Natural farming and rice planthoppers in Western Japan

Planthoppers are serious pests of rice in both tropical and temperate East Asia. In temperate regions, population density has been shown to be largely determined by the rate and timing of immigration from tropical areas. Historical records of planthopper outbreaks in western Japan over 1300 years suggest that the cropping system may influence planthopper populations as the likelihood of an outbreak has increased substantially since the conversion to intensive rice production during the second half of the twentieth century. To investigate this in more detail, planthopper populations were compared in naturally and conventionally farmed paddies in Hiroshima Prefecture. Naturally farmed paddies differed from conventionally farmed ones in the absence of synthetic chemicals and the use of plant compost as the sole source of fertilizer. Paddies farmed naturally for >8 years had less planthopper damage than conventional paddies, slightly lower densities of white-backed planthopper, much lower densities of brown planthopper, and higher rates of planthopper parasitism by the nematode Agamermis unka. Nematode density and parasitism showed a close correlation with variation in planthopper populations, suggesting that nematodes may control planthoppers in naturally farmed paddies.