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Food webs and spider efficacy in organic production systems Sandra Öberg Department of Entomology, SLU Background Many predatory insects and spiders are important for pest regulation (Sunderland & Samu 2000) Spiders are generalists Other predatory behaviour: “Wasteful” killing by hunting spiders Partial consumption Killing of non-consumed prey in webs Background Few studies on spiders in Sweden Lycosids and Linyphiids suppress bird cherry-oat aphid (Rhopalosiphum padi) Early colonisation to crop fields important Colonisation of Rhopalosiphum padi growth decline Number of R. padi establishment JUNE JULY Background Limiting factors on spider predation: Cannibalism Interference competition Intraguild predation Exploitative competition Interactions between spiders and Conspecifics Other spider species Predators other than spiders, such as carabids Food web Field Edge zones Generalist predator guild Spiders Movement Carabids Spiders Carabids Pest insects . Study organisms Money spiders, Linyphiidae Wolf spiders, Lycosidae Questions What is the timing of colonisation from adjacent habitats into crop fields? Exploration of spider food webs. Field Edge zones Generalist predator guild Spiders Movement Carabids Spiders Carabids Pest insects . Field observations 2003 Recolonisation of spiders to cereal fields Dispersal by air and ground Dispersal by air more random and faster than dispersal by ground? Difference between field sizes in colonisation Easier to colonise smaller fields? Abiotic factors Ballooning dependent on weather Methods 2003 8 cereal fields were chosen on organic farms 1,5-5 Ha 7-12 Ha Methods 2003 Traps were emptied twice a week Field Margin 10 m 50 m Pitfall trap with barrier • Start: April Air trap • End: August Field observations 2003 Species composition of spiders Relative abundance of spiders Time of reproduction for Lycosid spiders Field observations 2003 Carabids as natural enemies: Species composition Time overlap with spiders Relative abundance Composition and timing of prey: Aphids - pest insects Leafhoppers - alternative prey or pest insects Collembolans - alternative prey Recolonisation, cont. Repeat recolonisation study 2004 and 2005 Focus on the crucial period after sowing Questions What is the timing of colonisation from adjacent habitats into crop fields? Exploration of spider food webs. Field Edge zones Generalist predator guild Spiders Movement Carabids Spiders Carabids Pest insects . Field Edge zones Generalist predator guild Food web interactions Spiders Movement Carabids Spiders Carabids Pest insects . Study competition and coexistence between different predators: Pardosa and Trochosa, both wolf spiders Pardosa and a carabid Pardosa and Oedothorax, a money spider Manipulative experiments in the green house Manipulative experiments in the field Field Edge zones Generalist predator guild Food web interactions Spiders Movement Carabids Spiders Carabids Pest insects . Experimental design: Pardosa Other Pardosa + other Pardosa*2 Other*2 Control Intra- and/or interspecific competition? Additive or non-additive Emigration interactions? or closed Outcome of competition, interference or exploitation? Does emigration affect competition? Effect on aphid pop. Collaboration with other project Study predation by spiders and carabids by molecular detection of prey-DNA in predator gut content: Prey choice Predator rates Intraguild predation QUESTIONS? Thanks to my supervisors Barbara Ekbom, Jan Bengtsson and Riccardo Bommarco
