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Two-spotted oak buprestid (Agrilus biguttatus)

Present in UK

Not notifiable

Scientific name – Agrilus biguttatus

Two-spotted oak buprestid Agrilus biguttatus, Gyorgy Csoka, Hungary FRI, Bugwood.jpg

 Picture: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

The two-spotted oak buprestid (TSOB) is a UK and European native, bark-boring beetle which lives mostly on oak trees (trees in the Quercus genus).

In sufficient numbers, feeding activity by its larvae (grubs) can severely weaken trees, and might hasten their deaths. It used to be considered a rare and vulnerable species posing little threat, but its numbers appear to have been increasing in recent years.

There is also concern about its possible role in acute oak decline, a serious and often fatal condition of oak trees.

It is also known as the oak jewel beetle, oak splendour beetle and two-spot woodborer; and it was previously known by the scientific name Agrilus pannonicus.

Distribution

TSOB is present in Great Britain and most continental European countries; in Turkey, Syria and Iran in the Middle East; and Morocco and Algeria in North Africa.

In Britain it is most frequently recorded in areas where acute oak decline (AOD) occurs, and it appears to be restricted to central, southern and eastern England. A map of records of occurrence is available in the National Biodiversity Network (NBN) atlas – see 'Additional Resources' below.

Susceptible species

TSOB is primarily a pest of oak trees, especially pedunculate or English oak (Quercus robur), sessile oak (Q. petraea), downy oak (Q. pubescens), evergreen oak (Q. ilex), cork oak (Q. suber) and Turkey oak (Q. cerris). However, it has also been found on European beech (Fagus sylvatica) and European sweet chestnut (Castanea sativa).

It prefers mature oak trees with trunks greater than 30cm (1 foot) in diameter, and which are weakened or in decline caused by damage, disease, attack by other pests, stress from drought or flood, or old age. It is possible that in some cases TSOB might kill a tree which would otherwise recover from a state of decline.

The threat

Damage to trees occurs when the larvae feed on the vascular tissues in the inner bark, creating galleries. When the number of larvae is high, the galleries can act to restrict the flow of nutrients within the tree, impairing its health. If the damage to the vascular tissues girdles the tree it can sever the flow of nutrients, contributing to the death of the tree.

However, trees can recover if larval numbers are low and damage is light: the trees employ a range of physical and chemical defences to protect themselves, and can cover over the galleries with callus tissue growth.

Identification and symptoms

Adult TSOBs are difficult to observe because they feed (on oak leaves) and mate high in the canopy, and the larvae develop beneath the bark. However, it is possible to infer its presence from the characteristic D-shaped exit holes which the new adult beetles make as they emerge from pupation through the bark.

The adults (see top picture) are iridescent green/blue or bronzy/green. Two distinctive white spots on the wing-cases give rise to its name, and help to distinguish it from other native Agrilus species. At 10-13mm (0.4-0.5 inches) long, they are also larger than other native Agrilus species.

Exit holes in oak made by Agrilus biguttatus

 

The D-shaped exit holes (pictured above) and larval galleries are frequently found on trees showing symptoms of acute oak decline.

Larva of two-spotted oak buprestid Agrilus biguttatus, Gyorgy Csoka, Hungary FRI, Bugwood.jpg

Picture: Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

The larvae (above) are creamy-white and measure up to 40mm (1.6in) long. The first thoracic segment is enlarged. They have no legs, and have two spines on the last abdominal segment.

Galleries of two-spotted oak buprestid Agrilus biguttatus, Louis-Michel Nageleisen, Département de la Santé des Forêts, Bugwood.jpg

 

Picture: Louis-Michel Nageleisen, Département de la Santé des Forêts, Bugwood.org

The galleries (above) which the larvae make as they feed on the vascular tissue typically follow a zig-zag course.

Report a sighting

TSOB is a native species and difficult for non-experts to observe, so we do not require reports of suspected sightings from members of the public. We do, however, invite reports of suspected acute oak decline.

Life cycle

The adult beetles lay their eggs in crevices in the bark of declining trees in the early summer. After the larvae (grubs) emerge from the eggs they tunnel through the corky outer bark to feed on the nutritious inner bark and cambium, creating long, sinuous galleries.

Mature larvae eventually pupate in the outer bark plates before emerging through the bark as adult beetles in the late spring and summer, creating the characteristic D-shaped exit holes in the bark as they do so. The entire life cycle can last for two years.

The life cycle is presented in graphic form in this Forest Research poster.

Spread

TSOB beetles can fly several kilometres. However, their distribution seems to be restricted by cool summer temperatures, although this might expand under a warming climate.

Our research

Forest Research has been investigating TSOB’s life cycle and distribution, and the role that it plays in acute oak decline.

To date (January 2020) we have been able to determine the role of temperature in the development of the beetle, and the duration of its life cycle.

Our research is also investigating:

  • the potential impacts of climate change on the beetle’s life cycle and distribution;
  • whether TSOB is involved in the transmission of pathogenic (disease-causing) bacteria between or within trees; and
  • when oak trees become suitable for larval development.

A summary of our research findings, as well as more-detailed information about two-spotted oak buprestid, are presented in this article in the journal Forestry.

Origins and background

TSOB is native to the UK, and until recently its recorded numbers and impacts were low. Possible reasons for its apparent increase in population might include:

  • increased host availability, i.e. more weakened or diseased oak trees for the beetle to breed in due to, for example, acute oak decline, chronic oak decline, or root pathogens;
  • climate change, including warmer average summer temperatures assisting an expansion of the insect’s distribution, and more frequent periods of water stress (such as the droughts of 1990-92 and 1995-97) increasing the susceptibility of host trees;
  • more-frequent severe weather events, such as storms, damaging trees; or
  • a combination of two or more of these factors.

 

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