THE BRONZE BIRCH BORER AND ITS RELATION TO THE DYING OF BIRCH IN NEW BRUNSWICK FORESTS

Abstract

Most of the literature regarding the bronze birch borer discusses the insect as a pest of ornamentals. References to its activity in the forest suggest that it becomes numerous only on cut-over lands where the birch is already moribund from exposure and injury.During the past five years, yellow, white, and grey birch throughout most of New Brunswick have been dying in large numbers from the top down. In 1939 the typical mature yellow birch stand showed about 20 per cent of the trees dead and 35 per cent with over half the crown dead. Young stands were much less severely injured. Virgin stands showed almost as much damage as cut-over lands.Agrilus larvae or galleries were present in practically all injured branches. Some trees were found, however, with the early symptoms of weak foliage growth in the top and no borer attack. Occasional trees occurred with some dead branches and little or no borer attack. No evidence has yet been discovered of the presence of any other destructive organism likely to be the primary cause of death.The question, of the relation of the borer to the observed damage is discussed and the following general conclusions are drawn:(a) The young larvae need living cambial tissue but the development of the final stage to successful pupation is dependent on the death or the moribund condition of the part attacked.(b) Trees of almost all ages, growing normally and apparently healthy, can be attacked and the larvae can develop in them to a half-grown stage. If the attack is sufficiently heavy to bring about the dying of the branch at this stage, development may continue to the adult stage. If the attack is too light for this, galleries are healed over and normal growth is gradually resumed. Such "unsuccessful attacks" may occur several times in a tree's history. The success of an attack depends on the vigour of the tree and the numbers of the insect.(c) Normally, the insect prefers mature or overmature trees or trees in which some other factor than age has seriously reduced the vigour of the part attacked. Given a large supply of such material to breed in, it is capable of increasing in numbers and as a result can successfully attack greater numbers of healthy trees.(d) The injury caused by the galleries may be due chiefly to the destruction of the phloem and the resulting interference with the conduction of food materials in an upward as well as downward direction.The following factors appear likely to have contributed to the present situation in New Brunswick: (1) The presence of large areas of mature and overmature stands; (2) The damage to birch from exposure following cutting for softwoods and dying of softwoods from budworm attack; (3) Repeated attacks by defoliators. There is no clear evidence of climatic factors having caused direct injury.Five classes of injury are suggested as a basis for sample plot tallies to estimate the need for salvage. The present increase in the numbers of the borer, however, demonstrates our failure to develop a sufficiently wide and stable demand for hardwood, coupled with good silvicultural management. Prevention of the injury depends on eliminating overmature stands and maintaining vigour of growth. Severe thinning or leaving birch on cut-over lands should be avoided.