Control of Southern House Mosquito Larvae in Louisiana Papermill Log Ponds

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Abstract
Concentrations of 1.0 and 0.5 ppm Dursban® (O, O- diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate) in total water volume provided 100% control of Culex pipiens quinque/asciatus Say for 120 days in papermill log ponds. At 0.25 and 0.0015 ppm concentrations, 90 and 30 days of 100% control were obtained, respectively. Laboratory bioassays showed that the 1.0 ppm concentration caused 10.070 mortality to 4th-instar colony C. p. quinque/asciatus larvae for 120 days, while the 0.5 ppm concentration caused 81% mortality after 120 days. The 0.25 ppm concentration caused 94% mortality at 60 days and 63% mortality at 120 days. The 0.0015 ppm concentration caused 100% mortality at 30 days but had lost its effectiveness by 45 days. Bioassay tests to determine the effects of dilution on residual effectiveness of Dursban in pond water showed that the 1.0-ppm treatment contained a concentration that was between 0.00068 (known LO90) and 0.00136 ppm 90 days after application. The 0.5- and 0.25-ppm treatments retained a concentration that was between 0.00068 (known LD90)and 0.00136 ppm 30 days after application. At 14 days after application the 0.0015-ppm treatment contained a concentration of Dursban that was between 0.00068 and 0.00136 ppm. First-instar C. p. quinque/asciailis larvae had an LD90 = 0.000021 ppm and LD90 = 0.000052 ppm while the 4th-instar larvae had an LD50= 0.000135 ppm and LD90= 0.00068 ppm. These data indicate that effective control of 1st-instar larvae could be expected in th field for some time after laboratory bioassay indicated a forthcoming loss of control to 4th-instar larvae. The lengthy success in the field could be attributed to the effectiveness of the treatments in controlling the reinfest in 1st-instar larvae which require a lower concentration of toxicant.

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