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Food webs and spider efficacy in organic production systems

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

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

  2. 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

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

  4. establishment growth decline Number of R. padi JUNE JULY Colonisation of Rhopalosiphum padi

  5. 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

  6. Food web

  7. Study organisms • Money spiders, Linyphiidae • Wolf spiders, Lycosidae

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

  9. 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

  10. Methods 2003 • 8 cereal fields were chosen on organic farms 1,5-5 Ha 7-12 Ha

  11. Methods 2003 • Traps were emptied twice a week Field Margin 10 m 50 m Pitfall trap with barrier • Start: April • End: August Air trap

  12. Field observations 2003 • Species composition of spiders • Relative abundance of spiders • Time of reproduction for Lycosid spiders

  13. 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

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

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

  16. 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

  17. 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

  18. 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

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

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