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Abstract

Recent evidence suggests that insect symbionts may be more important hidden-players in insect-plantinteractions than is currently realised. In the last decade, it has become apparent that facultativesymbionts (i.e. not required for host survival) can be important mediators of direct and indirectinteractions between insects, their host plants and their natural enemies. The pea aphid (Acyrthosiphonpisum) is the main model organism for studying the role of facultative symbionts in insect herbivores.In this species facultative symbionts provide their hosts with several services. Hamiltonella defensa, forexample, protects aphids from parasitic wasps, an effect that has been extensively studied in thelaboratory. The role of this bacteria in a wider community context, however, remains relativelyunexplored. In a series of field and laboratory experiments, I will address costs, benefits and trophicweb consequences of pea aphid infection with H. defensa. I will also explore whether effects in thetrophic web indirectly affect other insect species (aphids in particular) through shared natural enemies(i.e. apparent competition) or shared host plants. Results suggest insect symbionts can be an importanthidden trophic level in plant-based insect communities.