Amy M. Treonis, Lorna A. Dawson, Susan J. Grayston, Philip J. Murray, Jasmine Ross, Shona Pratt, & Eileen Reid

Although root grazing by insect herbivores is a potentially important rhizosphere interaction, the effects of belowground root herbivory on soil processes and biota have not been studied extensively.  An objective of the NERC Soil Biodiversity Programme is to increase understanding of biological diversity in soil and the functional roles played by soil organisms.  As part of this Programme, the impact of feeding by leatherjacket larvae (Tipula paludosa) on plant above- and belowground biomass and rhizosphere microbial communities was investigated in a pot experiment using sieved soil from an upland grassland (Sourhope Research Station, Cheviot Hills, Scotland).

Ryegrass (Lolium perenne) and white clover (Trifolium repens) were grown in pots (23-cm diameter) alone or in combination.  Twenty leatherjacket larvae were added to half of the pots in a randomized block design, and the effects of their feeding were measured after 10 days.  Larvae reduced the aboveground biomass of both grass and clover indirectly via root feeding and directly through folivory, demonstrating that this organism can feed on the soil surface as well as belowground.  The presence of larvae was also associated with reduced root biomass, shorter root length, and increased root necromass for both ryegrass and clover, grown alone or together.  The larvae had a greater and more rapid negative effect on the biomass of clover compared to ryegrass, indicating a possible species preference.  In the soil, the presence of larvae was associated with a tenfold increase in populations of Psuedomonas spp., demonstrating a significant change in the structure of the rhizosphere microbial community.

To investigate the feeding patterns of leatherjacket larvae on the root systems of grass and clover, further experiments were conducted using a slant-board technique.  In clover, the larvae tended to bisect primary roots when they fed, thereby cutting off large sections of root from the plant and providing dead root material for decomposer microorganisms in the soil.  In grass root systems, the larvae appear to have concentrated their feeding on lateral roots.  These contrasting feeding patterns could potentially have an impact on rhizosphere communities and nutrient cycling and ultimately also on grassland vegetation composition.

These studies suggest that root herbivory by larval root feeders is an important process by which carbon resources are redistributed in the rhizosphere, thereby influencing the composition of microbial communities.  Removal and/or damage to root material by feeding larvae may cause quantitative and qualitative changes in root exudation, which in turn may affect microbial communities.  Further research will investigate these interactions.