Abstract
The prey-predator relationship between Sympetrum frequens nymphs and Anopheles sinensis larvae in rice fields was investigated in the suburbs of Omiya City in Japan during the period from June to July of 1978 and 1980. Sym. frequens developed into 5th-9th instar nymphs at the beginning of June, and they emerged by the beginning of July after developing into the 10th (and final) instar stage. The population sizes of the nymphs were as large as 50, 000 in Field-A (920m2) and 30, 000 in Field-B (1, 000m2) by early June. The density of An. sinensis larvae became higher toward the end of June when the density of Sym. frequens nymphs became low. The distribution patterns of Sym. frequens nymphs and An. sinensis larvae in Field-A were observed to have a non-overlapping tendency, indicating effective predation by Sym. frequens nymphs. By precipitin testing of An. sinensis larvae antiserum against Sym. frequens nymphs collected from the surveyed fields, it was confirmed that Sym. frequens nymphs predated An. sinensis larvae in the rice fields. Rates of Sym. frequens individuals positively detected to have An. sinensis in their guts were estimated to be 0-2.7% at the beginning of June and 33.3-56.5% just before or after June 20th. The rates of individuals of An. sinensis larvae predated by Sym. frequens nymphs inhabiting the surveyed fields were estimated to be 90-100% during the first half of June. It was concluded that Sym. frequens nymphs should be superior predators in the rice fields of this surveyed district.
