Indigenous generalist predators and plant residues: conservation biological control of pests in an organically managed cabbage field

Abstract

Plant residues are viewed as an important source for organic matter in farmlands. We studied if wheat straw and weed residues applied to an organically managed cabbage field by favoring alternative prey may improve the suppression of lepidopteran pests by indigenous generalist predators. The trophic characteristics of predators were determined by a stable isotope approach (δ13C and δ15N). The relative contribution of alternative prey in predators species' (i.e. wolf spiders) diet was estimated using a mixed model based on their stable isotope signatures. As expected, densities of alternative prey became significantly higher in the plots with added plant residues. Consequently, population densities of predators (wolf spiders, predacious carabids and Japanese tree frogs) also became relatively higher and pest densities lower in these plots. We also note that, besides lepidopteran pests, alternative prey accounted for approximately 80% of predator diets. This finding suggests that they may play an important role in population persistence of generalist predators. The application of organic matter to the field for the enhancement of natural enemies’ impacts through increased alternative preys, which might be useful to manage lepidopteran pests in organic cultivation of cabbage.