Prey choice by generalist predators in the field is a complex and dynamic process, involving a range of density dependent and independent factors including predator hunger, the spectrum of prey species available, relative prey densities, prey size, predator density, prey encounter rates, nutritional value of prey, previous feeding history, habitat architecture and many others. Laboratory feeding trials can tell us whether a predator is capable of attacking and killing any given prey but not whether it will choose to eat that prey in the field. Measurement of prey choice in the field is fundamental to our understanding of the dynamics of complex food webs, to identify potential biocontrol agents and to study predator responses to prey diversity.

The work of my group to quantify interactions between generalist predators and their prey in the field, using a range of novel molecular techniques, will be described. This will be illustrated principally with examples from two important groups of generalist predators, carabid beetles and linyphiid spiders. Techniques include the development of prey-specific monoclonal antibodies to study predation on important agricultural pests. More recently we have developed PCR-based approaches that now allow us to study the prey ranges of predators and the effects of non-pest prey on predator-target prey interactions. The first use of PCR to determine prey choice in the field is described and as is our development of rapid screening systems that allow us, for the first time, to identify all the prey in the guts of a predator in a single operation. A further technique will be presented which can be used to measure prey DNA diversity in the guts of predators as a separate parameter, and to compare this with diversity in the field.