Weevils: A Vegetarian Lifestyle
Originally denoting almost any kind of beetle, the name weevil is now used for the 500 species of the family Curculionidae.
Many are brightly coloured and all are vegetarians.
Most weevils are recognisable by the fact that the head is well drawn out into an elongated, beak-like snout, at the tip of which are situated tiny jaws. This elongation, called the rostrum, bears a pair of elbowed and somewhat clubbed antennae. In many weevils the antennae can be folded for protection into grooves at the sides of the rostrum.
Weevils are found on many kinds of plants, some being highly specific to their host-plant. According to their way of life, they may be leaf rollers, stem borers or bud eaters, and they may burrow into fruits and roots or just eat parts of the leaves. In fact, there is hardly any part of a plant that some leaf weevil will not eat. Because of this diet it is not surprising that some weevils are notorious pests of crops, and sometimes of stored food as well. Here we describe several species from four of the 26 British sub-families of weevils.
The leaf rollers
The red oak roller, one of our larger weevils, is a conspicuous red colour, except for its legs and head which are black. The female is about 6mm (1/4in) long, while the male is slightly smaller. This species lives mainly on the foliage of young oaks and is rarely found on mature trees. The conspicuous coloration is a warning, indicating inedibility or the possession of some effective weapon of defence or offence against a potential enemy such as a bird. The female lays her eggs singly on oak leaves, then rolls the leaves up to form protective homes for the larvae.
The birch leaf roller is related to the red oak roller, but is rather smaller and of a shining dark bluish-black colour. This weevil has much the same shape as its relative, but is only 3-5mm long. Looked at with a hand lens the thorax of the birch leaf roller is decidedly hairy; the male can be distinguished by his puffed out black femora (the long leg joint nearest the body).
Like the red oak roller, the birch leaf roller is occasionally found on other trees – beech, hazel, hornbeam and alder for example. But it is essentially an inhabitant of birch woods and it, too, rolls up leaves to house its larvae. It is widely distributed and quite common in the British Isles.
A related, but larger and hairier, weevil also occurs chiefly on birch. It is called Byctiscus betulae and it can be a dark metallic blue, green, or shining red-brown in colour. This weevil is much more local than the other two and is commonest in the south of England. Although it is unlikely to be confused with the birch leaf roller, it can be precisely identified because the male has a small, forward-projecting spike on each side of its thorax. Byctiscus is also a leaf roller.
There are seven species of weevils of the genus Cionus, all of them square in shape with a rather long rostrum on which the antennae are set well forward. The legs are short and stout, giving these weevils a compact appearance.
Cionus scrophulariae is closely associated with common figwort, water ligwort and the related common mullein. It is widespread over much of Britain. Its most distinctive feature is the light colour of its thorax, which is covered with whitish or pale yellow scales. There are also a number of square black spots on its grey wing cases, the central ones being larger than the others. The antennae are reddish with darker clubbed ends, and the legs are black with brown tips. This weevil is 4-4.5mm long and one of the largest in the genus. The thorax of the closely related C. tuberculosus is much the same colour as its wing cases (not lighter). This species occurs on the same food plants as C. scrophulariae but is much rarer.
Cionus weevils feed on the leaves of their food plants, and a number of them are usually found together. The larva is a greyish, sluglike creature, heavily coated with slime, which it produces from a gland near the end of its body. This slime undoubtedly helps to protect the otherwise rather vulnerable grub from bird predation, and especially from drying up. It is also used to form a tough cocoon, which is firmly attached to the food plant. Inside the cocoon the pupa is well protected for the short period of a week during which it undergoes the transformation into an adult beetle, although it may be attacked by tiny parasitic wasps. The fully formed adult bites its way out, making a neat circular hole at one end of the cocoon. Ultimately it goes into hibernation at the base of a tuft of grass or among leaf litter in a hedgerow.