Spider mites escape bacterial infection by avoiding contaminated food.

To fight infection, arthropods rely on the deployment of an innate immune response but also upon physical/chemical barriers and avoidance behaviours. However, most studies focus on immunity, with other defensive mechanisms being relatively overlooked. We have previously shown that the spider mite Tetranychus urticae does not mount an induced immune response towards systemic bacterial infections, entailing very high mortality rates. Therefore, we hypothesized that other defence mechanisms may be operating to minimize infection risk. Here, we test (a) if spider mites are also highly susceptible to other infection routes-spraying and feeding-and (b) if they display avoidance behaviours towards infected food. Individuals sprayed with or fed on Escherichia coli or Pseudomonas putida survived less than the control, pointing to a deficient capacity of the gut epithelium, and possibly of the cuticle, to contain bacteria. Additionally, we found that spider mites prefer uninfected food to food contaminated with bacteria, a choice that probably does not rely on olfactory cues. Our results suggest that spider mites may rely mostly on avoidance behaviours to minimize bacterial infection and highlight the multi-layered nature of immune strategies present in arthropods.