Bronze birch borer
July 30, 2015
Bronze birch borer is a pest of birch trees, especially white barked birches such as Betula papyrifera, B. populifolia, B. pendula and B. maximowicziana. B. papyrifera is much more tolerant of bronze birch borer than B. pendula. Insects overwinter as larvae in galleries in the vascular system and resume feeding in spring as the sap rises. Adults emerge over a period of about 6 weeks beginning in late May or early June when pagoda dogwood and ‘Winter King’ hawthorns are in full bloom. Females lay eggs on the bark, and larvae hatch out and begin boring into the bark around the time that European cranberry-bush viburnum or weigela are in bloom. Larvae form winding galleries in the cambium of the tree, girdling branches and disrupting the flow of water and nutrients in the tree. Larvae may take up to two years to complete their development.
Adults are similar in shape to two-lined chestnut borers (photo on page 49) but are 8-10 mm long and a dull metallic bronze in color.
Bronze birch borer injury includes dieback that begins in the upper portion of the tree, a lumpy appearance to branches where galleries are present, and D-shaped exit holes in the bark created by emerging adults. Rusty-colored stains may also be visible on bark in the area of entrance or exit holes.
Stressed trees are much more prone to injury. Avoid planting birches in hot, dry sites. Help keep trees healthy through proper mulching and watering. Once injury occurs, damage may be pruned out if it isn’t too extensive. Plant resistant birches, such as ‘Heritage’ (Betula nigra ‘Heritage’) and river birch (Betula nigra). If using insecticide treatments, target the young larvae before they tunnel into bark, around the time that Viburnum opulus or Weigela florida are in bloom.
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