Low water availability enhances volatile-mediated direct defenses but disturbs indirect defenses against herbivores

1. Interactions between plants and natural enemies of insect herbivores influence plant productivity and survival by reducing herbivory. Plants attract natural enemies via herbivore-induced plant volatiles (HIPVs), but how water availability (WA) influences HIPV-mediated defenses is unclear. 
2. We use tomato (Solanum lycopersicum), tomato fruitworm (Helicoverpa zea), and two natural enemies, the parasitoid wasp (Microplitis croceipes) and the predator spined soldier bug (Podisus maculiventris), to investigate the effect of WA on HIPV emission dynamics and associated plant defense. 
3. We show that low WA initially increases total HIPV emission by tomatoes on the first day of herbivore exposure and, in contrast, reduces HIPV emission on the second day. Low WA enhances HIPVs that are mostly found in tomato trichomes. Notably, some volatiles inhibited by low WA are known attractants of natural enemies. Evidence from Y-tube and in-cage behavioral assays indicates that changes in HIPV emiss...