Negative photoaxis of mosquito larvae as a potential tool in the rapid bilogical monitoring of aquatic wastes (Diptera: Culicidae)
A little-known approach to toxicity testing--based on negative phototaxis of larval Aedes aegypti--was investigated as a contribution to the search for rapid methods applicable to the field of water pollution control. Zinc and copper were the toxicants tested. All tests were conducted with a standard "synthetic" dilution water.
A mosquito colony was established to provide a uniform supply of test larvae. Preliminary tests were performed on the acute toxicity of zinc and copper against A. aegypti larvae, as well as tests on larval growth and development at various concentrations of the metals.
For the photomigration toxicity tests, two juxtaposed troughs were used, one containing the test solution, the other a control. Third instar larvae migrated away from a six-watt fluorescent light for two minutes per run. This was repeated at intervals until 50% were unable to migrate 50 cm in 120 sec. Photographs were taken of the larval migrations. From the pictures an empirical criterion was derived (the 40-cm, 60-sec ET₅₀) through a series of graphical interpolations. All inactivation analyses were based on this criterion.
From time-inactivation regression lines, exponential toxicity curves were obtained by interpolation. The curves were of an unusual shape, depicting the characteristic nature of the dosage-response.
The sensitivity of the inactivation technique was comparable to that of the acute toxicity tests. However, inactivation was far quicker; depending on concentration, it occurred within one to five hours. By all methods used in this study, zinc and copper were judged to be slow-acting and of low overall toxicity. Copper was, however, consistently more toxic than zinc by at least one order of magnitude. The ET₅₀ in 10 ppm Cu⁺⁺ was 147 min.; in 10 ppm Zn⁺⁺, it was 209 min.
Some possible improvements in technique were discussed. It was suggested that the photornigration approach to toxicity testing can be of definite practical use to biologists in water pollution control.