1. Pathogens can increase vulnerability to predation through their harmful effects on hosts. Recently it was shown that the mere activation of the immune system by pathogens may increase the host’s risk of predation. Here we test whether exposure to non-pathogenic bacteria also activates the immune system and thereby increases vulnerability to predation.
2. We exposed Enallagma cyathigerum damselfly larvae to a non-pathogenic strain of the bacterium Escherichia coli and measured immune defence, anti-predator behaviour and survival times in the presence of larval dragonfly predators. To evaluate whether non-pathogenic bacteria also generated energy-based trade-offs leading to other fitness costs, we also quantified growth rate and survival in the absence of predators.
3. Exposure to the non-pathogenic bacterium did not affect survival in the absence of the predator but increased growth rate, possibly a response to reduce exposure time to the bacterium. Larvae exposed to the bacterium activated their immune response as shown by an increase in the activity of phenoloxidase and the number of haemocytes. The bacterium affected anti-predator traits involved in avoiding detection by predators as well as traits involved in escape after detection. Pre-exposed larvae showed higher activity levels and further increased the number of feeding strikes in the presence of predation risk, possibly driven by energetic constraints. Pre-exposed larvae swam less often when attacked, but faster. This impaired anti-predator response came at the ecological cost of increased vulnerability to predation.
4. Our study demonstrated that exposure to non-pathogenic bacteria increases vulnerability to predation, which is a novel type of antagonistic interaction. This highlights the unexplored possibility that non-pathogens may play a role in maintaining variation in immune defence through insidious effects on predator-prey interactions. Since non-pathogenic bacteria can be very abundant, this unexplored ecological cost of immune system activation in terms of increased predation may have major consequences in natural systems and may provide an unexplored new force underlying variation in immune defence.