Fruit IPM
Factsheet Fruit IPM
Factsheet
Dogwood Borer
Scientific Name- Synanthedon scitula (Harr.)
Family- Sesiidae
Reference: Common Tree Fruit Pests, Angus Howitt, 1993
NCR 63, Michigan State University
The early 1960's the Dogwood Borer was found causing economic damage to blueberries, but it has since been only an occasional pest in them. Before this, the borer was noted only for causing occasional damage to ornamental trees and shrubs, such as dogwood. Oak and dogwood, which are especially common in the Midwest, are considered to be the borer's natural hosts.
Since the early 1980's, the dogwood borer has been a problem in the burr knots of apple varieties in Michigan, New York, New Jersey and Pennsylvania.
Life Stages
Egg: The egg is oval, measuring 0.5 by 0.3 mm. The egg incubation period is about nine days.
Larva: Body color ranges from near white to light pink
but may appear slightly orange when the larva is removed from the burr knot because of a
covering of sap from the feeding wound. The head capsule is sclerotized and deep
brown. Mature larvae measure 12 to 13 mm. There are seven instars.
Larvae of the dogwood borer and American plum borer are similar in size, but crochet location is different. Dogwood borer larvae have only one row of crochets in a line on the prolegs. Crochets on American plum borer larvae are arranged in a circle.
Pupa: The cocoon is made up of a silken
thread with fragments of frass adhering to it and is rather small. It is
positioned in the galleries the larva has made in the burr knot, usually close to the
surface and covered by a layer of reddish frass. The pupa is about 9 mm or slightly
shorter. When the adult emerges, the pupal case protrudes out of the tree slightly
and will remain visible for a long time.
Adult: The adult is a typical black and yellow sesiid moth similar to adult peachtree and lesser peachtree borers, but its wingspan of 1.4 to 2 cm makes the dogwood borer adults much smaller than the average-sized adults of either of the other two borers. The female has a wide yellow band on the fourth abdominal segment; the male has a much narrower band on the same segment.
Host Range
The dogwood borer is a native clear-wing moth found in all parts of the United States east of the Rocky Mountains. It occurs throughout the apple-growing areas of the eastern United States and Canada on a wide range of host plants, including oak and their galls, dogwood, black cherry, apple, mountain ash, hickory, willow, birch, American chestnut, beech, pine, elm and myrtle. It may also become a pest in pecans in the South and is in an occasional pest of blueberries. On apple, the borer was not an economic problem until the introduction of clonal rootstocks, which are much more prone to produce burr knots than conventional rootstocks.
INSERT GRAPH
Injury or Damage
On apple, dogwood borer larvae feed inside burr knots, which can develop on the aboveground portion of clonal rootstocks. All commercial dwarfing and semi-dwarfing rootstocks tend to develop burr knots. This tendency can be enhanced by low light conditions around the trunk due to shading by weeds, low limbs, suckers, opaque mouse guards and shallow planting. burr knots are aggregations of partially developed root initials that usually occur in clusters at or below the graft union. Reddish frass on the surface of a burr knot indicates an active infestation. The tunnels in newly infested burr knots are irregular, not well defined and usually quite shallow.
Feeding is initially confined to the burr know but sometimes spreads to healthy bark outside it. Feeding in the burr knot itself does little or no damage to the tree, but feeding below the bark is much more destructive and may eventually girdle the tree. Tree kills attributable to dogwood borer usually take several consecutive years of infestation, even thought several dozen larvae may be found on a single tree at one time. Persistent infestations over several years can contribute to a slow decline of the tree and reduce yields. Infestations probably also increase the chances of disease introduction.
Factors Affecting Abundance
Dogwood borers are very attracted to the grafting of new scion wood onto older trees. Populations of the insect build up quickly under these conditions, and the larvae can soon girdle the graft union area. Adults are attracted by dark take and grafting wax. Opaque mouse guards provide oviposition sites for females.
Life History
In the fall, the larvae enter a hibernaculum stage. The hibernaculum is generally found in a gallery the larva has made in or around the burr knot on the tree. The larva emerges from the hibernaculum in May and pupates in early June for about 25 days. After emergence, the empty pupal cases often protrude from beneath the bark and remain visible for up to a year. The adults emerge over a period of about two months beginning in early June and continuing until the end of July, peaking in late June to early July.
The moths, whose life span is six to eight days, are most active at dawn and
twilight. Mating and egg laying occur within a few days of emergence. The
female lays eggs on near burr knots. Trees that are already infested and that have
large numbers of burr knots are particularly attractive. The larvae hatch after
eight to nine days and seek entry into the plant. The burr knots consist primarily
of meristematic, non-lignified root tissue that larvae can easily penetrate and feed
on. On apples, the almost never enter healthy bark or pruning wounds to feed.
In October and early November, the larva constructs a hibernaculum and overwinters in the
larval state in this structure.
Monitoring
Look for reddish frass on the surface of burr knots. Use pheromone traps placed in trees in mid-June to detect the presence of adults and peak emergence.
Control
Dogwood borers can be controlled with diluted trunk applications of residual insecticides. Thorough coverage of the lower trunk and burr knots is necessary for good control. A single spray timed between first and peak egg hatch will give effective control.
Evidence from New York and Michigan seems to indicate that the timing of trunk sprays may not be as crucial for dogwood borer control as it is for other borer control on fruit trees. Some sprays that were applied to full-grown larvae even before the larvae pupated in Michigan and after peak egg hatch in New York still have good control. This is possibly because the spray can penetrate into the spongy burr knot tissue and be taken up by the relatively shallow feeding of the larvae.
NAA applied to the burr knots will destroy the knots but will not discourage attack by the dogwood borer. Killing the burr knot tissue with NAA is not desirable because it forces the larvae to feed at the edges of the burr knot in the healthy cambium. In addition, other borers such as the American plum borer may establish in the dead burr knots. New York studies on M.9, M.9/MM.106 and MM.111 rootstocks showed some difference in varietal susceptibility, but only MM.111 rootstocks had a considerably lower infestation level. The Empire variety seems to be the most heavily infested, possibly because it tends to have more burr knots per tree.
Planting deep and avoiding rootstock exposure will eliminate the burr knot problem. In already planted trees, place soil around the trunk up to the graft union to prevent borer access to the burr knots. This can be practical where the graft union is not too high off the ground. Burr knots and borers, however, have been found on Empires above the graft union and as high s 5 feet above the ground, so this may not always be possible.
Place the soil around the trunk in a wide mound rather than a narrow cone to avoid winter injury to the covered portion of the trunk. The additional root growth from the buried burr knots should improve anchorage and increase tree vigor.
