Chemical composition, insecticidal activity and persistence of three Asteraceae essential oils and their nanoemulsions against Callosobruchus maculatus (F.)

Abstract

Essential oils from three local plants Ageratum conyzoides, Achillea fragrantissima and Tagetes minuta were obtained by hydrodistillation and analyzed using gas chromatography (GC) and GC/mass spectrometry (MS). Nanoemulsions from the obtained oils were prepared using the High Pressure Homogenization (HPH) technique. The plant oils showed considerable ovicidal, adulticidal and residual activities against the cowpea beetle, Callosobruchus maculatus, where A. conyzoides oil was the most toxic. Following a 24-h fumigation and a 48-h post exposure period, eggs were less susceptible to the oils than adults, where LC50's ranged from 71.6 to 161.9 μl/l air and 19.2–77.8 μl/l air against eggs and adults, respectively. When mortality was counted 96 h after treatment, susceptibility of all stages was increased. When prepared as nanoemulsions and tested as fumigants, toxicity of oils was increased dramatically, where LC50 values 96 h after treatment ranged from 16.1 to 40.5 μl/l air and 4.5–24.3 μl/l air against eggs and adults, respectively. In a contact toxicity bioassay, adults were susceptible to the oil-treated filter papers, where LC50's 96 h after treatment ranged between (37.1–110.8 μl/cm2). When mixed with kaolin powder and tested as contact adulticides, activity of oils increased compared with their application alone. The plant oils showed a weak to moderate residual adulticidal activity, where A. conyzoides oil was the most effective. The study shows the potential of new ways of using the tested plant oils as natural grain protectants after the required toxicological assessments.