Abstract
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.
Highlights
Native predators and parasitoids can contribute significantly to control of insect pest populations, a process known as ‘conservation biological control’ (Thies et al 2011).The presence and state of local natural habitat could be instrumental in supporting this ecosystem service, because species at higher trophic levels are generally more sensitive to land-use intensity and habitat fragmentation than the insect pests they attack (Chaplin-Kramer et al 2011)
Our results suggest that natural vegetation is a Keywords: agroecology, Ceratitis, conservation biological control, ecosystem services, integrated pest management, mango malformation disease, natural enemies, natural vegetation, pest control
Using these three pest types, whose control depends upon different groups of natural enemies, we ask (i) does the density of these pests/pathogens increase with distance from natural vegetation, and (ii) for the insect pests, does this correspond to a decline in natural enemy action with distance from natural vegetation?
Summary
The presence and state of local natural habitat could be instrumental in supporting this ecosystem service, because species at higher trophic levels are generally more sensitive to land-use intensity and habitat fragmentation than the insect pests they attack (Chaplin-Kramer et al 2011). Natural vegetation harbours greater, more diverse populations of service-providing species associated with greater agricultural yields (Tscharntke et al 2005; Bianchi et al 2006). A recent review hypothesised that ecosystem services in complex habitats (>20% untransformed) may be no stronger than in simple habitats (1-20% untransformed), the authors conceded this may not apply outside of temperate ecosystems and called for more studies in high diversity regions (Tscharntke et al 2012). To provide predictive, traitbased hypotheses linking landscape composition and biological control efficiency, quantitative analyses of real systems from many different habitats and geographic locations are required (Thies et al 2011)
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