Entomological Special Study No. 31-002-71. The Effects of Diatoms on the Larvicidal Activity of Dursban (Registered Trade Mark).

A colony of field-collected diatoms was established in the laboratory and used to study interactions between the diatoms, 3rd to 4th instar larvae of Culex pipiens quinquefasciatus Say, and Dursban 0,0-diethyl 0-3,5,6,-trichloro-2-pyridyl phosphorothioate. Comparative observations of diatom cell density and the pH of the culture medium indicated that pH, in itself, could be used as a measure of increasing cell density and the metabolic state of the diatom culture. Dursban at a rate of 0.4 ppm had an adverse effect on the development of the diatoms. Diatoms had no discernable effect on the degradation rate of Dursban. Diatoms appeared to accumulate definite quantities of Dursban, but the amounts were minute relative to the total amount of insecticide present in the culture medium. Diatoms had no significant effect on the larvicidal activity of Dursban. Most of the loss of Dursban from the culture medium, and corresponding decrease in larvicidal activity, was attributed to ultraviolet degradation of the insecticide. Author