Rhizobacteria inoculation of plants for reducing insect herbivores: A meta-analysis on insect behaviour and fitness

Abstract Microbial communities in the plant rhizosphere – the soil region closely associated with plant roots – play critical roles in shaping plant growth, defence, fitness, and ecosystem processes. Inoculation of plants with specific rhizobacteria strains has shown promising potential for increasing crop yields. Rhizobacteria can also induce plant defences, resulting in reduced insect growth and reproduction, and can manipulate plant biochemistry to alter insect host-choice and recruit natural enemies of the insects. We present a meta-analysis examining the impact of rhizobacteria inoculation of plants on insect fitness and behaviour. Our findings indicate that rhizobacteria inoculation generally reduces herbivore fitness and affects host choice behaviours. However, effect sizes varied significantly depending on type of herbivore (chewing versus sucking), plant host, and rhizobacterial species. Bacillus spp. showed stronger effects than the commonly studied Pseudomonas spp. Rhizobacteria notably reduced traits such as host choice, leaf consumption, survival, and reproduction of chewing herbivores, while primarily impacting sucking herbivores by reducing reproduction. Single-strain inoculants tended to perform better, especially for sucking herbivores, suggesting potential strain incompatibility issues with multi-strain inoculants. Furthermore, field trials showed less impact on insect fitness reduction compared to experiments under controlled conditions, possibly due to soil diversity and environmental factors affecting inoculant persistence. Despite very limited experimental data, studies observed that inoculation with rhizobacteria can attract parasitoid wasps and predators to the plants, as an additional indirect benefit, but with variable results across the few studies. These results underscore the need for considering broader environmental interactions when developing effective rhizobacteria-based pest management strategies. Understanding specific and generalist rhizosphere interactions can aid in developing synthetic microbial communities with broad protective functions across various plants and environments.