Intraguild Predation Among Generalist Predators in Winter Wheat.

Abstract

Modern annual arable crops are susceptible to outbreaks of pests due mainly to the uniform habitats that agricultural fields represent. Cereals are particularly prone to infestations of aphids, which may damage the crop directly through herbivory, or indirectly by acting as vectors of disease. These insects, however, have a large range of invertebrate natural enemies, which under certain circumstances, can maintain infestations below economically damaging levels. Greater habitat complexity at landscape and farm scale usually leads to more diverse assemblages of natural enemies at the field scale, but such diversity less often translates to a higher risk for pests. When higher natural enemy diversity is associated with lower levels of pest control, intraguild predation (IGP) is often cited as one of the primary antagonsitic mechanisms. IGP occurs where predators not only compete for the same resource, but also partake in a trophic interaction with one another. Controlled experiments suggest that the niche proximity of predators relative to each other and their shared prey may help predict the outcome of multiple-predator interactions.
The primary aim of this thesis was to assess levels of IGP amongst generalist invertebrate predators and to elucidate their spatial patterns, in fields of winter wheat (Triticum aestivum), an important cereal crop in North-West Europe. The chief objective was to establish the regulatory abilities of these predators in the control of aphids. Post-mortem gut content analysis using PCR was used to establish the intensity of IGP by two polyphagous predators, the carabid beetles Pterostichus melanrius and P. madidus, on a number of insectivorous linyphiid spiders and their shared aphid prey. Each of the spiders tested was found to suffer IGP. Predation rates were adjusted using data from controlled feeding trials and resampled using Monte Carlo models to test the hypothesis that predation was density-dependent. In one experiment, the web-occupying linyphiid Tenuiphantes tenuis was consumed by up to a third of P. melanarius. Predation rates by carabids on the linyphiid Bathyphantes gracilis were consistently lower than expected. B. gracilis also relies principally on its web to capture prey, but builds these webs significantly higher in the wheat stem than T tenuis. Preferences for intraguild prey species more likely to hunt aphids on the ground, the tetragnathid spider Pachygnatha degeeri, and the linyphiids Erigone spp. (E. atra and E. dentipalpis), were less consistent than those species more dependent on their webs to hunt aphids and other prey. While many factors may contribute to the outcomes of multi-predator interactions, these findings broadly supported the hypothesis that niche proximity of intraguild predators is positively related to levels of disruption due to intraguild interactions.