Natural vegetation benefits synergistic control of the three main insect and pathogen pests of a fruit crop in southern Africa

Abstract

Summary: Most studies of the potential for natural habitat to improve agricultural productivity have been conducted in transformed, temperate regions, but little is known of the importance of agroecosystem services in biodiverse developing countries. Natural vegetation may promote the density and/or diversity of natural enemies of crop pests, but the strength of the effect varies, and few studies directly measure concurrent impacts on pest density. Considering multiple pest species within the same agroecosystem may help explain why some pests are more affected than others by landscape complexity. Here, we investigated multiple pest species (three species of Tephritidae fruit fly, leaf galling flies and pathogenic fungi Fusarium spp.) and their enemies in cultivated mango Mangifera indica, in north-eastern South Africa. The density of generalist Tephritidae fruit flies increased with distance from natural vegetation during harvesting months, and predation rate of pupae sharply decreased from ~50% at the edge with natural vegetation to 0% at 250 m into the crop. Parasitism rates of the cryptic, gall-forming fly increased with proximity to natural vegetation, but pest density was unrelated to distance from natural vegetation. Incidence of the fungal pathogen disease increased with distance from natural vegetation, possibly due to decreased predation of commensal mites. Although the relationship with distance to natural vegetation was significant for all species considered, the strength of this relationship varied across pest species and type of natural enemy studied, suggesting the benefits of natural vegetation depend on each natural enemy species' ability to disperse into the agricultural environment. Synthesis and applications. Our results suggest that natural vegetation is a net source of natural enemies in a region of South Africa that still contains much of its natural biodiversity. However, the decline in natural enemies, and increase in pests, with distance from natural habitat indicates that this biocontrol is limited by natural enemy dispersal. In landscapes like these that are still dominated by natural habitat, conservation biocontrol can still be improved by management aimed at providing corridors of key plants and habitat elements into the crops, to facilitate natural enemy dispersal. Our results suggest that natural vegetation is a net source of natural enemies in a region of South Africa that still contains much of its natural biodiversity. However, the decline in natural enemies, and increase in pests, with distance from natural habitat indicates that this biocontrol is limited by natural enemy dispersal. In landscapes like these that are still dominated by natural habitat, conservation biocontrol can still be improved by management aimed at providing corridors of key plants and habitat elements into the crops, to facilitate natural enemy dispersal.