Molecular Tracking of Arthropod Predator-Prey Interactions

Local generalist predators can be exploited in conservation biological control of agricultural pests. To conserve and promote the most important predators, it is critical to have detailed knowledge about their prey range under natural conditions. I have studied predator-prey interactions between lycosid spiders from the genus Pardosa and a major aphid pest in spring-sown cereals; Rhopalosiphum padi. I have also studied the link between Pardosa and springtails; a potential source of alternative food that may sustain generalist predators when pest species are absent. To reveal what the spiders had been feeding on, I collected them in farmers ́ fields and searched for preyspecific DNA remains in their stomach contents. In the same fields, I also assessed abundances of aphids and alternative prey, including springtails. For detection of R. padi remains I used available PCR primers, but to be able to study predation on springtails, I had to develop novel group-specific springtail primers. This thesis also evaluates how fast R.padi and springtail DNA is digested in laboratory-fed spiders and how temperature influences detection of prey remains in a model predator; the two-spotted lady beetle, Adalia bipunctata. Such knowledge is critical when applying PCR-based gut-content analysis to field-caught predators. By combining the consumption data with measurements of prey availability I was able to demonstrate that Pardosa feed on R. padi when it is most critical for successful biological control, i.e. when aphid densities are low. Spiders testing positive had consumed the pest shortly before capture, a conclusion based on results from the feeding experiment, which revealed that R.padi was digested rapidly in laboratory-fed spiders. I also found that springtails are an important source of alternative food for Pardosa and conclude that springtail occurrence may contribute to spider population maintenance and, indirectly, to enhanced predation pressure on co-occurring pests. However, I also found evidence that high abundances of alternative prey, temporarily, might divert spiders away from feeding on R. padi. Altogether, this work has generated new knowledge about the feeding habits of Pardosa spiders that will improve our understanding of the role of generalist predators as natural pest suppressors in agroecosystems.