Can volatile sulphides act as fumigant or contact toxicant in cowpea weevil, F.?: a comparative analysis.

For understanding the toxic action of volatile sulphides, we have evaluated the insecticidal efficacy of sulphide mixture (diallyldisulfide, diallylsulfide and diallyltrisulfide) against F. adults by fumigant toxicity (FT) and contact toxicity (CT) bioassays. Further, diffusional characteristics of sulphides inside the treatment vials were analysed by GC-FPD and microstructural impacts on treated beetles were observed using SEM. In the comparative analysis, maximum level of insecticidal toxicity (0.88 µl/L of LC), residue persistence (32.28 ppm/adult) and microstructural aberration in beetle elytra was observed in CT compared to FT bioassay (1.60 µl/L of LC and 22.67 ppm/adult of residue). Whereas, the GC-FPD based diffusion analysis indicated even distribution of volatiles inside the vials in both FT and CT bioassays. The study results suggests that volatiles act as fumigant/vapours in both bioassays with respect to even diffusion and toxicity occurs based on insect-volatile contacts inside the treatment chambers.