Macro-scale perspectives in conservation biological control: latitudinal differences in the effects of organic farming on insect pests and natural enemies in rice paddy ecosystems

Abstract

The pest control services provided by natural enemies are important for maintaining crop productivity in environmentally friendly agriculture with reduced use of pesticides. Organic farming is generally considered to increase the abundance of natural enemies and to enhance ecosystem services, but species and guild compositions vary from region to region, and so the population levels of pests and their natural enemies can be expected to vary geographically. Here, we surveyed the abundance of rice insect pests and spiders as their natural enemies in 51 organic and 45 conventional paddy fields at 22 locations in Japan and examined latitudinal differences in the effects of farming practices on their abundance. Independently of latitudinal differences, organic farming had positive effects on the abundance and species diversity of spiders caught in nets and on Tetragnatha and lycosid spiders. It increased the abundances of the leafhopper Nephotettix cincticeps and the white-backed planthopper Sogatella furcifera, and decreased that of the small brown planthopper Laodelphax striatellus. Both spiders and insect pests showed diverse responses to the interaction between farming practice and latitude. In particular, insect pests distributed in warmer regions, such as So. furcifera and Ne. cincticeps, tended to be more abundant in the south. The ratios of spider to pest abundance, as indicators of pest suppression, were lowest in organic paddies at lower latitudes for So. furcifera and Ne. cincticeps; at higher latitudes they were higher in organic paddies than in conventional paddies for La. striatellus. These results suggest that the effectiveness of pest suppression by natural predators in organic paddy fields varies by region depending on the ecological characteristics of each organism. As such, it is necessary to devise field management strategies appropriate to each region to balance crop production and biodiversity.