Insecticidal products from Azadirachta indica and Plectranthus glandulosus growing in Cameroon for the protection of stored cowpea and maize against their major insect pests
Botanical insecticides are more biodegradable than synthetic chemicals and could be a source of more environmental- friendly insecticides. Accordingly, the effectiveness of oils from Azadirachta indica seeds and pulverized seeds of this plant and leaf powder of Plectranthus glandulosus were tested against Callosobruchus maculatus on cowpea seeds and Sitophilus zeamais on maize grains at fixed laboratory conditions. The azadirachtin A contents of A. indica seed oils and powders from sun-dried kernels, shade-dried kernels, sun-dried seeds and shade-dried seeds, and the chemical composition of P. glandulosus powders from sun-dried leaves, were determined, before admixing each product with cowpea seeds or maize grains for the toxicity and progeny reduction bioassay studies. Mortality counts were determined after 6 d (C. maculatus) or 7 d (S. zeamais). After the mortality counts, the grains were kept until all the emerging F1 progeny were recorded. From the results, Azadirachtin A content was higher in A. indica seed oil than in powder. The average content of Azadirachtin A in the seed powder was 1.20 g/kg and this irrespective of the drying regime. On the contrary, the oil from the shade-dried seeds (3.69 g/kg) had a higher azadirachtin A content than that from the sun-dried seeds. About 45 volatile compounds were found in the sun-dried leaves of P. glandulosus. Generally, P. glandulosus powder caused greater mortality to C. maculatus and S. zeamais than A. indica seed powder, but the neem seed oil was more active towards both insects than the powders. The A. indica products were more effective in suppressing progeny emergence in both insects than P. glandulosus leaf powders. Insecticidal products from the present study could form a major component of the integrated storage protection package for cowpea and maize against beetle infestations.
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