Sitona obsoletus (Gmelin in Linnaeus, 1790)
Clover Root Weevil

Widespread and generally common throughout Europe except for the most northerly parts of Fennoscandia, and extending across the Palaearctic region to the far east of Russia and Japan, this species has become established and is now widespread across the United States and, from introductions first discovered in 1996, is established, widespread and locally abundant in New Zealand. In the UK it is generally common and often abundant throughout England and Wales, rather local and scarce in Scotland north to Orkney, and there are a few scattered records from the north of Ireland. Typical habitats are grassland and arable land but the species can tolerate a wide range of conditions, from dry to permanently damp, and adults readily move to new sites when conditions become adverse e.g. when the host plants are ploughed or when land is physically disturbed, they are recorded from road verges, hedgerows and wooded margins, parkland, peat bogs, dunes and salt marsh margins as well as disturbed areas such as gardens, parkland, quarries and waste ground. Host plants include a range of legumes (Fabaceae); typically clovers (Trifolium hybridum L., T, repens L, T. hybridum L and T. pratense L. and various cultivars), Alfalfa (Medicago sativa L.) and various peas (Pisum L) although it appears that some species or cultivars e.g. T. repens, are preferred while others are largely ignored. Adults are present year-round; they overwinter among litter and tussocks or in the soil, usually away from crops or natural aggregations of hosts plants, and are active from March until October, peaking in abundance during July and August. Mating occurs in the spring after a period of feeding on host foliage and females oviposit in the soil or on lower host leaves over a long season from spring until late summer, their fecundity varies widely and seems to depend on local conditions but numbers of eggs are often high among dense host plants growing in damp conditions. Larvae work their way down into the soil where they will feed both externally and internally on host roots; early stages tend to attach nodules while older larvae feed more generally on roots, stolons and nodules. Larvae are fully-grown after about six weeks, they pupate in a subterranean cell and adults emerge after a week or so. Large local populations may build up where clover is grown for several years and while the weevils can be a minor pest they rarely have any financial impact. Larvae from eggs laid in spring and early summer produce adults later in the year but larvae from late-summer eggs may overwinter in the soil and complete their development during the following spring, but in temperate northern regions there is only a single generation each year. While the species is rarely a serious pest in its native range it has quickly become the most damaging of clover pests in New Zealand, especially on the North Island where there are commonly two generations each year and huge numbers (up to 1800 per sq. M.) of larvae have been found among arable crops. Here they have no natural enemies and the economic impact could threaten the sustainability of various clovers in the arable system. Surveyed in New Zealand soon after its discovery in 1996, the species was considered widely established and impossible to eradicate, it was hoped that the parasitiod Microctonus aethiopoides Loan, 1975 (Hymenoptera, Brachonidae), itself introduced as a biocontrol agent for the very widespread alfalfa pest Sitona discoideus Gyllenhal, 1834, but results have been mixed and are still being evaluated. Adults can be swept from clover in almost any situation but they usually occur among larger numbers of other superficially similar members of the genus and so will need to be examined very carefully, fortunately they soon become distinctive with a little experience.