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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:
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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
establishment growth decline Number of R. padi JUNE JULY Colonisation of Rhopalosiphum padi
Interference competition Background • Limiting factors on spider predation: • Cannibalism • Intraguild predation • Exploitative competition • Interactions between spiders and • Conspecifics • Other spider species • Predators other than spiders, such as carabids
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 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 • End: August Air trap
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.
Food web interactions • 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
Experimental design: Pardosa Other Pardosa + other Pardosa*2 Other*2 Control Intra- and/or interspecific competition? Additive or non-additive interactions? Outcome of competition, interference or exploitation? Does emigration affect competition? Effect on aphid pop. Emigration or closed Food web interactions
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