Curculio nucum Linnaeus, 1758
This is a widespread and generally common species throughout Western Europe from the northern Mediterranean to southern Scandinavia and the U.K. Here it is very local and scarce through central and southeast England, and rare and sporadic further north and west, in Wales there are only a few southern coastal records. The typical habitat is deciduous woodland, parkland and mixed hedging where the host, hazel, occurs, and they may be abundant on individual trees growing on calcareous grassland. Adults may be found among the foliage from April or May to late August. Females feed upon foliage or leaf buds for 4-8 weeks in the spring before they become sexually mature and mating occurs from June with oviposition beginning shortly after. A single egg is laid on a developing kernel, while it is still soft, and each female will lay between 20 and 30 eggs, larvae emerge after a week or so and begin to feed inside the nuts where they induce the formation of a gall, they feed for about a month, consuming most of the contents, and after the nut has fallen they bore through the shell and burrow into the soil where they build a cell. Here they pass the winter and most will then spend the summer in diapause, pupation occurs late in the summer and adults eclose in the autumn, overwintering in the pupal case and emerging the following spring. Thus the life-cycle is spread over three years but this varies with larvae spending from one to three years in diapause. This may be advantageous over an annual cycle as it could allow adults to avoid variation in nut production or the build up of predators and parasites following a population explosion. In warmer continental areas this species continues to be a serious pest of hazel nut crops, the larvae destroy large quantities of nuts while the adults act as a vector for brown rot fungus, an invasive species affecting a wide variety of commercially grown fruit. Examining old fallen hazel nuts for exit holes is a good way to find host plants, and beating or sweeping foliage from late spring is the best way to sample the adults.
Curculio nucum 1
Curculio nucum 2
This species is very distinct among the U.K. fauna but care must be taken as there are a few similar species although the host plant is a very good guide to the species. Only two of our species have the scales raised over the suture towards the elytral apex; the present species and C. venosus (Gravenhorst, 1807) which occurs on oak. C. venosus is distinguished from all other U.K. species by the elongate scutellum, in C. nucum it is quadrate.
6-8mm. Entirely black but for the antennae and the rostrum beyond the antennal insertions which are red. The eyes are large and round, meeting the rostrum anteriorly, the antennae very long and slender and the rostrum glabrous and expanded just before the apex in side view. The dorsal scales are creamy to bronze; on the pronotum lying transverse to the centre-line which is slightly raised, on the scutellum conspicuously denser, on the elytra directed posteriorly with 2-5 transverse bands of slightly broader scales which form a pattern and is best appreciated in lateral or oblique view, those adjacent to the suture raised towards the apex. The legs are dark but for the tibiae, especially the pro-tibiae, which are variously lighter.