Agrilus biguttatus, one of the biotic factors that contributes to oak decline.
(This species of jewel beetle was formerly known as Agrilus pannonicus).
The ecological status of the two spotted oak buprestid, Agrilus biguttatus, has changed greatly over the past 25 years. In 1987 this native beetle was listed in the British Red Data Book as ‘vulnerable’, meaning that extinction was thought likely if no change occurred. However, since then it has been increasingly sighted and in the early 1990s its presence became a striking feature during an oak decline episode in England at that time. Distinctive D-shaped exit holes are created in the bark as new adults leave the host tree, where they remain and indicate a successful emergence of the canopy dwelling Agrilus beetles, which are elusive and rarely seen themselves. At present exit holes are commonly found at Acute Oak Decline sites.
Left to right:
- A fully grown Agrilus biguttatus larva
- An adult Agrilus biguttatus beetle
- Agrilus biguttatus larval feeding results in a network of galleries beneath the bark
- A D-shaped exit hole is created when adult beetles emerge from the bark. Its characteristic form is flat on one side and otherwise curved, shaped like a capital letter “D”.
Although Agrilus biguttatus is not considered to be capable of attacking healthy oaks, weakened trees may be utilised as sites for egg laying and larval feeding. On the continent Agrilus biguttatus is regarded as a harmful pest of oak, while in Britain, up until the early 1990’s, it was considered a rarity only associated with over-mature and veteran oaks. Since then however, it has been found in increasing numbers, with individual trees exhibiting high densities of exit holes as they decline and die. The known distribution of Agrilus biguttatus is restricted to southern England.
The adult beetles lay eggs in the bark crevices of declining trees and the newly hatched larvae tunnel through the corky outer bark to feed on the nutritious inner bark and cambium creating a lattice of sinuous galleries hidden beneath the surface. When the number of larvae is high, the galleries may act to restrict the flow of nutrients within the tree reducing its health. This “girdling” effect has been implicated in tree mortality, although if the number of larvae are low, trees can recover, producing callus tissue to heal over galleries. Mature larvae eventually pupate in the outer bark plates, before the adults emerge creating the characteristic “D” shaped exit holes.
Agrilus biguttatus is the largest of four Agrilus species found on oak trees in Great Britain, its development taking place on the main stem. In contrast, the larvae of the two smallest species, Agrilus laticornis and Agrilus angustulus, are found in smaller branches. The fourth species Agrilus sulcicollis was only discovered in England in 1992.
If, as it appears, Agrilus biguttatus has become more common over the past two decades, it may be an important component in oak decline. Further research is required to investigate when and how weakened trees become susceptible to attack, and the additional role that larval feeding may play in reducing oak health.