Impact of non-pathogenic bacteria on insect disease resistance: Importance of ecological context

1. The aerial surface of plants is a habitat for large and diverse microbial communities; termed the phyllosphere. These microbes are unavoidably consumed by herbivores, and while the entomopathogens are well studied, the impact of non-pathogenic bacteria on herbivore life history is less clear. 2. Previous work has suggested that consumption of non-entomopathogenic bacteria induces a costly immune response that might decrease the risk of infection. However, we hypothesised that insect herbivores should be selective in how they respond to commonly encountered non-pathogenic bacteria on their host plants to avoid unnecessary and costly immune responses. 3. An ecologically realistic scenario was used in which we fed cabbage looper, Trichoplusia ni Hübner, larvae on cabbage or cucumber leaves treated with the common non-entomopathogenic phyllosphere bacteria, Pseudomonas fluorescens and P. syringae. Their constitutive immunity and resistance to a pathogenic bacterium (Bacillus thuringiensis; Bt) and a baculovirus (T. ni single nucleopolyhedrovirus) were then examined. 4. While feeding on bacteria-treated leaves reduced the growth rate and condition of T. ni, there was no effect on immunity (haemolymph antibacterial and phenoloxidase activities and haemocyte numbers). Phyllosphere bacteria weakly affected the resistance of T. ni to Bt but the direction of this effect was concentration dependent; resistance to the virus was unaffected. Host plant had an impact, with cucumber-fed larvae being more susceptible to Bt. 5. The lack of evidence for a costly immune response to non-entomopathogenic bacteria suggests that T. ni are probably adapted to consuming common phyllosphere bacteria, and highlights the importance of the evolutionary history of participants in multi-trophic interactions.