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Bronze Birch Borer is a Buprestid beetle that attacks all birches, but is most serious on European White Birch. Poplars, aspens, willows, and cottonwoods may also serve as hosts for this beetle. The adult beetles are ½ inch long with an iridescent metallic sheen. The larvae are about 1 inch long with a wide area behind the head. Emergence in Utah normally begins mid May depending on climatic conditions. The beetles congregate on host trees and feed on the foliage for two weeks before laying eggs in cracks and crevices or under lose bark. The damage done to the tree by the adults is of no consequence. The larval tunnels beneath the bark cause the dying limbs. In severe cases large limbs and even entire trees are killed by the pests. After hatching the larvae bore into the cambium layer, feed until the following year and then emerge. Emergence holes have a characteristic "D" shape. As with many pests, the damage is already done when these holes appear, as they are formed as the pests leave, not when they enter the tree. Control begins by reducing stress on the tree. Birches are not well adapted to alkaline soils nor to hot dry conditions. Water regularly and treat with iron if the leaves start to yellow. Remove dead branches as these serve as breeding sights for the pests. Insecticidal control is aimed at killing the young larva before they enter the tree. Once the larva is inside the tree, control is virtually impossible. Carefully check birch trees and when new "D" shaped holes appear, the trees should be sprayed within two weeks. Check with local nurseries for registered insecticides. Although a single, well-applied cover spray was satisfactory in the past unless borers were present in large numbers, that is no longer the case. About the only products registered are the synthetic pyrethroids. These typically do not last for a long period of time so follow label directions as to the frequency of application. Under most situations, multiple applications of the product might be needed. The pests attack only larger branches and trunks so leaf sprays are not needed. Larry A. Sagers Regional Horticulturist Utah State University Extension Thanksgiving Point Office
