Factors influencing the numbers and distribution of the Brown hairstreak, Thecla betulae l., (lepidoptera, lycaenidae) and the black hairstreak, Strymonidia pruni., (lepidoptera, lycaenidae).

An autecological study of Thecla betulae L. and Strymonidia pruni L. has been made to discover why these closely related butterflies should be so local and rare when the foodplant of both, Prunus spinosa L. is so widespread and common. Information was also required to formulate conservation plans for both insects. The distribution and status, habitat characteristics, general biology and behaviour, and population dynamics of each species was studied from 1969 to 1973. T. betulae was found to be widely distributed throughout southern Britain, but to be very local. In some regions it has declined considerably in the last 100 years. Most colonies are on lowlying soils, especially clays, and most are near woods. T. betulae oviposits near the ground along wood edges and especially hedges, although there is some evidence that adults require woods, where they aggregate. The population dynamics of a colony of T. betulae at Cranleigh, Surrey was studied for four years. The heaviest mortalities occurred in the pupal stage probably due to predation by small mammals. The failure of females to lay their full potential of eggs was also important in regulating numbers. This appeared to be dependent on the length of time females were on the site, but climatic conditions were also important. S. pruni is restricted to the east Midlands in England where it has been recorded from about 50% of the larger woods on the lowlying land between Peterborough and Oxford. These are mostly on clays. S. pruni colonies usually occur within woods or along sheltered edges. They are very local and the adults rarely stray, so that colonisation of new habitat appears to be very slow. This is probably a major reason why S. pruni is so local, for colonies can only survive very gradual forestry operations and it is only in the east Midlands in Britain that coppice cycles have been long for several centuries. Recent changes in forestry practices have destroyed several colonies. At Monks Wood heavy mortalities were found in the late larval and pupal stages of S. pruni and attributed to bird predation. There is evidence that the temperature during these stages (in May and June) indirectly affects adult numbers.