New vice-county records of Clitostethus arcuatus (Rossi) (Col.: Coccinellidae) and a new association with wild cabbage.
The ladybird Clitostethus arcuatus (Rossi) is a specialist whitefly predator easily identified due to the pale horseshoe-shaped marking on the light- to dark-brown elytra. It is native throughout the western Palaeartic particularly around the Mediterranean but not in Scandinavia or the Baltic (Booth & Polaszek, 1996). In northern Europe, it was historically encountered only on walls, cliffs or in woodlands in thermally optimal areas, usually being recorded from beating of ivy, though its geographical and host plant range in Germany appear to have expanded in the last half century (Pütz et al, 2000). Records from the UK are limited in number but extend as far north as Yorkshire (Roy et al, 2011). The species has been categorised as RDB 1 - Endangered in the UK due to its very limited occurrence (Hyman & Parsons, 1992). In 2010, during the course of a survey of the distribution and ecology of the wild cabbage Brassica oleracea (L.) and the whitefly Aleyrodes proletella (L.) (Hemiptera: Aleyrodidae) along 15 km of the Kent coast, individual adults and larvae were found occasionally in association with these species (VC15). However, high numbers (up to five adults or larvae per leaf) were found on heavily whitefly-infested cabbage plants around Dover along sheltered, south-east facing areas of cliff. While the suitability of A. proletella as prey has long been established (Bathon & Pietrzik, 1986), the beetle is rarely found in association with even heavily infested cultivated brassicas, presumably due to the ephemeral nature of the crops. Though the presence of C. arcuatus on this host plant is unsurprising, it appears that this is the first report of such an association. In 2011, wider recording of whiteflies produced further UK records. In a garden in Greater Manchester (VC58), larvae and pupae were found on Welsh poppy, Meconopsis cambrica (L.), feeding on Aleyrodes lonicerae Walker. Aleyrodes proletella was also present in the area. Small populations were also found on mature honeysuckle (Lonicera periclymenum L.) in various woodland and scrub locations across Medway, Kent (VCs15 & 16), again feeding on A. lonicerae. Whether these are relict populations, introduced, or the result of range expansion is unknown. The southerlyfacing locations and extensive stable populations of host plant and whitefly on the coast would permit long-term occupation, though the influence of international transport should not be discounted. The Medway locations are small fragments of ancient contiguous woodland, while the Manchester site is a former long-term agricultural area. In Germany, the species was believed to have recently spread into urban areas (Bathon & Pietrzik, 1986), presumably due to the advantageous thermal conditions for both the predator and for prey species. In spite of these new records, British populations are most likely small, highly locally distributed and vulnerable to disturbance. The limited diversity and abundance of whitefly species in the UK, which are also mostly specialists at the edge of their range, may be a limiting factor. The species is relatively small, cryptic in habit and is probably overlooked (Shirt, 1987; Hyman & Parsons, 1992). The populations described here were all located through targeted searching of known whitefly host plants, rather than during general surveys; this may be the most efficient method to determine the species distribution in the UK. However, German records include specimens from Malaise traps, car nets, leaf litter and an unoccupied nest box (Pütz et al, 2000). If a range expansion is taking place, this species may well prove to be less inconspicuous in the future.