Efficacy of polydimethylsiloxane against Culex pipiens (Diptera: Culicidae)

We evaluated, under controlled laboratory conditions, the insecticidal activity of polydimethylsiloxane (PDMS) for the control of the house mosquito, Culex pipiens. In a first series of bioassays, we tested the residual effect of different PDMS doses to control 3rd instar larvae. The label dose caused high mortality rates (>80%) even after 50 days from the initial application. Mortality levels at half the label dose were significantly higher compared to control, during the entire experimental period following a gradual decrease over time. Similar trends were observed when the 1/4 and the 1/8 of the label dose were applied with a much steeper decrease at long time intervals since the initial application especially for the lowest dose. However, after the 10th day of the bioassays and until the end of the experiments, mortality rates in the case of the label dose were higher compared to all other doses after the initial 10 days post application. Conversely, the insecticidal efficacy of PDMS against pupae was generally lower in comparison with larvae. Indeed, 10 days after the application, mortality was notably reduced, regardless of the dose rate tested. In a second series of bioassays, we assessed the efficacy of short exposures of different instars of larvae and nymphs to registered formulation and label dose. Mortality was well over 95% for 2nd instar larvae at exposure intervals ranging from 1 to 4 days. Significant mortality was recorded to 3rd instar larvae even at 15 min of exposure. Exposures longer than 30 min caused high mortality rates to 3rd instar larvae. Similar results were also recorded in pupae. Finally, in a third series of bioassays, the efficacy of PDMS in egg hatch was tested. Egg hatch was completely suppressed in the treated substrate (all egg rafts were "sinking" at the bottom of the test cups). Moreover, there was no oviposition in the case of the application of the label dose, while some egg rafts were recorded at one-eighth of the label dose. In both cases, it was recorded that PDMS acts as an oviposition deterrent. Τhe results of this study show that PDMS is effective in a wide range of conditions and application scenarios.