Pyrochroa serraticornis (Scopoli, 1763)

Red-headed Cardinal Beetle

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POLYPHAGA Emery, 1886

TENEBRIONOIDEA Latreille, 1802

PYROCHROIDAE Latreille, 1806

PYROCHROA Geoffroy, 1762

Although this is generally the commonest European member of the family there has been a decline in recent decades and its occurrence is now sporadic; it remains common across much of central and southern Europe from Spain east to Ukraine and extends sporadically north into southern Scandinavia and the UK, but it is rare in some northern regions e.g. Poland and Denmark. Here it is more widespread and common than the Black-headed cardinal, occurring throughout England and Wales including Anglesey although less commonly so in the north and there has been a decline in many areas for obvious reasons. Adults are active from April until late June or July, they typically occur in rather cool and shaded places near to woodland or in wooded parkland with a supply of fallen and undisturbed timber, early in the season they may be seen basking on herbaceous vegetation, e.g. on large dock leaves or among nettle beds exposed to the sun, and later on become more active, dispersing by flight and occurring on flowers, especially umbels on wooded borders. They are predatory, hunting small insects on flowers and also feed on pollen. Mating occurs early in the season and females oviposit among or under dead bark on fallen timber, typically where the bark is rather tight to the wood. Eggs are laid in small batches and hatch within a few weeks, the tiny larvae will remain in close proximity throughout their lives, feeding upon detritus and insect remains but they are also carnivorous and are known to be cannibalistic at high densities. Larval development takes at least two years and during this time the bark will loosen and accumulate a thick layer of compost; larger larvae will almost always be found among this compost, and when fully grown in the spring they will clear a small oval area in which to pupate. Sometimes a particular area of bark will remain attractive for two seasons and more eggs may be laid so that larvae of two generations occur together and for a while in the spring small larvae may be found alongside fully-grown larvae and pupae. Pupae are very distinctive with the body segments and appendages obvious, they are pale yellow or creamy at first, darkening to black as the beetles develop, they mature rapidly and adults eclose after a few weeks; the pale adults remain in the pupal cell for up to two weeks as they harden and develop the distinctive colour pigments and then emerge from under the bark during the first warm spells from the middle of April. Adults live exposed lives among vegetation, they often bask in the sun and are ignored by birds and other predators, protected by their aposematic colouration, they contain the poisonous substance cantharidin which is synthesized by males and passed to females during copulation, and then to the eggs during oviposition. There are records of Pyrochroa species being seen on the back of an oil beetle, suggesting that cantharidin may also be obtained from meloids as occurs in many tropical species.

This large and very distinctive species should not be confused with any other in the UK; the entirely red body will distinguish it from superficially similar species such as lycids and the lily beetle, all of which have black heads, similarly our other cardinal beetles have black heads. Among the longhorns Pyrrhidium might also be a candidate for confusion but here the expanded femora, filiform antennae and pubescent elytra, among other things, are distinctive.  Smaller than P. coccinea, 10-14mm, and less brightly coloured, generally rather dull red or orange as opposed to the much brighter red or scarlet of that species, serraticornis should be immediately obvious in the field. The sexes are easily distinguished; antennae pectinate in the male, serrate in the female. Male with deep depression between the eyes, in female much shallower.

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