The productivity of grasslands in England for Oscinella frit (L.) (Chloropidae) and other stem-boring Diptera

Abstract

Dissections of over 93,000 grass stems obtained by random core-sampling of natural grasslands at a site in Berkshire largely confirm earlier conclusions regarding the associations of different species of dipterous stem borers and grasses. Allowing for the number of stems of each grass species per unit area, the densities of larvae were greatest in Anthoxanum odoratum, Poa and Lolium and lowest in Dactylis glomerata. Oscinella frit (L.) was less abundant than Cetema spp. and Opomyza germinationis (L.); it was not found in D. glomerata but was most abundant in Lolium and Festuca. Sampling showed that overwintering larvae of O. frit suffer at least 50 per cent, mortality between September–December and January–March.Previous studies have largely concentrated on populations in different species of grasses, but it is advantageous to consider the production of O. frit in terms of short (grazed or regularly mown) grasslands and long grasslands (hay fields, uncut commons and rough grazing); comparisons of figures obtained by core-sampling for larvae and emergence traps for adults indicate a production from short grassland and long grassland of about 11 and under 1 overwintered adult per sq. yd., respectively. Having regard to the greater productivity of common leys, dominated by Lolium and Poa, a national average for the first figure would be 15. There is little increase from generation to generation in grassland populations during the year, in contrast with the situation in oats.From tentative estimates of the acreages and productivities of the different sources of O. frit in England and Wales as a whole, it is clear that the majority of the adults of O. frit are produced by grasslands in the spring and summer and by the oat panicles in the autumn, the latter population being much the greater. As O. frit is a migratory insect, such a conclusion is especially significant and indicates that during the autumn the ‘ population pressure ’ in the grasslands will be very different to that during other seasons.