5. Neem as a cost-effective and potent biopesticide against the diamondback moth Plutella xylostella L. (Lepidoptera: Plutellidae) and the cabbage webworm Hellula undalis F. (Lepidoptera: Crambidae)

Cabbage is an important cash crop to the resource-poor farmers in sub-Saharan Africa and offers a good source of vitamins and minerals. The diamondback moth (DBM), Plutella xylostella L. and the cabbage webworm, Hellula undalis F. are major pests causing significant losses to brassica crops worldwide. During the major and minor seasons of 2015, an experiment was carried out at the University of Ghana Soil and Irrigation Research Centre (SIREC), Kpong to determine the effect of some pesticides (synthetic insecticides-chlorpyrifos and lambda-cyhalothrin, botanicals - hot pepper fruit extract, aqueous neem seed extract, local insecticidal soap - ‘alata samina’ and water as control) in controlling the diamondback moth and the cabbage webworm on cabbage. Cabbage seedlings were transplanted onto 3m x 3m plots, and plots were labelled by randomly assigning treatments to them. The experiment was laid out in a complete randomised block design, consisting of six treatments in three replications. Treatments were applied weekly, two weeks after transplanting and data on the population of the diamondback moth and the cabbage webworm, multiple head formation were collected weekly. At the end of each season the resulting yield was assessed for marketability, and cost benefit analysis carried out to determine the cost: benefit ratio. The results revealed that the highest population of the diamondback moth was recorded in the plots sprayed with chlorpyrifos and lambda-cyhalothrin, with neem recording the least number of diamondback moth and cabbage webworm populations. The highest marketable yield of 13.82t/ha and 28.36t/ha was recorded for the neem sprayed plots with a cost: benefit ratio of 1:48.6 and 1:137.1 for both seasons, respectively, followed by pepper extract (3.92t/ha, 1:10.5) for the major season and ‘alata samina’ (8.86t/ha, 1:36.4) for the minor season. The aqueous neem seed extract can be used by resource-poor farmers in Ghana as the most cost-effective biopesticide against the diamondback moth and the webworm on cabbage.