Food webs and spider efficacy in organic production systems Sandra Öberg Department of Entomology,...
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Transcript of Food webs and spider efficacy in organic production systems Sandra Öberg Department of Entomology,...
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
JULYJUNE
Num
ber
of R
. pa
di
establishment growth decline
Background
Limiting factors on spider predation: Cannibalism Intraguild predation Exploitative competition
Interference competition
Interactions between spiders and Conspecifics Other spider species Predators other than spiders, such as carabids
Food web
Field Edge zones
Spiders Spiders
Carabids
Movement
Pest insects
.
Generalist predator guild
Carabids
Study organisms
Money spiders,
Linyphiidae
Wolf spiders,
Lycosidae
Questions
What is the timing of colonisation from adjacent What is the timing of colonisation from adjacent habitats into crop fields?habitats into crop fields?
Exploration of spider food webs.Field Edge zones
Spiders Spiders
Carabids
Movement
Pest insects
.
Generalist predator guild
Carabids
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
10 m
50 m
Pitfall trap with barrier
Air trap
Field Margin
• Start: April
• 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.Exploration of spider food webs.Field Edge zones
Spiders Spiders
Carabids
Movement
Pest insects
.
Generalist predator guild
Carabids
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
Field Edge zones
Spiders Spiders
Carabids
Movement
Pest insects
.
Generalist predator guild
Carabids
Food web interactions
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
Field Edge zones
Spiders Spiders
Carabids
Movement
Pest insects
.
Generalist predator guild
Carabids
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