Southeastern Apple Production
ENTOMOLOGY
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Southeastern Apple ProductionENTOMOLOGY |
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The dogwood borer (DWB) is the larva of a small clearwing moth that infests burr knots and trunk bases. Fully grown larvae are about 1/2 inch long and vary from white to light pink with a dark brown head capsule. Adult moths bear a striking resemblance to small wasps, with a black and yellow color pattern and a wingspan of about 1 inch. The yellow band on the fourth abdominal segment is much wider on the female than on the male. Eggs are small, ovoid, ridged, and usually laid singly in bark crevices.
 Larva |
 Adult |
 Adult |
DWB overwinter as nearly mature larvae. A cocoonlike hibernaculum, in which the larva remains during the winter, is formed inside the feeding gallery. In the spring the larva feeds for a short time and then spins a silken cocoon, which is covered with frass (debris or excrement) just below the surface of the burr knot. Adults emerge about 25 days later, and may be present from May through July. Mating occurs a short time after the adults emerge from the cocoons. Females lay eggs on or near burr knots, showing a preference for sites previously infested. Eggs hatch in 8 or 9 days and larvae enter the burr knot.
There is ongoing research into the specific life cycle of DWB in the southeastern US, as it is currently unclear how many generations occur in the region and how populations can be best monitored and controlled.
DWB larvae feed almost exclusively within burr knots and trunk bases. Burr knots are aggregations of partially developed root initials that usually occur in clusters at or below the graft union. The Malling and Malling-Merton series of rootstocks have a tendency to produce burr knots.
The first indication of a DWB infestation is an accumulation of frass, which appears as a reddish brown sawdustlike material on the surface of burr knots. At times, golden brown pupal cases (exuviae) may be found protuding from the burr knot. This is a sign of adult emergence.
Feeding within the burr knot does not harm the tree directly. It may produce an entry for disease organisms but otherwise causes little or no damage. Occasionally, the larvae may begin to feed under the bark outside the area of the burr knot, which can girdle the tree and cause death. In most cases continuous infestation by DWB larvae for several years may cause a slow decline of the tree and reduced yields.
 Burr
knot |
 Frass at gallery entrance |
 Larva
at gallery entrance |
 Exuviae |
Pheromone traps can be used to monitor adult emergence. Traps should be placed about 4 feet above the ground - catches are significantly reduced if placed much higher or lower. (Many commercial DWB lures are not fully effective and may catch other species; a current research project is isolating the DWB pheromone and will result in more effective lures.)
Burr knots can be controlled with plant growth regulators such as NAA, but this is not desirable in most situations since it will force the DWB larvae to feed under the bark at the margins of the burr knot. Other borers may also attach at the site of a dead burr knot. Of the Malling and Malling-Merton rootstocks, only MM 111 shows any indication of resistance to DWB attack.
White latex paint applied to the lower trunk before egg laying begins will reduce infestation levels significantly. Proper planting to avoid production of burr knots below the graft union will prevent infestations from becoming established.
Trunk sprays with residual insecticides will help control DWB. Generally, a single application at peak emergence will provide adequate control. A low pressure application that thoroughly wets all surfaces of the lower trunk and the burr knots is needed to ensure good control.
 DWB trap |
Insect and Mite Management Overview
Insect and Mite Index
Southeastern Apple Production Mountain Horticultural Crops Research & Extension Center 455 Research Drive Mills River, NC 28759 Phone: 828.684.3562 ~ Fax: 828.684.8715 Email: jim_walgenbach@ncsu.edu |
Web Crafters: Anne S. Napier and
Steve Schoof |
Updated March 22, 2007