Effects of Bacillus thuringiensis, Beauveria bassiana, Neem, and a Tomato Inter-crop on Diamondback Moth (Plutella xylostella L.) (Lepidoptera: Plutellidae) in Tharaka-Nithi County, Kenya

Abstract

Cabbage (Brassica oleracea var. capitata) is an important crop in Kenya, ranked 3^rd among the exotic vegetables. Diamondback moth (DBM) is a major cabbage pest that can cause a 100% loss in yield if not well managed. Farmers indiscriminately use synthetic pesticides associated with environmental, health, and pest resistance concerns to manage the DBM. To increase the availability of technologies for effective management of DBM in Kenya, three biopesticides; BioPower (Beauveria bassiana 1.0 x 108 CFUs/g), Dipel DF® (Bacillus thuringiensis subsp. kurstaki), Neemraj Super 3000 (Azadirachtin 0.3% EC), and a tomato inter-crop were evaluated. Cabbages (30 plants) spaced at 40 cm x 60 cm were established in 6 m x 6 m plots in 3 replicates per treatment. The treatments applications were; B. bassiana 100 g/20 L, B. thuringiensis 20 g/20 L, and neem 20 mL/20 L. A row of tomato inter-crop was established in every two rows of cabbage and distilled water was used on the untreated control.  Data on the number of damaged leaves, live larvae, and marketable yields for two seasons was collected from five leaves of ten randomly selected cabbage plants. Results showed significant differences (P ≤ 0.05) between the control and treatments in the mean number of live larvae and the number of damaged leaves. However, there were no significant marketable yield differences between the treatments (P ≥ 0.05). In conclusion, the three bio-pesticides and the tomato inter-crop were effective against the DBM damage on cabbage leaves and can be part of integrated pest management (IPM) packages for DBM management on cabbage. There is a need to determine why the treatments did not result in meaningful outcomes against the DBM population and cabbage head weights. In addition, a cost-benefit analysis of each treatment is necessary.