Potential effects of earthworms on leaf-chewer performance

1. Detritivores affect plant performance and therefore have the potential indirectly to affect above-ground herbivore populations. 2. In a microcosm study we asked whether changes in the performance of two plant species, Cardamine hirsuta L. and Veronica persica Poiret., arising from the activity of various earthworm communities influenced the consumption rate and development of a leaf-chewer, Mamestra brassicae L. 3. Earthworms increased soil nitrate and foliar nitrogen concentrations. There was no associated increase in plant biomass. 4. There was no significant effect of earthworm treatment on the biomass of M. brassicae larvae. However, highest larval mortality occurred in microcosms without earthworms, while the proportion of larvae that had reached the final instar was lowest. In a 24 h feeding trial, the consumption rate of V. persica foliage by M. brassicae was higher when plants were grown in soil containing earthworms. The complexity of earthworm communities did not affect M. brassicae consumption rates and consequent development. 5. This is the first study to show the potential of detritivore communities indirectly to affect leaf-chewing herbivores. It highlights the need to explore the interactions that occur between above- and below-ground components of plant-insect communities.