The biology of Simulium austeni Edwards 1915 : with special reference to feeding in the larval stage

Since the early 1960s, the biting habits of Simulium austeni (Diptera: Simuliidae) have caused a medical problem along the Stour valley, Dorset. After a consideration of control measures employed against simuliids, the biology of the pest species is described. Behavioural studies on the adult ~lies were carried out. Male swarms are usually large and associated with mating, whilst blood-seeking females form small swarms around potential hosts. Mating precedes blood-feeding. Oviposition is probably into smooth-flowing water. Females are most active during the warmest part of the day, but male activity extends for a longer period.S. austani produces a single generation each year; larvae are found for approximately six weeks and pupate in late April - early ''lay. Eggs occur in river sediment for most of the year. In spring, the species is numerically dominant amongst simuliid populations along much of theR.. Stour; in smaller tributaries-and in comparable rivers nearby, S. austeni is rare or absent.The -Stour possesses a true phytop & Arkton with. spring and autumn chlorophyll peaks; in some years, S- austeni larvae are-able to exploit this phytoplankton. Bacteria attained higher concentrations in the Stour than in the Fronts during April, 1975. Larvae of S.-austeni-and S. (Wilhelmia) sp. in the Stour feed more slowly.. than larvae of.the latter species in other Dorset water courses. .In the laboratory, S. austeni fed on diatoms grew more and survived better than those supplied with any other food; larvae given bacteria did not increase significantly in length. Water source influenced growth rate but not survival. Radiotracer experiments suggested that S. austeni larvae assimilate a smaller proportion of ingested carbon than do S. (llilhelmia) sp. larvae. The importance of this and other factors in controlling the abundance and distribution of S. austeni are discussed.