Allyl isothiocyanate-induced tritrophic responses: Suppressing Myzus persicae and enhancing biological control in Brassica

Insect herbivores present a significant challenge to agricultural production, hindering efforts to meet the demands of the growing global population. Economically important Brassica crops are particularly susceptible to herbivorous pests. Among these pests, the aphid Myzus persicae Sulzer (Hemiptera: Aphididae) is a major threat. Although insecticides have been a common pest control strategy, their limitations—including insecticide resistance and non-target effects, highlight the need for sustainable alternatives. This study explores the potential of allyl isothiocyanate (AITC), a plant-released volatile compounds, as a defense elicitor to mitigate aphid infestations. We hypothesised that AITC treatment would reduce aphid performance while enhancing the effectiveness of natural enemies, such as the parasitoid Aphidius gifuensis and the predator Harmonia axyridis. Brassica plants were treated with three concentrations of AITC (50, 75, and 100 mg/L), while control plants received deionized water. Bioassays conducted 72 h after treatment assessed aphid performance (mortality, fecundity, development, weight, and preference) and the behavioural responses of natural enemies (foraging, parasitism, and olfactory preference). Our results showed that higher concentrations of AITC (75 and 100 mg/L) significantly reduced aphid performance, particularly in terms of weight and development. In olfactometer bioassays, aphids exhibited a lower preference for plants treated with 100 mg/L AITC. Conversely, natural enemies demonstrated enhanced efficacy and preference for plants treated with 100 mg/L AITC, except in the aphid consumption bioassay. These findings suggest that AITC has the potential to serve as a sustainable pest management tool for Brassica crops, possibly reducing aphid performance through direct toxicity and/or volatile-mediated signaling, while supporting biological control.