L.A. Dawson, S.J. Grayston, P.J. Murray and A.M. Treonis

Soil invertebrates are an important component of the soil food web. Their biomass can exceed that of the corresponding livestock on upland pasture. While much is known about above-ground plant responses to both vertebrate and invertebrate herbivory, there is a paucity of information on effects below-ground. In particular, little is known about the effect to the root system caused by invertebrate root herbivory, the impact on rhizosphere C flow and the consequent effects on the soil. It is in this area that our research is focused.

The larvae of clover weevils (Sitona spp.) have been shown to inflict considerable damage to the roots of white clover (Trifolium repens L.). The root biomass can be reduced by up to 50% due to larval feeding and nitrogen fixation has been shown to be impaired. The larvae of the cranefly, Tipula spp. one of the more prevalent pests in the uplands, can also affect pasture species. The larvae appear to preferentially choose clover over grass, which has implications for vegetation dynamics in the field. Results show that larvae reduce length of clover roots by severance and also feed on clover nodules. Root death rate is also accelerated in both clover and grass in the presence of Tipula larvae. The soil microbial communities are altered by the presence of Tipula larvae, with increased pseudomonad populations and greater carbon utilisation. These changes in the microbial community could be a result of increased detrital inputs to the soil from root severance, by larval faecal deposits or indirectly from altered rhizosphere carbon flow. Only through such integrated research will a better understanding of the processes operating below-ground in upland pastures be achieved.