The jasmonate pathway and water loss in rice leaves induced by a stem‐borer inhibit leaf‐feeder growth

Plant‐mediated interactions between herbivores play an important role in regulating the composition of herbivore community. The fall armyworm (FAW), Spodoptera frugiperda, which has become one of the most serious pests on corn in China since it invaded in 2018, has been found feeding rice in the field. However, how FAW interacts with native rice insect pests remains largely unknown. Here, we investigated the interaction between FAW and a resident herbivore, striped stem borer (SSB, Chilo suppressalis) on rice. The infestation of rice leaf sheaths (LSs) by SSB larvae systemically enhanced the level of jasmonic acid (JA), abscisic acid (ABA), and trypsin proteinase inhibitors (TPIs), reduced relative water content (RWC) in leaf blades (LBs), and suppressed the growth of FAW larvae. In contrast, because FAW larvae infested LBs and did not affect defence responses in LSs, they did not influence the performance of SSB larvae. Using different mutants, together with bioassays and chemical analysis, we revealed that SSB‐induced suppression of FAW larvae growth depended on both the SSB‐activated JA pathway and RWC in LBs, whereas the ABA pathway activated by SSB larvae promoted the growth of FAW larvae by impeding water loss. These results provide new insights into mechanisms underlying plant‐mediated interactions between herbivores. Summary Statement Striped stem borer (SSB) larvae infestation (at rice leaf sheaths [LSs]) suppresses the growth of fall armyworm (FAW) larvae (feeding on rice leaf blades) by systemically activating JA and trypsin proteinase inhibitors, as well as reducing relative water content in rice. In contrast, infestation by FAW larvae had no impact on the performance of SSB larvae because it did not influence defence responses in LSs.