Effects of Pesticide Application on Chironomid Larvae and Ostracods in Rice Fields

Abstract

Benthic macroinvertebrates were surveyed in rice fields in which three pesticide applications were adopted ; no pesticide, herbicide only, and herbicide+insecticide+fungicide.Abundant taxa were chironomid and ostracod. The populations of these animals fluctuated widely in the pesticide-treated rice fields. Larval populations of odonates and dytiscids which prey on chironomid larvae and ostracods, were depressed according as various pesticides were applied. The low density of predators presumably allowed the large increase in the populations of chironomids and ostracods. However, competition between chironomids and ostracods, as well as the direct toxic effect of pesticides, may have suppressed the increase of their numbers in some degree. Benthic algae decreased with most applications of herbicide. The algae showed a slight increase probably due to heavy grazing by chironomids and ostracods and to herbicide toxicity in the pesticide-treated fields.