The Cryptocephalus beetles, sometimes commonly referred to as ‘pot’ beetles, are a fascinating genus of leaf beetles. Worldwide, the genus is enormous with approximately 1500 described species, 18 of which are found in the UK and 14 of these are listed as endangered, vulnerable, rare, notable A or notable B.
What makes them so fascinating is their remarkable life history. As adults, they resemble any other group of leaf beetles, perching on vegetation and eating leaves, however, from egg laying right up until the adults emerge in the early summer these beetles live a life which is very different from the leaf beetle norm. A mated female perches on a leaf or twig and lays an egg which she holds with her back feet. With delicate precision she rotates the egg, sticking tiny plates of faeces to it until it resembles miniature fir cone (figs 1 and 2). Once finished, the female flicks the encased egg away from her perch and it comes to land amongst the leaf litter. On hatching the larva nibbles the end away from its egg case and sticks its head and legs into the open air. With its abdomen partly curled inside the case the larva travels with ease amongst the leaf litter, withdrawing into its case at the first sign of danger. The protection afforded by the case is enhanced by the larva’s hardened, angular head capsule that fits the case like a stopper. On a diet of leaf litter and low growing vegetation, the larva grows and after each moult it enlarges its case by adding its own faeces and moulding them with its mandibles (figs 3 and 4). A case of a fully grown larva is a perfectly formed flask or ‘pot’ and when the time is right for pupation it seals itself inside its little home and turns around to face away from the case opening. After pupation, the adult beetle chews a perfect lid in the case (fig 5) and takes its leave of the larval habitat. Larval development can take as long as three years for the larger species and the adults are found as early as April through to August, although the peak time of adult activity is in May and June.
The fact that female Cryptocephalus beetles make no obvious choice of the habitat where their larvae will develop is one reason many Cryptocephalus species are very rare. As adults they are dependent on very warm micro-climates and any habitat that is open to the wind will be devoid of these beetles. Their rarity is compounded by their limited dispersal ability.
On the afternoon of the 28th of May, 2008, a single female Cryptocephalus sexpunctatus (Figs 6 & 7) was discovered in Shrawley Wood at GR 381265.The weather was overcast and the beetle was sitting on the low vegetation at the side of the forest track.
C. sexpunctatus (RDB 2 and UK BAP) is certainly one of the rarest species of this genus in the UK. It has not been seen for many years in any considerable numbers and you would be hard pressed to find any site in the UK where you could guarantee to see one, which makes the discovery in Shrawley Wood all the more important, not only as an new locality for this species, but also as an indicator of the insect diversity in this wood. The actual location for the discovery of this beetle in Shrawley Wood is slightly atypical and it is very probable the recently emerged female had been blown from elsewhere in the wood. Many of the rare Crytocephalus beetles are found on short host trees or the lower parts of taller host trees and always in very sunny, sheltered locations, such as glades, woodland rides and similarly warm micro-habitats. These ‘edge’ habitats often support diverse scrub vegetation and most of the rare Cryptocephalus beetles will eat a small selection of host plants and C. sexpunctatus will probably eat hazel, birch, sallow and oak.
Shrawley Wood undoubtedly supports a population of this beetle and surveys should be conducted in May and June to find the core of the population so that sympathetic management can be conducted to benefit this species and the numerous other insects which depend on ‘edge’ type habitats. The presence of C. sexpunctatus in Shrawley Wood also raises the possibility that the wood supports other rare Crytocephalus species. C. coryli (RDB 1), C. nitidulus (RDB 1) C. punctiger (Notable A) C. bipunctatus (Notable B) and C. parvulus (Notable B) also depend on the same sort of habitats preferred by C. sexpunctatus and two or three of the species are often found together. Targeted surveys in the wood will undoubtedly yield some interesting results.
Location of female Cryptocephalus within Shrawley Wood © Ross Piper
Fig 8: Female Cryptocephalus © Ross Piper
Fig 9: Female Cryptocephalus © Ross Piper
Fig 10: Female Cryptocephalus © Ross Piper