Thursday, July 24, 2014

Dogwood borers on young pecan trees

   Last weekend I was unwrapping bark grafts and I discovered a graft union covered by  a pile of nasty insect frass (photo at right). Looking closely at the grains of insect excrement, I found they were held together by fine silken threads. I pulled out my pocket knife to scrape off  all the debris when I discovered a cream-colored larvae with a red head.
    The larvae was a dogwood borer (Synanthedon scitula). This larvae definitely didn't like being out in the sunshine and immediately attempted to burrow down under the remaining frass piles (photo at left). Cleaning off all the frass, squashing the larva, and painting the graft union probably saved this graft union from being girdled by the dogwood borer.
    Once I found a dogwood borer in one of my grafts, I started to look at other trees for sign of borer activity. Dogwood borers feed on the inner bark and cambium of numerous hardwood trees and can become a serious pest in young pecan orchards. Adults are clear-winged moths that lay eggs on the bark of susceptible tree species. After hatching, young larvae search for a tree wound, branch attachment crevice, or graft union to enter the tree and star feeding on nutrient rich inner bark. The red arrow in the photo at right points to a column of reddish-brown frass pushed out of a dogwood borer's feeding gallery deep inside a branch attachment crevice. The frass pile usually takes on a tubular shape and is held together by fine, silken strands.

    I also found evidence of dogwood borer infestation on the trunks of young trees. the yellow arrows in the photo at left point to tree wounds created by dogwood borer activity. Piles of reddish-brown frass have pushed out of active borer sites while old borer wounds seem to ooze tree sap (dark black staining on bark).
   Dogwood borers seem to more of a problem in young, non-bearing orchards. Problems with this insect seem to disappear when trees begin nut production and a regular insecticide program is adopted to control nut feeding pests. Insecticides aimed at pecan nut casebearer and pecan weevil will serve to control both dogwood borer adults and emerging larvae.
   Because I found extensive dogwood borer damage all across my young orchard, I will be applying a trunk spray of chlorpyrifos (Lorsban) to knock back the existing borer population. However, if I had been paying closer attention and recognized the borer threat earlier, an early May trunk spray would have been most effective.