Bug!

Once people discover that you are a naturalist they will often reveal some of their own interests in the subject. While such interests are usually rather casual in nature, they are invariably sufficient to prompt questions about things that have been seen in their garden or while out walking in the countryside. Just the other day, for example, I was asked to identify a ‘bug’ that someone had found on their arm. A quick glance at the resulting smartphone photograph revealed it to be the final instar nymph of the forest bug Pentatoma rufipes, one of the large, robust and aptly named shieldbugs.

As with other shieldbugs, this particular species goes through a number of moults as it grows. Insects, having a chitinous exoskeleton, have to shed their rigid outer layer if they are to increase in size and this individual would soon moult through into the adult form. The forest bug is one of our largest shieldbug species, the adults may reach 14 mm in length, with a body that is shaped like an elongated heraldic shield that has projecting shoulders (picture an American footballer viewed from the back). The nymph is more rounded in appearance, mottled in shades of grey, green and black, and with patterning which helps it to blend in with the tree bark on which it lives.

Unusually for a shieldbug, the forest bug overwinters as a small nymph, which means that come spring it is already well developed and can quickly gain maturity. The first adults are seen in the last week of June but numbers don’t peak until the first half of August. The forest bug is a common enough species, associated with deciduous woodland and often encountered along the edge of woodland rides. It is thought that the species is mostly vegetarian in its habits, feeding on sap and honeydew, but it will feed on dead insects when the opportunity arises. What I particularly like about this species, and other shieldbugs, is the unusual structure of the forewings. These are part hardened and part flexible, the hardened section when folded over the back forms the ‘shield’ that gives these bugs their name. They are well worth a glance through a magnifying glass if you get the chance.