Do host-plant interactions and susceptibility to soil cultivation determine the abundance of graminivorous sawflies on British farmland?
Larvae of the sawfly genera Dolerus and Pachynematus feed on cereals and grasses in farmland habitats and can cause crop damage. In Britain they are not economic pests, but they have a role in the wider ecology of agricultural ecosystems as important sources of food for birds. They have a patchy distribution across the landscape and are particularly associated with areas of cereals undersown with grass. In a survey of British farmland, larvae of these two genera were significantly more abundant in fields of sown perennial rye-grass, Lolium perenne L., than in crops of winter wheat, Triticum aestivum L., and spring barley, Hordeum vulgare L. Host-plant relationships and the effect of soil cultivation on overwintering survival were considered as factors underlying this distribution. In oviposition trials, perennial rye-grass was the preferred host of most species tested, and winter wheat was avoided. Growth rates and survival of larvae in performance trials were significantly higher on rye-grass than on cereals. Soil cultivation during the overwintering period caused up to 50% additional winter mortality of pupae in some years. However, in other years there was no additional mortality, indicating that another factor must be involved following the soil disturbance, such as winter frosts. The observed differences in host-suitability could strongly influence larval distribution on farms, depressing abundance in cereal habitats. Post-cultivation mortality may reinforce this effect. Varying cultivation regimes and availability of undisturbed grassland could be used to manipulate the abundance of these sawflies on farms.