The dogwood borer, Synanthedon scitula , is a clear-wing moth in the Sesiidae family. It enters apple tree trunks through burr knots. Burr knots are the result of many partially developed root initials that form just below the graft union on some dwarfing rootstocks. As with any borer, dogwood borer can contribute to a decline in the health of trees. Larvae feed initially on burr knot tissue, but when that is consumed they will move into other bark tissue eventually girdling the tree. As they feed, this damage likely causes yield reductions.

Borers overwinter as full-grown larvae under the bark. Pupation occurs in the spring and adults begin emerging in June in Ontario (Gardner , 2003). Emergence is extended through July and often early August, with peak flight occurring in early July. Females lay eggs on the surface of the burr knot. Once hatched the tiny larvae bore into the burr knots and begin feeding. As they feed, reddish brown frass is evident at the entry hole (Figure 1).

Figure 1. Dogwood borer larvae in burr knot

Tree guards, such as the plastic spiral-type, have been shown, in West Virginia , to increase dogwood borer infestation indirectly (Pfeiffer et al. 2005). Plastic tree guards create a damp environment favorable for burr knot formation. Tree guards with a more open mesh facilitate air circulation and exposure to sunlight, promoting drying of tissue which is susceptible to burr knot formation.

At the time this study was proposed, the extent of infestation of dogwood borers in Minnesota orchards was unknown. At the University of Minnesota 's Horticultural Research Center in Chanhassen, dogwood borers are found in many of the young plantings of Honeycrisp on dwarfing rootstocks, particularly Bud.9. Young plantings of Honeycrisp on other dwarfing rootstocks, M.9 and M.26, have been shown to be particularly susceptible to dogwood borer infestation in Massachusetts (Clements, 2001). We surveyed members of the Minnesota Apple Grower's Association at their summer field day to determine the extent of the problem. We then did in-field surveys to determine the extent of dogwood borer infestation in orchards.

We asked growers attending the summer field day of the Minnesota A pple Grower's A ssociation a few questions concerning dogwood borers and their management practices. We first asked “Have you seen dogwood borer damage in your orchard?” If they answered yes, then we asked about age of trees, types of tree guards and ground cover management. From this survey, 50% of the growers did not know if they had dogwood borer in their orchard. A pproximately 20% of the growers overall had noted some damage due to dogwood borer. If the trees were younger, between the ages of 3 to 8 years, there was a greater likelihood that there would be damage. Tree guards did not seem to be related to damage. Ground cover management did seem to have an effect. Of the growers surveyed, 63% indicated the presence of dogwood borer when bare ground was used as a ground cover system whereas 38% said dogwood borer was absent when they had some type of ground cover in their orchard.

We then visited farms across the apple growing area of Minnesota . We focused on Honeycrisp trees of varying ages and on varying rootstocks. Of the nine orchards that were surveyed, the level of damage varied from 0 to 61% infestation, with only 2 farms not having any infestation. There was also a trend that greater infestation was seen with more dwarfing rootstocks and with the use of plastic tree guards.

Dogwood borer has clearly established itself in Minnesota orchards. Damage was noted but the effect on long term survival of the trees is unknown. We will continue to follow this infestation to observe its impact on the Minnesota apple industry.

References:

Clements, J., 2001. Dogwood Borer in Dwarf Apples. Accessed September 2005. Web site: www.umass.edu/fruitadvisor/clements/articles/dogwoodborer.html

Gardner, John. 2003. Apple Borers. Accessed September 2005. Web site: www.gov.on.ca/OMAFRA/english/crops/facts/apborers.htm

Pfeiffer, D.G., L.A. Hull , D.J. Biddinger, & J.C. Killian. Accessed September 2005. Burr knot borers [Dogwood Borers, Synanthedon scitula (Harris), and Apple Bark Borer, Synanthedon pyri (Harris)] and related clearwing species. Reprinted from Mid-Atlantic Orchard Monitoring Guide , published by NRAES, 152 Riley-Robb Hall, Ithaca , New York 14853-5701 . Web site: www.ento.vt.edu/Fruitfiles/burrknot.html