Odours from killed conspecifics affect survival and population dynamics in a spider mite

Physiological and behavioral responses of prey to predation risks, the non-consumptive effects, have ecological impacts that can be as great as or even greater than direct predation. In nature, the ovipositing females in a large population may be continuously exposed to olfactory cues from the dead conspecifics of different developmental stages, even in the absence of predators. In this study, we investigated how odour cues from killed conspecifics of different developmental stages affect survival, fecundity, offspring development, and population growth of the spider mite Tetranychus ludeni Zacher (Acari: Tetranychidae), an invasive agricultural and horticultural pest, by continuously exposing the mated females to olfactory cues from killed eggs, female deutonymphs, or female adults. Results show that females exposed to these cues shortened longevity by 21-31% and reduced fecundity by 25-51% compared to control, with the strongest effects from killed eggs. Daughter production declined by up to 80%, leading to significant reductions in net reproductive rate (R₀) and intrinsic rate of increase (rₘ) of T. ludeni. Females perceiving killed adult-borne cues peaked their reproduction early, exhibiting a terminal investment strategy; while those detecting killed egg-borne cues displayed a flattened reproductive trajectory with a delay of egg hatching. These responses of T. ludeni females demonstrate that killed conspecific-borne cues elicited stage-specific effects, with killed eggs signalling the greatest ecological threat to the ovipositing females. Our results highlight that T. ludeni could integrate conspecific chemical information into reproductive decisions under risk environments. This study delivers insights into the mechanisms of non-consumptive stress mediated-regulation in suppressing prey population growth, suggesting that non-consumptive effects induced by killed conspecifics may amplify the impacts of predators on spider mite population dynamics.