Do host-plant requirements and mortality from soil cultivation determine the distribution of graminivorous sawflies on farmland?
There are many arthropod species on farmland that are not economic pests but which have an important role in the wider ecology of agricultural ecosystems. Few of these have been studied in detail. This study investigated the factors underlying the distribution of one such group, graminivorous sawflies (Hymenoptera: Tenthredinidae). In a farm survey in southern England in 1993-96, sawfly larvae of the genera Dolerus and Pachynematus were over six times more abundant in fields of sown perennial rye-grass Lolium perenne than in fields of spring barley, winter wheat or long-established grazing pastures. In rearing trials the growth rates and survival of larvae of the two genera were high on L. perenne and low on two cereals, spring barley and winter wheat. Differences were sufficiently marked to have had a strong influence on the distribution of larvae between crops in the field. In an experimental study, nearly twice as many adults emerged from uncultivated as from cultivated ground, suggesting that disturbance of the overwintering prepupae and pupae in the soil caused about 50% extra mortality. This mortality will have reinforced the effects of host-plant requirements, suppressing sawfly numbers in annually cultivated cereal fields whilst numbers built up in less-frequently cultivated grass fields. Sawfly abundance is known to have declined on farmland. The results suggest that changes in patterns of crop rotation have been the main cause of this decline. Farming practice in Britain has moved away from mixed rotations alternating cereals with undersown grass fields. Modern intensive farms grow monocultures of cereal crops or of heavily grazed grass, both poor habitats for sawflies. Increasing insecticide use in cereals has probably been only a minor influence on population abundance of sawflies as wheat and barley fields are not high-quality habitats. Provision of undisturbed grassy habitats would promote the conservation of graminivorous sawflies on farmland.