Occurrence of American tadpole shrimp, Triops longicaudatus (LeConte), in Paddy fields cultivated under conventional and organic farming methods

Abstract

The American Tadpole Shrimp, Triops longicaudatus, a possible biological weed control agent, was studied in paddy fields using conventional farming (with pesticides and chemical fertilizer) and organic farming methods at three localities in Hiroshima Prefecture. The results indicate that T. longicaudatus did not occur in six fields using organic farming, but occurred in five of 8 fields using conventional farming. To understand this difference, the seasonal abundance of T. longicaudatus and its predator aquatic insects was investigated. Preliminary results suggested no impact of predator aquatic insects on the abundance of T. longicaudatus in conventional and organic paddy fields. An experimental inoculation of 50 metanauplii of T. longicaudatus was conducted in artificial paddy containers using soil from conventional and organic farming methods. In the organic containers, all metanauplii died within 6 days of inoculation, but about 50% survived in the conventional containers. The water pH in the organic containers was approximately 4.9, much lower than that in the conventional containers, which was more than 6.3. A similar difference in pH was observed between conventional and organic paddy fields at HigashiHiroshima. These results suggest that the low pH of water in paddy fields using organic farming affects the survival of T. longicaudatus, especially young larvae.