Bioactivity of powders and essential oils of three Asteraceae plants as post-harvest grain protectants against three major coleopteran pests

Abstract

Powders and essential oils were obtained from Achillea biebersteinii, A. fragrantissima and Ageratum conyzoides and tested for their insecticidal activity against Sitophilus oryzae, Rhyzopertha dominica and Tribolium castaneum. Composition of the oils was identified by gas chromatography (GC) and GC/mass spectrometry (MS). The tested plants showed adulticidal activity, where toxicity varied with dosage of the plant product, the insect tested and the exposure period. When mixed with grains as ground powders, a dosage of 20g/kg grains killed 88.2, 73.6 and 64.4% of S. oryzae after 12days of exposure with 79.6, 63.3 and 48% reduction in progeny for powders of A. biebersteinii, A. conyzoides and A. fragrantissima, respectively. Under the same assay conditions, A. biebersteinii and A. conyzoides powders killed 100% of R. dominica. T. castaneum was less susceptible to the plant powders. The LC50 values for powders were 22.8, 31.6 and 39.7mg/g grains for A. biebersteinii, 27.1, 35.4 and 47.8mg/g grains for A. conyzoides and 36.9, 48.3 and 78.6mg/g grains for A. fragrantissima against R. dominica, S. oryzae and T. castaneum, respectively. Upon fumigation, a dose of 60μl/L air of A. biebersteinii and A. conyzoides oils was sufficient to kill 100% of R. dominica after 12days of adult exposure, while 91.3% adult mortality was recorded in the case of A. fragrantissima oil. S. oryzae and T. castaneum showed a great susceptibility to the plant oils. Powders exhibited a moderate to strong residual activity where A. conyzoides showed the greatest grain protecting activity.