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

For understanding the toxic action of volatile sulphides, we have
evaluated the insecticidal efficacy of sulphide mixture (diallyldisul-
fide, diallylsulfide and diallyltrisulfide) against Callosobruchus macu-
latus 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 com-
parative analysis, maximum level of insecticidal toxicity (0.88 µl/L of
LC50), residue persistence (32.28 ppm/adult) and microstructural
aberration in beetle elytra was observed in CT compared to FT bio-
assay (1.60 µl/L of LC50 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 bio-
assays with respect to even diffusion and toxicity occurs based on
insect–volatile contacts inside the treatment chambers.