Dogwood Borer Synanthedon scitula (Harris)

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Superorder	Holometabola
Order	Lepidoptera
Superfamily	Sesiodea
Family	Sesiidae
Subfamily	Sesiinae
Tribe	Synanthedonini
Genus	Synanthedon

Scientific Name

Synanthedon scitula

Common Name

dogwood borer

Solomon, J. D.; Payne, J. A. A guide to the insect borers, pruners, and girdlers of pecan and hickory. Gen. Tech. Rep. SO-64. New Orleans, LA: USDA Forest Service. Southern Forest Experiment Station; 1986. 31 p.

Importance

Although the dogwood borer, Synanthedon scitula (Harris), is perhaps best known for its economic damage to the flowering dogwood for which it is named, it sometimes causes serious damage to pecan (Herrick 1904, Moznette and others 1931). It has also been found attacking hickory, oak, and many other species. It is known from southeastern Canada southward over the Eastern United States west to Texas. Damage results from the larvae feeding just beneath the outer bark in the phloem and cambium. Individual branches and even young trees may be completely girdled and killed, but most often the stem is only partially girdled or patches of bark killed. Attacks may occur at any point on the trunk and branches of trees of all sizes. Trees that are wounded, diseased, or in poor vigor are most susceptible to attack. It often attacks grafted and budded trees, destroying much of the cambium and callus tissue and preventing the union of scion and stock. Topworked trees have suffered serious damage in the past.

Description

The adult is a delicate bluish-black clearwing moth with yellow markings on the thorax, yellow-banded legs, and yellow stripes on segments two and four of the abdomen (fig. 3A) (Herrick 1904, Pless and Stanley 1967). Wings are transparent with bluish-black margins and have a wingspan of 14 to 20 mm. Eggs are pale yellow to brown, elliptical in shape, and about 0.5 mm long and 0.4 mm in diameter. Larvae are off-white to cream colored with reddish -brown heads, and range in size from less than 1 mm when newly hatched to 14 mm when mature (fig. 3B). The prothoracic shield has two dorsal yellowish-brown spots. The pupa is brown and remains inside a cocoon until time for adult emergence.

Evidence of Infestation

The most easily recognized early sign of attack is the presence of fine frass that has been extruded from the tunnels through small openings in bark crevices (Herrick 1904, Underhill 1935). By cutting away the outer bark, larval tunnels and feeding larvae can be exposed (fig. 3B). When inspecting trees for damage, it is important to examine the lower trunk of young trees, especially around and just above the groundline. Trees wounded, diseased, or in poor vigor are most susceptible to attack and should be inspected frequently. After trees have been infested for a year or so, the dead bark over the borer tunnels begins to loosen and break away, sometimes exposing the wood beneath. Trees that are badly infested may have an unhealthy appearance, exhibit dieback of branches in parts of the crown, and send out sprouts from near groundline. Mortality occurs if trees are completely girdled. Graft failures should be inspected closely for evidence of infestation. Frass around fresh wounds and points of grafting and budding can mean the beginning of an infestation. Small brown pupal skins may be found protruding from the bark from April to October (fig. 4B). Exposure of infested sites by prying the bark loose frequently reveals one to a dozen or more larvae of various sizes feeding close together in localized tunnels.

Biology

Adults emerge from April to October with the greatest number appearing in the early part of the year (Pierce and Nickles 1941, Pless and Stanley 1967). Eggs may be deposited on the bark anywhere along the trunk or brances, although most are placed close to wounds, on or adjacent to frass, near openings produced by other borers, and around grafts or buds. The eggs hatch in 8 to 9 days. Newly hatched larvae are small, fragile, and very sensitive to humidity; many die from dessication before locating a suitable niche to begin boring. The newly hatched larvae are able to move short distances and usually seek wounds, fresh grafts, and tunnels of other borers to become established, although a few are able to successfully establish burrows at uninjured sites. Larvae make irregularly shaped tunnels or burrows that spiral upward in the bark and cambium. Although the sapwood may be etched, no burrows are made into the sapwood. Larvae of various sizes overwinter in their burrows and are able to withstand a wide range of temperatures. The following spring the immature larvae continue feeding, while the mature ones pupate. The mature larvae usually prepares a cocoon of frass and silken threads in the burrow beneath the outer bark for pupation. Pupae within cocoons are rarely found in the leaves and trash around the base of the tree. The pupal stage lasts 8 to 12 days. There is one generation per year, with some evidence of a second generation in its southern range.

Control

Except under conditions favorable for the borers, natural enemies usually keep infestations in check (Pless and Stanley 1967, Underhill 1935). Parasities that have been recovered are four braconids, Apanteles sesiae Viereck, Agathis buttricki Viereck, Microbracon sanninoidae Gahan, M. mellitor Say, two ichneumonids, Phaeogens ater Cress., Scambus (Itoplectis) conquisitor Say, and one eulophid, Hyssopus sanninoidae Gir. In some cases, up to 50 percent of the larvae are parasitized. A disease caused by the fungus Cordyceps sp. has been found but is not prevalent. Woodpeckers excavate a small number of larvae. Adhering to recommended cultural practices for keeping trees vigorous and free of bark injuries is important in preventing serious damage due to dogwood borers. Direct control is seldom needed in natural stands but is sometimes needed in groves, nurseries, and ornamental trees (Pierce and Nickels 1941).