Food webs and spider efficacy in organic production systems Sandra Öberg Department of Entomology, SLU

Background S Many predatory insects and spiders are important for pest regulation (Sunderland & Samu 2000) S Spiders are generalists S Other predatory behaviour: S“Wasteful” killing by hunting spiders SPartial consumption SKilling of non-consumed prey in webs

Background S Few studies on spiders in Sweden S Lycosids and Linyphiids suppress bird cherry-oat aphid (Rhopalosiphum padi) S Early colonisation to crop fields important

Colonisation of Rhopalosiphum padi JULYJUNE Number of R. padi establishmentgrowthdecline

Background S Limiting factors on spider predation: S Cannibalism S Intraguild predation S Exploitative competition Interference competition S Interactions between spiders and S Conspecifics S Other spider species S Predators other than spiders, such as carabids

Food web

Study organisms S Money spiders, Linyphiidae S Wolf spiders, Lycosidae

Questions S What is the timing of colonisation from adjacent habitats into crop fields? S Exploration of spider food webs.

Field observations 2003 S Recolonisation of spiders to cereal fields S Dispersal by air and ground SDispersal by air more random and faster than dispersal by ground? S Difference between field sizes in colonisation SEasier to colonise smaller fields? S Abiotic factors SBallooning dependent on weather

Methods 2003 S 8 cereal fields were chosen on organic farms 1,5-5 Ha7-12 Ha

Methods 2003 S Traps were emptied twice a week 10 m 50 m Pitfall trap with barrier Air trap FieldMargin Start: April End: August

Field observations 2003 S Species composition of spiders S Relative abundance of spiders S Time of reproduction for Lycosid spiders

Field observations 2003 S Carabids as natural enemies: SSpecies composition STime overlap with spiders SRelative abundance S Composition and timing of prey: SAphids - pest insects SLeafhoppers - alternative prey or pest insects SCollembolans - alternative prey

Recolonisation, cont. S Repeat recolonisation study 2004 and 2005 S Focus on the crucial period after sowing

Questions S What is the timing of colonisation from adjacent habitats into crop fields? S Exploration of spider food webs.

Food web interactions S Study competition and coexistence between different predators: SPardosa and Trochosa, both wolf spiders SPardosa and a carabid SPardosa and Oedothorax, a money spider S Manipulative experiments in the green house S Manipulative experiments in the field

Food web interactions S Experimental design: SPardosa SOther SPardosa + other SPardosa*2 SOther*2 SControl ïIntra- and/or interspecific competition? ïAdditive or non-additive interactions? ïOutcome of competition, interference or exploitation? ïDoes emigration affect competition? ïEffect on aphid pop. Emigration or closed

Collaboration with other project S Study predation by spiders and carabids by molecular detection of prey-DNA in predator gut content: SPrey choice SPredator rates SIntraguild predation

QUESTIONS? Thanks to my supervisors Barbara Ekbom, Jan Bengtsson and Riccardo Bommarco