The Jewel beetle Agrilus biguttatus (FABRICIUS, 1777)

1.   Introduction

In Central Europe as well as in Germany the genus Agrilus comprises a large number of species, which are often very similar and difficult to determine, especially the females.

The 13 mm large oak buprestid beetle Agrilus biguttatus, in the US called oak splendour beetle, is the largest species of the genus Agrilus in Germany and one of the few that can be easily determined. Confusion is possible only with Agrilus ater and Agrilus guerini. The typical pubescent white spots on the elytra and on the abdomen of these three species are absent in all other species of the genus. In other pubescent species of the genus Agrilus, such as in Agrilus cinctus, the pubescence does not form spots, but extends over a larger area, is very sparse and therefore seems to be rather grey than white.

The three similar species Agrilus biguttatus (A), Agrilus ater (B) and Agrilus guerini (C).
Photograph of Agrilus guerini © Petr Zábranský, www.zabra.at, use with written permisson.

For those who are familiar with the most important characteristics of these species, the differentiation of the three species is easy. The elytra of Agrilus guerini form long divergent sharp tips at the apex. Those of Agrilus ater end in divergent and sharp tips too, but the latter are rather short. The elytra of Agrilus biguttatus are rounded at the tips. There are a few more characteristics in the white marks of the elytra. Agrilus guerini shows three white scaly patches on each elytron, one on the shoulders, one in the middle and one at the beginning of the last third of the elytra, all together at the same distance. Agrilus ater exhibits three white scaly patches too, and more or less even in similar positions, but the distance between the patch on the shoulder and the patch in the middle of the elytra is bigger than the distance between the patch in the middle and the patch at the beginning of the last third. Finally Agrilus biguttatus has only one white scaly patch on each elytron somewhere at the end of the second third and beginning of the last third. All other white scaly patches are not located on the elytra.

The genus Agrilus is widespread with approximately 2500 species throughout the world. There are almost 40 species recorded from Central Europe. The development of all species takes place in deciduous trees.

2.   Distribution

The natural range of Agrilus biguttatus stretches from Southern Russia in the east, the Caucasus, Persia and Asia Minor to Southwestern Europe (northeastern parts of Spain and the northernmost parts of Portugal) and to Northern Africa. In the north, the distribution area goes as far as Norway and Sweden. The distribution in Spain, Portugal and Northern Africa is insular, as it is in Norway, too. In Sweden, only the southernmost parts are populated. Here the species is present where its food plant grows. In Germany Agrilus biguttatus is known to occur in all Federal States. In the mountains it is found in Europe below 1000 m above sea level (Southern Europe). In Germany the species is frequent up to approximately 500 m above sea level, above 500 m and 700 m it is only encountered sporadically, and above 700 m it is very rarely found.

3.   Development cycle

The larva of Agrilus biguttatus develops exclusively in different species of oak (Quercus sp.). In Germany mainly common oak (Quercus robur) and sessile oak (Quercus petraea) are populated. From Southern Europe there are reports from downy oak (Quercus pubescens), holly oak (Quercus ilex), turkey oak (Quercus cerris) and cork oak (Quercus suber), too. There are records of development in other deciduous trees like beech (Fagus sylvatica), chestnut (Castanea sativa) and a single record from Germany in fluttering elm (Ulmus laevis) by BÜCHE.

The female deposits its eggs in clusters of approximately six on stems and thick branches of minimum 12 cm in diameter. Most of the species of the genus Agrilus produce a protective cover for the egg batch. This is yet not known for Agrilus biguttatus, but it is possible that this species also produces a protective cover. After hatching the larva enters the inner bark and causes a small place forming burrow by feeding between bark and the cambium layer, which is subsequently filled with liquid. From here five to seven burrows lead away in a star shaped form. The larvae feed between the bark and sapwood and produce galleries in irregular, twisting transverse directions during their development. This can lead to partial or complete girdling of branches or the whole tree and causes them to die off. For pupation the mature larva enters the bark and produces a puparium. Then the larva prepares hatching and produces a burrow to the outside. At the end of this burrow a thin layer of approximately 5 mm is left. The larva hibernates in the puparium. Pupation takes place in spring. The pupal period lasts about 14 days. The beetles can be found from May to August.

The development cycle is usually one year, but can take up to two years under unfavorable conditions.

4.   Lifestyle

After hatching the beetles need a maturation feeding. Therefore they consume a small quantity of foliage in the crowns of oaks. At sunny and warm weather the beetles sit on the stems and leaves of the breeding trees, on trunks or bushes and trees nearby, on freshly harvested timber and shoots of the stumps. The beetles are not found on blossoms. They fly very well and reach new breeding trees in this way. The beetle's lifespan is about three weeks.

Agrilus biguttatus is regarded as forest pest, because it harms trees with its activities, either directly or indirectly by paving the way for fungi causing the tree to die off. They can only populate trees, which have already been primarily damaged and subsequently get secondarily affected by Agrilus biguttatus. Healthy trees react with irregular wood formation (callus formation) to the feeding activity of the larvae, then with cortical interruptions and finally with secretion of sap into the burrows that kills the larvae. The burrows become overgrown but are still evident years later because of the darker color of the affected areas. Therefore Agrilus biguttatus is regarded as a secondary pest, killing trees, which would have possibly survived without its influence. Due to the fact that it can only develop in weakened trees like many other saproxylic and phytophagous beetles, the species is called a weakness parasite. Under normal conditions, as generally prevailing in Germany, the damage resulting from Agrilus biguttatus is negligible. But in warm and dry years, especially if several of them occur in succession, the beetle causes damage even in Germany. The devastation of the oaks in the forest 'Bienwald' in Rhineland-Palatinate caused by the gypsy moth (Lymantria dispar) in 1993, 1994 and 1995 in combination with warm, dry summers and high population densities of Agrilus biguttatus caused severe damage to the oaks.

The successful search for Agrilus biguttatus also reveals one of the few known predators of the beetles and their larvae: Woodpeckers remove the top scales of the bark without damaging the cambium to reach the hibernating larvae in the puparium, or the pupae in spring. Such trunks and branches show a very typical pattern which can be seen from large distances.

The European red-bellied clerid - Thanasimus formicarius - (family Cleridae - Checkered Beetles) is another known predator of the larvae of Agrilus biguttatus. Besides, there are a few parasitic wasps, that parasitize the larvae, one parasitic wasp (family Ichneumonidae) Dolichomitus imperator, and two Brack Wasps (family Braconidae) - Atanycolus sculpturatus and Doryctes rex.

5.   Red List

Agrilus biguttatus is currently recorded from everywhere in Germany and is almost common. The breeding plant (oak) is far away from being endangered, so the beetle is not included in the Red List of Germany (1998). A threat to Agrilus biguttatus is not recognizable. German legislation put all jewel beetles (family Buprestidae) under protection, except a small number of species, like Agrilus biguttatus, because of their pest potential.

6.   Sources

1. BRECHTEL, F, KOSTENBADER, H. (eds.) et al. (2002): Die Pracht- und Hirschkäfer Baden-Württembergs, Ulmer Verlag
2. CHATENET, du G. (2000): Coleopteres phytophages d'Europe, family Buprestidae, S. 127 ff.
3. HARDE, K. W. (1979) in FREUDE, H., HARDE, K. W., LOHSE, G. A.: Die Käfer Mitteleuropas, Volume 6, Diversicornia, p. 204 ff.
4. NIEHUIS, M. (2004): Die Prachtkäfer in Rheinland-Pfalz und im Saarland, GNOR-Eigenverlag