Contact and fumigant toxicity of some essential oil constituents against a grain insect pest Sitophilus oryzae and two fungi, Aspergillus westerdijkiae and Fusarium graminearum

Abstract

Bioassays were performed to determine the antifungal and insecticidal activity of clove essential oil (EO), several botanical compounds (eugenol, carvacrol, allylisothiocyanate (AITC) and ethyl formate (EtF)) against mycotoxigenic fungi (Aspergillus westerdijkiae and Fusarium graminearum) and the rice weevil, Sitophilus oryzae. Antifungal activity was quantified by measuring conidia germination inhibition and mycelial growth inhibition zone to determine the minimum inhibitory concentration (MIC). Mortality of insects was determined through either contact toxicity assay (impregnated filter paper) or fumigation toxicity assay (airtight exposure chamber). A four-parameter logistic regression of fungi inhibitory growth rate to the dose of tested substances, either by the agar diffusion assay or the micro-atmosphere test, showed that AITC had antifungal and sporicide activity. The concentration at MIC for A. westerdijkiae and F. graminearum was 24.2 and 19.8 μl l−1, respectively, whereas clove essential oil (EO) was 755 and 352 μl l−1 after 72 h incubation period. Bioassays with S. oryzae showed that clove EO contact insecticidal activity was similar to pure eugenol and carvacrol: LD90s were 366, 385 and 442 μl dm−2, respectively. The fumigation insecticidal activity of AITC was LC95 = 10.8 μl l−1. For EtF and clove EO, only the LC50 could be accurately determined after 24 h exposure time which was observed at 41 and 210 μl l−1, respectively (vs. 6.4 μl l−1 for AITC LC50). The combined antifungal activity on mycotoxigenic seed-borne fungi and insecticidal activity against the rice weevil demonstrated that AITC in vapour phase may be a promising active substance for the preservation grain stored in unsafe conditions with a risk of fungal growth.