Variation in the susceptibility of two Callosobruchus species to essential oils

GC/MS analysis of essential oils extracted from two Cymbopogon species revealed that limonene (23%) and p-mentha-2,8-dien-1-ol in cis (14.3%) and trans (5.6%) forms were the main compounds in Cymbopogon giganteus oil whereas citronellal (31%) and geraniol (24%) were identified in Cymbopogon nardus oil. The toxicity tests performed by fumigation on eggs and adults of Callosobruchus maculatus and Callosobruchus subinnotatus using both essential oils showed a variation in bruchid susceptibility. Essential oil of C. giganteus was more toxic to adults of both bruchid species while essential oil of C. nardus showed the better ovicidal activity. Comparative susceptibility analysis suggested that eggs and adults of C. subinnotatus were two-fold more tolerant to essential oils than C. maculatus in both stages. Oviposition of treated females was strongly reduced in the presence of essential oils. Callosobruchus subinnotatus was more affected than C. maculatus by the essential oil of C. giganteus (oviposition reduction by at least 91% v.s 81% in C. maculatus at 5 μL/L) but the two species were affected similarly by the essential oil of C. nardus.