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Estimation of absolute numbers of damselfly nymphs (Odonata: Coenagrionidae) by dipper sampling in California rice fields with seasonal, spatial distributions and vegetation association.

Estimates of relative and absolute density in rice field populations of damselfly nymphs (predominantly Enallagma civile with few Ischnura denticollis) were compared using the regression method. An equation, X = Y divided by 0.0016, allows estimation of absolute density (X) from a relative density index (Y, dipper count). In the rice growing area of Fresno, California, nymphal population peaks appeared during June and August approaching 3-5 million per 0.405 ha (1 acre). Spatial distribution was theoretically represented fairly well by a negative-binomial distribution. The degree of clumping is one of overdispersed types; it is especially classified as a model of randomly distributed colonies with mean colony size fixed. Presence or absence of submerged vegetation markedly affected damselfly nymphal density but the biomass of submerged or emerged vegetation was not a significant factor.