D3373: Geographical trends in the response of insect pests and spiders to organic farming in Japanese rice paddy fields

Organic farming can increase the abundance and diversity of arthropods. The responses of pests and natural enemies, however, are predicted to differ geographically because the species and guild composition varies with areas. We investigated the abundances of spiders and rice insect pests at organic and conventional paddy fields in Japanese archipelago, and examined their responses to farming practice and latitude. Total abundance of spiders was higher at organic farming paddies than conventional paddies and in lower latitude areas, while the main spider guilds showed various responses. Organic farming increased the abundance of Tetragnatha spiders. At higher latitude, linyphiid spiders were less abundant, while Pachygnatha spiders were more abundant. Notably, an interaction between farming practice and latitude was observed in lycosid spiders; the abundance decreased with latitude in conventional paddies, whereas there were no latitudinal trends in organic paddies. The effects of farming practice and latitude differed between insect pests. Organic farming increased the abundance of Naranga aenescens and Stenotus rubrovittatus. Higher abundance of Laodelphax striatellus was observed at higher latitude areas. An interaction between farming practice and latitude was also observed in abundance of Nephotettic cincticeps and Sogatella furcifera; the abundance in conventional paddies was merely affected by latitude, but it was significantly higher in organic paddies at lower latitude areas. These results suggest that the effectiveness of organic farming practices in controlling pests by natural enemies varies through areas. Therefore, it is necessary to propose suitable farming practice for pest management depending on regions.