Evaluation of the ecological and environmental factors associated in sediment bioassay in inducing cannibalistic behavior in the midge larvae, Chironomus tentans (Diptera: Chironomidae).

Abstract

Aquatic insects are frequently employed as contaminant biomonitors. Chironomus tentans, a representative of the dipteran family Chironomidae, usually inhabits the bottom mud of shallow eutrophic ponds and is easily maintained in the laboratory. These traits make this species ideally suited for evaluating contaminated sediments. The standard procedure is based on a presumed correlation between survival rate and toxicant concentration. Results from sediment toxicity tests, however indicated two causes of death, one attributable to the contaminants, and the other to cannibalism. This behavioral characteristic cannibalism, has not been documented for this species and thus may affect test results. Therefore, a series of experiments were designed to evaluate environmental effects influencing the cannibalistic behavior in C. tentans. The environmental factors found to influence the cannibalistic behavior of C. tentans are summarized in the order of importance. Shelter proved to be the most important factor for minimizing cannibalistic behavior among midges. Whereas, increased exposure time provided midges more opportunities to engage in cannibalistic activities. Population density, larval age (or size) and toxicant concentration were also shown to be relevant. Finally, temperature, food and light were determined to play only a minor role in enhancing cannibalism. Obviously, the occurrence of cannibalism among midges reduces the accuracy of sediment toxicity tests. One possible solution is to place larvae in separate containers. However, this procedure is unlikely to be cost effective. It may be possible to have multiple exposures in which each individual is separated from others by a screened-in cage that allows free transfer of sediment components. Another alternative is to ensure adequate space to minimize potential contact with others. Also, midges could be transferred one day prior to initiation of test to allow each to bury itself in the substrate. The best solution would be to seek a different species which has the advantages of C. tentans, but is not prone to cannibalism.