Transcript Food webs, landscapes and natural enemy efficacy in organic

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