Agrilus biguttatus (Fabricius, 1777)
POLYPHAGA Emery, 1886
BUPRESTOIDEA Leach, 1815
AGRILINAE Laporte, 1835
AGRILINI Laporte, 1835
AGRILUS Curtis, 1825
ANAMBUS Thomson, C.G., 1864
This very widespread native Palaearctic species occurs throughout Europe north to the UK and southern Fennoscandia, it also occurs across Mediterranean North Africa, the Middle-East and Asia Minor, and further east it extends through Russia to Siberia. It is regularly imported into the United States among wooden crates and packing materials and has become established in several areas e.g. Michigan. Here it is generally common the south of England and the midlands, becoming more scattered and local further north to Yorkshire but is absent from the West Country and Wales although it may be under-recorded as the adults are very elusive; they feed and mate high up in tree canopies and when they do descend tree-trunks, usually in bright sunlight, they move very quickly and fly readily and fast when disturbed. Over much of the European range it has increased in abundance and this seems to be the case here as well and in recent years it is thought to be at least partly responsible for the decline in oak trees in various areas. Host trees are various oaks including Quercus robur, Q. pubescens, Q. ilex, Q. suber, Q. petraea and Q. cerris and occasionally beech, Fagus sylvatica and sweet chestnut, Castanea sativa, it has also been reported from various poplars but this association is doubtful. Adults are active from May until July or , rarely, August and occur around oaks in woodland and parkland, they usually frequent mature trees but we have recorded them from dense foliage on coppiced oaks in otherwise predominantly hornbeam woodland, they are active in warm sunny spells and only descend to low levels to oviposit on trunks. Eggs are laid in batches of 5 or 6 during May and early June among thick bark on south-facing parts of trunks on old trees, typically around 80 years old, the females always choose moist living bark and will ignore areas of bark that have dried out; they are secondary pests and will choose trees, or areas of trees, that have been stressed by other factors such as defoliation or damage to the bark caused by other insects, late spring frosts, fungi or drought, and so in severe infestations Agrilus may kill the tree before it has a chance to recover. In warmer southern parts of the range the larvae develop in a single season and
the species is univoltine but further north, including the UK, they overwinter twice and so the life cycle takes two years. Larvae emerge from the eggs and bore through the bark to the cambium layer where they produce undulating galleries up to 150cm long and pass through five instars; after the first year they measure about 10mm and when fully grown, after a year and a half, between 25 and 45mm. Larvae continue to develop through the winter and in the spring excavate pupal chambers about 15mm long under areas of thick bark. Freshly eclosed adults remain under the bark for a while to harden before gnawing their characteristic D-shaped emergence holes; these are larger than those produced by our other buprestids, measuring about 4mm by 3mm. Newly emerged adults usually fly up to the crown of the tree where they will mature and feed on parenchyma tissue of young foliage. On the continent large populations have been reported from stressed trees; 38 emergence holes per 0.5 metres of bark and in a Polish study a single oak trunk some 28m long produced more than 700 specimens. It is the larval stage which damages the tree; young larvae produce longitudinal galleries but later instars burrow transversely and may completely girdle the trunk, the first indication of their presence may be areas of dead twigs or branches higher up on the trunk or areas of foliage in otherwise dead areas of crown. Younger and more vigorous trees can repair larval damage by reactions in the bark which produce a callus and shows as dark cracks, often with a sappy efflux. Sampling adults will require very quick reflexes with an aerial net around dense foliage in hot weather or the employment of flight-interception traps, the emergence holes are distinctively large and indicate the beetle’s presence but also indicate that it is too late to use emergence traps.
The large size and distinctively marked elytra make this species quite unmistakable among the UK fauna. 8.3-13.0mm. Dorsal surface strongly sculptured and densely and often vaguely punctured; metallic green, golden-green, blue or violet, elytra each a small white spot near the suture in the apical third. Head with obvious pale pubescence, otherwise the entire insect has very short and sparse pubescence. Head transverse with large convex eyes that are continuous with the outline, vertex longitudinally grooved and frons with a wide and shallow depression. Antennae dark metallic with elongate basal segments and serrate from the fifth segment. Pronotum transverse with two median depressions and variously depressed inside the posterior angles, lateral carinae only weakly developed. Lateral margin evenly curved and basal margin very strongly sinuate. Scutellum large; parallel-sided in the basal half and strongly narrowed to a sharply acute apex. Lateral parts of the abdomen and, rarely, the anterior pronotal angles with small white spots. Elytra long and slender, parallel in the basal half and strongly narrowed to separately rounded and finely serrate apical margins. Legs entirely dark metallic. Males are distinguished by the larger eyes and narrower frons and longer and denser frontal pubescence.