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Borers
The most common borers that I have seen in commercial apple orchards in Vermont are the dogwood borer (DB) and the roundheaded apple tree borer (RATB). The former is a clearwinged moth that usually lays its eggs around burr knots; the latter, a beetle whose larvae can tunnel in the lower part of the trunk for 2-4 years causing extensive damage.
Insecticide sprays for Plum Curculio and Apple Maggot will help to control the RATB. These insects are thought to be attracted to weak trees -- therefore it is important to provide your trees with proper nutrition and water. Also, it is important to keep the base of the tree weed free and to remove any plastic vole guards at petal fall so that the base of the trunk is exposed to insecticides. In addition, a clear area around the base of trees allows woodpeckers, a natural predator, access to borer larvae. A 60:40 mixture of white latex paint and water painted on the base of trees will repel egg-laying of the RATB and also makes it easier to find infested trees. Any Thiodan or Lorsban sprays that are applied in the orchard when 1500 - 1800 degree days have accumulated from March 1 (base 50F), will help to kill newly hatched RATB. As of July 22, we were at 1350 DD at the UVM Hort.Res.Center. Prevention is the best strategy because once the larvae are established in the tree, "worming" -- removing the borers with a knife or piece of wire with a hooked tip -- is a very time consuming option.
Infestations of dogwood borers are associated with burr knots or graft unions that are planted too high above the ground. Again, weeds or plastic mouse guards provide protection for these insects and block insecticide sprays from reaching oviposition sites. Feeding of the dogwood borer larvae at the graft union can weaken the tree and contribute to a slow decline in health. Management options include painting the exposed portion of the rootstock with white latex paint to deter new infestations and applying Lorsban or Thiodan as a dilute trunk application between July 15 and August 15, keeping in mind the pre-harvest interval requirements of these insecticides.
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