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