Field evaluation of aqueous indigenous plant extracts against the diamondback moth, Plutella xylostella L. and the rape aphid, Brevicoryne brassicae L. in brassica production

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Small-scale vegetable farmers in southern Africa are battling with increasing pressure from insect pests due to limited sustainable control options. Concerns are growing about human safety and environmental impact of conventional synthetic pesticides commonly used against the pests. Two experiments were therefore conducted to investigate the insecticidal efficacy of crude aqueous extracts of two indigenous plants, Maerua edulis and Bobgunnia madagascariensis in controlling the most damaging pests of cabbage and rape; Plutella xylostella and Brevicoryne brassicae, respectively. Experiment I was on-station in which aqueous extracts of M. edulis leaf and B. madagascariensis fruit powder were applied separately on rape and cabbages at 3–20% w/v compared to negative water control and synthetic pesticide positive controls. Concentrations of 5 and 10% w/v M. edulis and 15 and 20% w/v B. madagascariensis were further tested on-farm in Experiment II. On-station results showed that extracts from M. edulis and B. madagascariensis at 5–20% w/v and 15–20% w/v, respectively, showed significant insecticidal effect (P<0.05) compared to the negative control. In both experiments, there were no significant differences in pest counts between plant extracts and the positive controls. In both experiments, vegetable yields from plant extract treatments differed significantly (P<0.05) from the negative control at M. edulis 5–20% w/v and B. madagascariensis 15–20% w/v, but not from the positive controls. The commercial potential of local plant extract sprays for vegetable pest management on small-scale farms is discussed.