It seems dogwood borer is an increasing problem in dwarf apples in the Northeast. This has also been observed in Michigan where use of size-controlling rootstocks such as M.9 and M.26 that form burrknots, which are attractive to the borers, has increased the incidence of this serious pest there too.
Growers are advised to scout their young plantings of dwarf apple trees for the presence of dogwood borers NOW! They are most likely to be found in the above-ground, exposed portion of the rootstock, particularly in and around burrknots. A pocket knife can be used to dig-away the bark and frass to find the borer larvae buried just beneath the rootstock/burrknot surface. A severe borer infestation can result in girdling and death of young trees. Again, the dwarfing rootstocks that form burrknots easily, such as M.9 and M.26, are most attractive to the borers, and that is the place to start looking!
Cultural practices such as clean herbicide strips, white trunk paint, removing plastic mouse guards, and deep planting or mounding of soil around the rootstock shank are helpful in keeping trees unattractive to borers. If, however, dogwood borer larvae are found in the rootstock, an immediate pesticide application is advised. Lorsban-4E has a supplemental label to use on borers, but only when applied to the trunk of apple trees (i.e., no foliar or fruit contact). This treatment is somewhat effective at killing borer larvae even though they are not exposed directly to the spray, and will also prevent adults from laying eggs.
For more information on this important pest of dwarf apples, visit the following web publications:
Cornell Tree Fruit IPM Insect Identification Sheet No. 14, Dogwood Borer
Michigan State Univerity's Fruit Crop Advisory Team Alert, October 1, 1999, Dogwood borers and the health of dwarf apple trees