Indigenous botanical pesticides for use in integrated pest management (IPM) of insect Pests of brassicas by small scale farmers in Kenya

Abstract

A survey of the knowledge of indigenous farmers in the selected locations of Kiambu, Maragua and Machakos districts in Central and Eastern Provinces of Kenya was carried out to determine their understanding of practical aspects of crop pest management procedures and methods. To test the insecticidal efficacy of the syrup concentrations made from the African marigold plant (Tagetes eracta) and of the chilli plant (Capsicum frutscens) on the diamondback moth (DBM) (Plutella xylostella L.) and the cabbage aphids (Brevicoryne brassicae L.), quantities of 300g/1 and 50g/1 fresh aqueous extracts of these plants were prepared in a laboratory and sprayed after 24 hours in kale crop fields against the DBM and aphids. Field experiments to determine the effects of reduced frequency use of chemical insecticides were also carried out alternating with chilli aqueous extracts to determine the effect of reduced use of chemicals by 50 percent on crop yields. Laboratory experiments to determine best dose of chili freeze-dried extracts were carried out by testing various concentrations of chili aqueous extracts for their efficacy against the laboratory reared 3rd instars DBM larvae. Antifeedant effects of the extracts were assessed by introducing DBM larvae on a treated food (leaf). It was found that the farmers in the districts studied had time-tested and in-depth knowledge in farming practices and pest control strategies but lacked knowledge on modern (chemical) insect pest control issues. Repellent effects and insecticidal properties of liquid concentrations of the African marigold plant (Tagetes eracta) and of the chilli plant (Capsicum futscens) on diamondback moth (DBM) (Plutella xylostella L.) larvae and cabbage aphids (Brevicoryne brassicae L.) were observed. The field experiments revealed that both DBM larval intensity and cabbage aphids' scores were significant between the treatments. The effects of the treatments on the actual crop yields varied. Results from replicate experiments showed significant differences in yields between the treatments. Treatments with marigold had a negative impact on the kale leaves in that all the plots sprayed with marigold syrup showed leaf scorching which could have contributed to less crop yield compared to all the other treatments. It was also observed that these botanicals had no impact on the natural enemies of the insect pests and on diseases such as virus and bacterial black rot. Results from the second field experiment for the reduction of the frequency use of diazinon showed that differences in the harvested leaves from the weekly sprayed plots with chemicals were not significantly different from those of the plots sprayed with chemical one week and alternated with chilli in the following week. Drastic reduction of food intake of the insect pests in the laboratory experiment was observed as concentration increased. Repellent effects of the chilli extracts were particularly significant as the concentration increased. However, larval mortality was not significant but it was proportional with the increment of the extract concentration. The combination of repellent effects, antifeedant and stomach poisoning of the chilli extract would make chilli products an alternative strategy on the vegetable insect pest control and could reduce the frequency use of chemicals for environmental safety