Fumigant toxicity is the major route of insecticidal activity of citruspeel essential oils.
In laboratory experiments, treatment (> 10 ml/mg against Callosobruchus maculatus or Sitophilus zeamais; >20 ml/kg against Dermestes maculatus) with Citrus (lime, tangerine [mandarin] and grapefruit) peel oils reduced oviposition or larval emergence through parental adult mortality, but had no residual activity on the eggs or larvae produced by survivors. Oil-treated cowpeas (7 ml/kg against C. maculatus) or dried fish (28 ml/kg against D. maculatus) which caused 100% mortality 1 h after application lost all activity within 24 h, thus confirming the non-residual nature of the effects. The activity of lime peel oil against test insects was found to be dependent on the time interval between the application of oil and start of bioassays. The non-volatile residues of lime peel oil were not toxic to insects on glass and dried fish surfaces. Topical toxicity trials against D. maculatus adults also illustrated the relative unimportance of contact toxicity of Citrus oils, as appreciable mortality (at application rates of up to 2 µl per insect) was obtained only when treated insects were confined in airtight glass chambers. The volatility of toxic constituents in the oils was further illustrated by mortality of untreated C. maculatus adults confined in airtight chambers with topically treated D. maculatus. It is suggested that a more efficient way to use Citrus peel essential oils to control insects would be as a fumigant in relatively enclosed or airtight systems.