Rhinocyllus conicus (Frölich, 1792)
This native species of southern and central Europe, western Asia and North Africa has spread in recent years to many northern areas including the U.K. During the 1930’s populations from France were released in several locations in Canada and the United States to help control the noxious invasive milk and plumeless thistles, also introduced from Europe, albeit accidentally, but this was too successful and the weevil began destroying native species and so its use is no longer recommended and in many areas its release is prohibited. In the U.K. formerly restricted to southern coastal areas from Cornwall to Kent but following a recent and continuing expansion it is now locally common across England and Wales, and it seems to be increasing in abundance. The species occurs on a range of thistles, Asteraceae, including members of Carduus, Cirsium, Onopordum and Silybum; during the summer the adults are readily observed on stems just below the flowers, often several to a stem. Locally we tend to find them mostly on spear thistle, Cirsium vulgare. They become established in open sites wherever the host thrives; waste ground, wood and arable borders and unmanaged grassland etc. Adults overwinter among plant debris near the host and become active in late spring when they feed on host foliage for a short while before dispersing by flight. Mating and oviposition begins in early summer; eggs are laid individually onto flower bud bracts and protected with a covering of chewed leaf material, these ‘caps’ turn pale brown after a few days and become easily visible around the flower. Each female will lay up to 200 eggs and a number may be deposited around a single flower. Larvae emerge after about ten days and feed below the developing seeds, development takes about six weeks and fourth (final) instar larvae construct a tough pupal cell from chewed plant material and frass; many cells may be present on a single flower and may fuse to form a large dark mass. New generation adults eclose after a week or two but remain in the cell for several weeks before chewing an exit hole to emerge, they will feed on leaves near the tops of stems producing characteristic round holes over the entire leaf surface. These adults will go on to overwinter but in some areas those emerging in midsummer with a day length >16 hours will produce a second generation.
Adults are medium sized, 4-7mm and very distinctive weevils with the body and rostrum black and the legs dark brown, in fresh specimens the elytra have an attractive tessellated pattern composed of patches of pale grey, creamy yellow or golden pubescence but this soon fades and becomes abraded; new generation specimens are thus easily recognized. The head and pronotum are variously pubescent, sometimes densely so. The head is broad and roughly punctured, with transverse and weakly convex eyes and a short and broad rostrum which bears a longitudinal fine and glabrous keel along the centre of the dorsal surface, the scrobes extend obliquely from the dorsal surface near the apex to the base of the eyes. The antennae are not geniculate, a character that will distinguish Rhinocyllus from other superficially similar species in the U.K., the basal segment is short and broadened anteriorly, 2-8 are transverse and 9-11 form a narrow club. The pronotum is transverse, rounded and without borders laterally, and produced backwards posteriorly, the dorsal punctures are wide and shallow, varying in size. Elytra parallel or widened behind the middle, the shoulders distinct and raised, the striae distinct to the apex and with relatively large punctures spaced about a puncture’s width apart, the interstices flat and rugose, especially towards the base, and very finely punctured. The legs are robust and scaled throughout; femora without a tooth internally, the tibiae straight, parallel and abruptly expanded at the apex and each has a small spur on the inner apical angle. The second segment of the meta-tarsus is shorter than the bilobed third segment in both sexes. Males are usually smaller and narrower than females; in males the first two abdominal ventrites have a median longitudinal impression.
Rostrum without keel
Rostrum longer and cylindrical
Generally broader and darker