Clearwing borers (Lepidoptera: Sesiidae) are common and destructive pests of woody plants throughout the continental United States. Up until the 1980's, clearwing borer (Synanthedon culiciformis (L.)) damage was not commonly encountered in Alaska even though clearwing borers are probably present throughout Alaska wherever birch is present. As of 1989, borer damage has been more prevalent in south-central Alaska, especially the Anchorage area. Borer damage is seldom encountered in undisturbed forested areas. Expanding urbanization and increased use of birch as an ornamental, however, has led to an increase in clearwing borer damage.
Adult borers are day-flying moths which mimic bees and wasps in their coloration (Cover photo). Although they cannot sting, this coloration provides protection against birds and other predators. Adult borers seek out stressed
Figure 1. Mature clearwing borer larvae
and/or recently wounded birch or alder and lay eggs near trunk wounds or in bark crevices. Young borer larvae bore into the tree. Once beneath the bark, borers are seldom detected until serious damage has been done.
This leaflet describes the general biology, damage, and control techniques for these borers. Management practices which help minimize borer problems in landscape trees are discussed.
LIFE HISTORY: Adult clearwing borers fly in late spring through midsummer and are most active during the warmer daylight hours. Adults occasionally visit flowers to feed on nectar. The adult life cycle is short, generally lasting a few weeks at most. Clearwing moths rarely injure healthy trees in their native environment; however, when trees such as birch are used as landscape trees, environmental stress factors such as soil compaction, moisture deficiency, and wounding can weaken them. Such trees are more susceptible to borer attack. Adults are attracted to fresh wounds near the base of the tree. Eggs are laid singly near these wounds. Young larvae hatch after 1-2 weeks and immediately bore through the bark to feed on the nutritious phloem tissue of the tree (Figure 1). As many as thirteen larvae have been encountered feeding beneath a two-inch-long wound. Larvae continue to grow throughout the summer and hibernate beneath the bark through the winter. The following spring, larvae transform, via a pupal stage, into adult moths which then emerge and begin another life cycle. Wounded areas on a birch can be attacked in successive years, resulting in almost complete girdling of the tree (Figure 2).
DETECTION AND DAMAGE: Infestation sites are often marked by scars, callus formation, and areas of cracked or loose bark. In the spring, borer larvae expel brownish-red frass (mixed fecal and woody boring material) which accumulates around the wound, in bark crevices, or at the base of the tree (Figure 3). Empty, tan-colored pupal skins, left partially protruding from the bark by emerging adults, are a sure sign that borers are present. Older trees may persist in an unhealthy condition and be reinfested in successive years. Such trees may exhibit off-color foliage and/or dieback of the crown.
Damage to infested trees occurs in several ways. First, the destruction of phloem by larval feeding can seriously reduce the tree's ability to transport nutrients from the leaves downward. Second, the tree becomes stressed and is more susceptible to additional insect and disease attacks. Wounds which are infested and reinfested by clearwing borers do not heal, and provide entries for decay-causing fungi. Trees which have been infested for consecutive years are quite susceptible to wind breakage (Figure 4).
Figure 3. Clearwing borer frass expelled from tree wound
Figure 4. Wind breakage of birch which had been heavily infested with clearwing borers
CONTROL MEASURES: Problems with clearwing borers can be minimized with proper shade tree management. To ensure good tree vigor, the following steps should be taken:
Fertilization helps promote vigor. The Cooperative Extension Service recommends 1-2 pounds of fertilizer for each inch of trunk diameter. Any complete lawn or garden fertilizer that is high in phosphorus is adequate. Regular watering and controlling other pests such as aphids and caterpillars maintains tree vigor.
Prevention of bark injuries greatly reduces the risk of borer infestation. Use lawn mower guards and remove grass and weeds by hand rather than risk bark injuries from string trimmers. If wounding occurs, an application of pruning seal may prevent borer infestation.
Remove heavily infested trees which serve as reservoirs from which borers emerge to reinfest nearby trees. Once a tree has been infested, control is difficult. A large birch can probably withstand one season of attacks. If symptoms of attack are apparent, a persistent contact insecticide may be applied to the lower tree trunk to prevent additional attacks. Timing of insecticide treatment is critical to ensure proper protection. Chemical treatment should be applied no later than mid-May.
Inspect susceptible trees regularly. Apply preventive insecticides no later than mid-May. Check with your local Cooperative Extension Service for current registered pesticides. Please read all label information carefully and apply the product in a safe manner.
Clearwing Birch Borers, by Edward H. Holsten, Entomologist, State and Private Forestry, Anchorage, AK, and Fred Sorensen and Beth Schulz-Blitz, Pest Scouts, Cooperative Extension Service, Anchorage, AK.
Additional information on this insect can be obtained from your local USDA Cooperative Extension Service office, Alaska State Forestry office, or from:
Forest Health Protection
State and Private Forestry
USDA Forest Service
3301 C. Street, Suite 202
Anchorage, AK 99503-3998.
Phone: (907) 743-9455
Forest Health Protection
State and Private Forestry
USDA Forest Service
2270 Sherwood Lane, Suite 2A
Juneau, AK 99801
Phone: (907) 586-8811
CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants, and fish or other wildlife-if they are not handled or applied properly. Use all pesticides selectively and carefully. Follow recommended practices for disposal of surplus pesticides and pesticide containers. Mention of a pesticide in this publication does not constitute a recommendation for use by the USDA, nor does it imply registration of a product under the Federal Insecticide, Fungicide, and Rodenticide Act, as amended. Mention of a proprietary product does not constitute an endorsement by the USDA.
