Salinity undermines biocontrol: A tri-trophic assessment of plant signaling and parasitoid fitness under salinity stress

Soil salinization and insect herbivory pose significant threats to global food security. While biocontrol agents like parasitoid wasps are a key pest management strategy, their efficacy may be compromised by abiotic stressors that cascade through trophic levels. This study investigates the tri-trophic interactions among tomato plants ( Solanum lycopersicum ), the caterpillar pest Helicoverpa zea (Lepidoptera), and its specialist endoparasitoid Microplitis croceipes (Hymenoptera: Braconidae), under salinity stress. In Y-tube olfactometer assays, M. croceipes were significantly more attracted to no salt-treated, herbivore-damaged plants compared to salt-treated, herbivore-damaged plants. The volatile blend emitted by plants did not differ between treatments but showed differences in volatile emissions of α-copaene, 3E-Hexenol, methoxy phenyl oxime, and TMTT, which could have driven parasitoid choices. We then investigated whether salinity induced changes to plants extended to host quality for parasitoids. When presented with a choice between two hosts, wasps showed no stinging or oviposition preference between “non-salty” and “salty” caterpillars. However, parasitoid development was significantly delayed in “salty” hosts. Salt stress also did not affect parasitoid sex ratio of offspring. Our findings demonstrate that concurrent soil salinity and insect herbivory reduce plant attractiveness and host quality, which can diminish top-down control exerted by parasitoids, undermining their use as biocontrol agents. This highlights the critical need to account for abiotic factors when designing and implementing biological control strategies for herbivore populations in saline environments.