory, oak, chestnut, beech, birch, black cherry, elm, mountain-ash, viburnum, willow, apple, loquat, ninebark, bayberry. It is a notorious pest on flowering dogwoods and pecans and is also extremely adaptable (more so than any other species in the family Sesiidae) to different unrelated food plants, including deciduous trees, shrubs, and occasionally vines (Engelhardt 1946).

Range

Generally distributed from southeastern Canada throughout the eastern United States westward to Texas and Minnesota (Engelhardt 1946).

Description

Adult

Bluish black and yellow clearwing moth. Forewings narrow and nearly devoid of scales, except dorsally where larger veins are marked with black scales. Body length of adults 8 to 10 mm and wingspan 16 to 18 mm. Overall body color variable, but generally dark blue to black, with second and fourth abdominal segments yellow dorsally (additionally yellow banding in southern populations). Femora dark but remaining leg segments mostly yellow. Noticeable anal tufts marked laterally with yellow. Egg. Pale yellow, elliptical, about 0.5 mm long by 0.4 mm wide, covered with fine reticulations.

Larva

Cream colored with reddish brown head and resinous appearance. Prothoracic shield with two dorsal reddish brown spots. Larvae from 1 mm or less at hatching to 15 mm when mature (Pless and Stanley 1967).

Pupa

Brown; remains inside frass-covered cocoon under bark until adult emerges.

Biology

Adults emerge over a 4-month period. Emergence begins in March (Engelhardt 1946) in the extreme South, in late April in eastern Tennessee (Pless and Stanley 1967), in mid-May in Virginia (Underhill 1935), and late in May in Connecticut (Schread 1965). Emergence continues through September. Adults live 7 to 9 days. Eggs frequently are laid next to wounds or on frass produced by other borers (Pless and Stanley 1967). Newly hatched larvae are small, fragile, and very sensitive to low humidity; many die from desiccation before locating suitable niches. Young larvae can move only short distances, usually seeking wounds, fresh grafts, and mines of other borers to become established, although some successfully burrow in uninjured sites. Small larvae may feed for several weeks in the bark before reaching the cambium. Throughout development (six instars), larvae feed in an irregular course in the cambium. They may etch the surface of the sapwood but excavate no galleries in it. Generally, only one larva occupies a gallery; when more than one is present, cannibalism may occur. Because larvae are present in trees throughout the year, some entomologist have suggested more than one brood per year, though most report one generation. Pupation normally occurs just beneath the bark within the larval mines and last 8 to 12 days. Pupae develop in cocoons made of frass and bark particles held together with silken strands.

Injury and Damage

Presence of borders usually is indicated by sapstain and fine frass on the trunk and branches in late summer (Coleman 1968). Sloughing of loose bark is another early symptom of attack (Johnson and Lyon 1988). By fall and winter, coarse brown frass is extruded from gallereis. Removal of outer bark reveals larval burrows in the cambium. Infested trees often have swollen, knotty, callused, or gall-like areas on the lower trunk (Schread 1965). Borer injuries sometimes are prevalent at the juncture of trunk and primary branches or smaller twigs and branches. After 1 year of infestation, dead bark over galleries begin to peel, exposing the wood (Pless and Stanley 1967). These borers often infest abnormal growths on stems and branches, such as insect galls, disease-caused galls and cankers, and mechanically caused wounds (Engelhardt 1946). In Kentucky, infestation increased significantly with exposure to sunlight and stem wounds (Potter and Timmons 1981), suggesting why dogwoods in the forest understory are much less subject to attack than open-growth ornamentals. Badly infested trees usually appear unhealthy and may have dieback in parts of the crown and sprouts near the groundline. Small brown pupal skins may protrude from bark and from frass-covered cocoons beneath the bark from spring to fall. Before the 1930's, when this species was known as the pecan-tree borer, it was so destructive to buds that is seriously hindered efforts to reproduce pecan varieties vegetatively. It destroys much of the cambium and callus in grafted and budded pecans, preventing the union of scion and stock. Feeding larvae reduce leaf area, change leaf morphology, and hasten leaf senescence in flowering dogwood (Heichel and Turner 1973). Thousands of dogwoods in Tennessee nurseries have been rendered worthless by one generation of this insect (Pless and Stanley 1967). In Virginia, 4,000 dogwoods were killed or badly damaged in nurseries over 4 years (Underhill 1935). In New York, 30 apple trees in 83 orchards were infested (Riedl and others 1985).

Control

Internal insect parasites are important natural enemies. Up to 50% of larvae were reported to be parasitized by the braconid wasp, Apanteles sesiae Viereck (Underhill 1935). Other insect parasites include Agathus buttricki Viereck, Hyssopus sanninoideae Girault, Microbracon mellitor Say, M. sanninoidae Gahan, Phaeogenes ater Cresson, and Scambus (Itoplectis) conquisitor Say. A fungus, Cordyceps sp., has been found but is not prevalent. Predators, including birds, are of some value as natural controls. Excessive sap flow in spring kills many young larvae (Underhill 1935), and both larvae and pupae are highly susceptible to desiccation during drought (Pless and Stanley 1967). Control on dogwood is not practical or economically feasible in forests. On ornamental dogwoods, monthly applications of insecticide to trunks and lower branches, from late April to mid-September, prevent attack (Coleman 1968). Cultural practices that keep trees vigorous and free of bark injuries are most important.

Gallery

3066009
Photo by James Solomon, USDA Forest Service, Bugwood.org

View in Bugwood Image Database

References

Solomon, J.D. 1995. Guide to insect borers of North American broadleaf trees and shrubs. Argic. Handbk. 706. Washington, DC: U.S. Department of Agriculture, Forest Service. 735 p.

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