Economic analysis of habitat manipulation in Brassica pest management: wild plant species suppress cabbage webworm

Chemical insecticide application has been the most widely used form of insect pest management in last six decades and resulted in well-documented negative impacts. Habitat manipulation based on intercropping to exert direct effects on pests and promote biological control has been explored in various systems as a more sustainable option. A range of intercrop plants have been evaluated, with many studies reporting successes in terms of enhanced natural enemy density, reduced pest numbers and, less commonly, reduced crop loss. Economic benefit and cost-effectiveness are less frequently explored, despite the importance of these criteria in farmer adoption and scope to scale. Here we quantify the effects of contrasting experimental habitat management treatments on densities of the pest, cabbage webworm, Hellula undalis Fab. (Lepidoptera: Crambidae), crop yield and quality, profitability and cost: benefit ratio in a cabbage Brassica oleracea var. capitata (Brassicaceae) production system in West Africa. Six wild plant species were evaluated as intercrops to promote pest suppression. Ageratum conyzoides, Tridax procumbens (Asteraceae), Crotalaria juncea (Fabaceae), Cymbopogon citratus (Poaceae), Lantana camara (Verbenaceae) and Talinum triangulare (Talinaceae) intercrop plantings were compared with a no-plant control treatment in a randomised block experiment over three growing seasons in Ghana. Costs of gathering plant propagules and establishing each intercrop plant were recorded, as was the market value of cabbage from each treatment, allowing the calculation of precise cost: benefit ratios. Hellula undalis larvae were numerous in the control compared with the intercrop plant treatments. The intercrop treatments had higher undamaged yields compared with the control. Reflecting this, and establishment costs, intercrop plant treatments had better cost: benefit ratios than the control, ranging from 1:2.8 for C. citratus to 1:46.6 for A. conyzoides. These findings provide an important evidence base for considering the economic case for habitat manipulation control and highlight that the identity of plants used in this type of agricultural intervention has a major effect on the economic outcome