Investment in multiple defences protects a nematode-bacterium symbiosis from predation

The act of predation often comprises multiple sequential steps whereby prey can employ defences at all or some of these stages to deter predation. However, investment in defences is costly unless they are outweighed by conferring some benefit to the bearer. One system that employs multiple defences is that of the entomopathogenic nematode Heterorhabditis bacteriophora and its symbiotic bacterium Photorhabdus luminescens. This nematode-bacterium complex infects and kills soil-dwelling insect larvae, in which they then reproduce and juveniles emerge 2 weeks later. Predation of the infected host cadaver at any point during infection is fatal for the parasitic colony inside. Infected individuals, however, turn red, produce a chemical defence, bioluminesce and smell strongly at various stages of the infection process. We tested whether these colour and scent cues conferred a benefit to the infecting nematode-bacterium complex, utilizing feeding trials of nematode-infected waxworms, Galleria mellonella, with wild-caught great tits, Parus major. We tested for multimodality, as the cues are in different sensory modalities, and found no overall benefit in terms of initial attack on the first prey item, although this does not rule out the possibility of multimodality within this system. We then examined the first five prey attacked and found that scent overshadowed colour at various stages of infection, in terms of reducing levels of attack, but not when both signals were in concert in terms of consumption of infected individuals.