Biotic factor that contributes to oak decline
Prior to the Great Storm of 1987, the buprestid beetle Agrilus pannonicus, was listed in the British Red Data Book as ‘vulnerable’, meaning that extinction was thought likely if no change occurred. However, one striking feature in the oak decline episode of the early 1990s in England was the highly visible evidence of Agrilus attack on many of the declining trees.
Although Agrilus is not considered to be capable of attacking healthy oaks, trees in decline may fall victim and early stage attack by this insect is often marked by the presence of dark exudates on the bark surface (sometimes known as tarry spots or bleeding). The adult beetles lay eggs in the bark of declining trees and the resulting larvae tunnel through the bark to feed, and this can girdle infested trees. When the larvae eventually pupate, characteristic D-shaped exit holes are made by the emerging adults.
If, as it appears, Agrilus has become much more common over the past two decades, it may be an important component in oak decline if trees already debilitated by other decline inducing agents are then attacked by the beetles for breeding purposes.
