Dogwood borer
Synanthedon scitula (Harris)
From: 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.
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.
Damage attack at rootcollar. James Solomon,
USDA Forest Service. |
Adult(s) female. James Solomon,
USDA Forest Service. |
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