The Relationship Between Bronze Birch Borer and Birch Dieback

Abstract

European white birch (Betula pendula) was the species most affected by birch dieback. Grey birch (B. populifolia) was occasionally affected. Paper birches (fi. papyrifera) showing symptoms of decline and dieback were rare. European white birches in good health rarely contained larvae of the bronze birch borer, Agrilus anxius. Trees in early stages of decline sustained borer larvae evenly distributed throughout the bole and main stem with lower numbers in the branches near the junction with the main stem. Trees exhibiting dieback symptoms contained high populations of borer larvae evenly distributed throughout the living cambium in the bole, main stem and live branches near the junction with the main stem, adult bronze birch borer beetles emerged only from areas where the cambium had died during the winter and early spring. Emergence did not occur from healthy trees or trees displaying early decline symptoms. Trees died back from branches in the crown; thus, adult beetles emerged in low numbers from branches of trees exhibiting dieback. The largest concentrations of adult beetles emerged from the boles and main stems of trees exhibiting advanced dieback symptoms.

The Shigometer as Predictor of Bronze Birch Borer Risk

“…Birch dieback is a serious problem of birch in the northern half of the United States. One study showed that over 40% of the European white birch (Betula pendula) in one midwestern city were exhibiting crown dieback (Ball and Simmons 1980). Birch dieback begins with the tree being altered by environmental stress (Batch and Prebble 1940).…”

“…We believe that only certain trees have been sufficiently stressed to be vulnerable to attack. During the past several years, we have observed that birch dieback is a progressive process, and we have developed a crown vigor classification system to better illustrate this progression (Table 1) (Ball and Simmons 1980). A tree may alternate between class 1 and 2 for many years.…”