Factors affecting the abundance of shoot-flies (Diptera) in grassland.
The shoot-fly larvae overwintering in cut and grazed grass in Northern Ireland were studied over a five-year period. The commonest species found were Cetema elongata (Mg.), Chlorops hypostigma Mg., Opomyza germinationis (L.), Oscinella frit (L.) and O. nitidissima (Mg.). The shoot-fly complex, although not every species, showed a strong preference for Poa trivialis. Some reasons for, and the significance of, this are discussed. Larval numbers were determined by sward density and the proportion of P. trivialis present, and population differences between various sites on one farm, different years, cut and grazed grass and different fertiliser levels were largely explicable by these factors. The only observed exceptions were that the abundance of Cetema and Chlorops in grazed grass was not entirely due to sward composition, and Cetema, Opomyza and Oscinella frit tended to be more numerous close to the sites of previous oat or wheat, wheat, or oat crops, respectively.