Rhizobacterial Modulation of Maize Defense Alters Herbivore Performance and Parasitoid Behavior

The association between plants and beneficial soil microorganisms can profoundly influence aboveground trophic interactions, yet these effects are often context-dependent. Although plant growth-promoting rhizobacteria (PGPR) are increasingly used in maize production, little is known about how these microorganisms affect maize defenses against herbivores and their natural enemies. Here, we investigated the effects of the PGPR Azospirillum brasilense Ab-V5 on maize interactions with the fall armyworm Spodoptera frugiperda and its larval parasitoid Aleiodes ceres. Our findings revealed that inoculation with the PGPR increased root length and foliar P concentration, which coincided with greater larval and pupal weight and reduced developmental time of S. frugiperda. Volatile analyses revealed changes in maize volatile profiles associated with A. brasilense inoculation, including altered emission of indole, TMTT and β-caryophyllene. Consistent with these changes, A. ceres females were preferentially attracted to volatiles emitted by inoculated plants. Together, our results demonstrate that, although A. brasilense inoculation improves plant nutrition, it may also enhance herbivore performance while strengthening plant-mediated indirect defenses. These findings highlight ecologically relevant trade-offs arising from microbial inoculation and underscore that PGPR effects vary across trophic levels.