If you look at acorns and hazel nuts in autumn you may find some with a small round hole bored in the shell. This is the work of weevils whose larvae live inside the shells and feed on the kernels.
The weevil, or Curculionidae, family has the distinction of being the largest in the animal kingdom; in the British Isles alone there are over 500 species of weevils. These insects are beetles (belonging to the order Coleoptera) and most are characterised by a long protruding snout (rostrum) and clubbed. ‘elbowed’ antennae. The majority are covered in fine scales, and all are vegetarians – both as adults and larvae. The long snout, with the mouthparts (mandibles) situated at the tip, is used for boring or piercing into plant tissue, especially stems, fruits and, and for feeding on leaves.
Weevil antennae, situated about halfway along the snout, have a long first segment – the scape – which the insect can fold back into channels or grooves – the scrobe – on the sides of the snout. In the species described here, members of the genus Curculio – the sexes can be distinguished by the length of the snout: the female’s snout is longer than the entire rest of her body and curves downwards. While that of the male is straight and shorter than his body.
Four nut-boring weevils
The hazel nut weevil (Curailio nucum) has rusty coloured legs and antennae and a black upperside flecked with yellow-brown scales. The adults, out and about in May and June, feed on hawthorn flowers and are locally common in woodland and hedgerows in England and Scotland. This species lays its eggs in young hazel nuts.
Three other members of the genus Curculio lay their eggs in acorns. Curculio villosus, a black weevil with a red scape, frequents England and parts of Scotland and is about 4mm (1/5in) long with grey, hair-like scales. C. venosus is very similar to C. nucum in appearance, but is exclusively English, being found no further north than Nottinghamshire. Another English species. C. glandium, a rusty or brownish coloured weevil 4-5mm (1/5in) long, is found in counties south of Derbyshire.
In early summer the female hazel nut weevil seeks out a hazel tree and finds a young green nut. She bores deep into the soft shell with her snout, then deposits a single egg into the hole.
The larva that hatches within the nut is a stout, limbless grub that is white or yellowish-while with a darker head. It spends the summer feeding on the nut kernel, moulting several times in order to increase in size. At this stage you cannot deduce the presence of the weevil larva in the nut because the small round hole bored in the shell by the female has healed.
In the autumn the nut falls to the ground and the larva gnaws its way out of the shell, leaving a neat round hole, and burrows into theto pupate. It stays in the safety of the soil throughout the winter, emerging in spring as a fully-formed adult (imago).