Organic rice is twice as nice
Investigator: Dong-Kyu Lee
Thursday Aug 15th, 2002
by John Bonner
Growing rice by organic methods is not just good for the environment, according to Korean researchers. Avoiding pesticides in paddy fields encourages certain fish, which effectively control mosquitoes that spread malaria and Japanese encephalitis.
Dong-Kyu Lee of Kosin University in Busan, South Korea, described the results of a study comparing the ecological effects of cultivating rice by organic and conventional methods. He counted the actual number, and relative abundance, of the aquatic insects found in both types of paddy.
As expected, the paddies in which no insecticides were used had a richer variety of insect life. But the actual numbers of larvae of two species of mosquitoes, Anopheles sinensis and Culex tritaeniorhynchus, were significantly lower in the organic sites, Lee found.
The former is the main vector species for malaria in the region, while the latter transmits the potentially fatal viral infection, Japanese encephalitis.
Lee suspected that the larvae were being eaten by a small fish, the muddy loach Misgurnus mizolepsis, which was much more abundant in the insecticide-free fields. So he examined the fish's effectiveness as a biological pest-control agent.
When placed in tanks containing 1200 larvae of both types of mosquitoes, the tiny fish were voracious predators, Lee found. The fish ate an average of 1121.8 larvae in 24 hours, and 1195.8 larvae within 36 hours.
The fish showed a slightly higher predation rate on Culex mosquitoes than on Anopheles, but "muddy loach are effective biological control agents for both vector species," Lee said.
Lee's findings have convinced the Busan local authority to try and use the fish to control mosquito larvae living in small ponds and ditches within the municipality. The fish is not the only form of biological pest control that his research suggests could be effective.
Lee's work studies show that the king snail, an aquatic species, would be just as effective as herbicides in controlling the growth of weeds in the rice crop.
The snail is not native to the area, having been originally imported from South America. However, there is no risk that the snail could itself become a pest because the mollusk is not hardy enough to survive the ferocious Korean winter. Rice farmers wanting to use the snails in their fields, Lee said, would have to buy a fresh supply of snails each year.
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