Curculio Linnaeus, 1758
This small genus includes about 30 species distributed throughout the Holarctic zone although some have spread by commerce e.g. the western Palaearctic C. rubidus (Gyllenhal, 1836) has recently become established in the United States. Members are commonly referred to as nut weevils or acorn weevils as some have become serious pests of commercial crops, most occur on wild trees of the Fagaceae, Juglandaceae and Betulaceae. They are characterized by the long, slender and curved rostrum which is most developed in the European C. elaphus (Gyllenhal, 1836) and some Nearctic species, and the compact and fusiform body. Most are covered with dense elongate brown to grey scales which in some form pattern on the elytra, and in some those along the suture towards the apex are distinctly raised, affording a good character for identification. The head is rounded in dorsal view, partly retracted into the thorax and follows the outline of the pronotum, with large and finely faceted eyes which are well-separated by the frons. The rostrum is long; often as long as, or longer than, the body and longer and more curved in the female, the slender and curved mandibles mounted at the apex are unusual in that they move up and down, these are adaptations to cutting into and feeding from deep within acorns etc. prior to oviposition. The antennae are inserted towards the base or the middle of the rostrum, are very long and slender with a clubbed scape, 7-segmented funiculus and a pubescent 3-segmented club. The pronotum is transverse to quadrate, broadest at the base and narrowing to distinct anterior angles, the lateral margins lack borders and the base is sinuate and produced backwards medially. The prosternum is transverse and wide in front of transversely-oval procoxal cavities which are closely approximated and open posteriorly, the metasternum is raised in front of widely separated and oval mesocoxal cavities, and the metasternum is large and flattened, truncate between the mesocoxae and emarginate posteriorly. The procoxae are conical
and project well-above the cuticle, the mesocoxae only weakly raised and the metacoxae weakly raised and widely transverse, almost reaching the elytral margin. There 6 visible abdominal ventrites, the first is not completely divided by the metasternum and the ovipositor is often visible beyond the elytra from above. The elytra are broadest behind the shoulders and taper towards a rounded or truncate apex, the surface is punctured and microsculptured between well-impressed and narrow striae, the lateral margins curve around the abdomen but distinct epipleura are missing or only narrowly visible towards the base. The hind wings are well-developed and all species are good fliers. The legs are long and robust with thickened femora that generally have a distinct ventral tooth but this may be missing e.g. in C. rubidus (Gyllenhal, 1836) or, as in some eastern Asian species, there may be two. The tibiae are parallel-sided and widened into a tooth at the apex which is fringed with stiff setae; in life the weevils may be seen running the ends of the pro-tibiae along the rostrum to clean it. The tarsi are 5-segmented with the third deeply bilobed and the fourth small. The claws are toothed at the base and free.
All species follow a similar lifestyle with females boring into developing nuts to feed and oviposit and larvae developing within as the nut grows, some are known to induce the growth of a gall within the kernel, after the nuts fall the larvae emerge and burrow into the ground and overwinter, then diapause to pass the following summer, sometimes this may extend into a second year, and pupation occurs in late summer or autumn, adults eclose soon after and pass the winter in the pupal cell before emerging the following spring. This unusual life-cycle may allow them to avoid fluctuations in nut production or the increase in predators or parasites following population explosions of the weevils. Some species e.g. C. villosus (Fabricius, 1781), have an alternative lifestyle, living among hymenoptera galls, but their position in the genus is questioned and they seem to be closer to Archarius Gistel, 1848, members of which also develop in galls. Although they are not a problem in the U.K several species of Curculio are serious pests of nut crops.