How a specialist and a non-specialist insect cope with dimethyl disulfide produced by Allium porrum

Damaged Allium plants produce and release sulfur allelochemicals, presumably to prevent insect herbivory. Defensive sulfur compounds, particularly dimethyl disulfide (DMDS), are highly toxic for non-adapted species. The toxicity of DMDS in these insects is due to disruption of the cytochrome oxidase system of their mitochondria. The purpose of this study was to compare susceptibility to DMDS in a specialist and a non-specialist insect of Allium plants, i.e., Acrolepiopsis assectella Zeller (Lepidoptera: Acrolepiidae) and Callosobruchus maculatus Fabricius (Coleoptera: Bruchidae) a specialist insect of Leguminosae. Results showed that A. assectella larvae are less susceptible to DMDS than C. maculatus adults. This bruchid becomes more tolerant after a first exposure to 0.2 mul l(-1) of DMDS, with second-exposure toxicity depending on the time lapse between exposures. Higher second-exposure tolerance could be due to selection and detoxification. To answer this question, the activity of glutathione S-transferase (GST), a key enzyme in the detoxification system, was analyzed after DMDS exposure of C. maculatus adults and larvae and of A. assectella larvae. Exposure to DMDS increased GST activity in C. maculatus adults and larvae. This finding implies that induced GST is involved in C. maculatus tolerance to DMDS. Exposure to DMDS had no effect on GST activity in A. assectella. Adaptations underlying A. assectella tolerance to sulfur compounds are discussed.