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The joint toxicity of coprecipitated, free-acid, nondispersible and dispersible formulations of erythrosin B and fluorescein was studied on fourth-instar Culex pipiens quinquefasciatus Say. When used alone or in combination, the dispersible formulation was more toxic than its nondispersible counterparts. The dispersible 1:1 and 1:3 (erythrosin B: fluorescein) combinations were more toxic and faster acting than any of the components used alone. The nondispersible coprecipitated formulation failed to enhance toxicity. However, the dispersible combinations showed synergistic action, with the 1:1 combination exhibiting a greater degree of synergism than the 1:3 combination. The 24-h dark incubation treatment increased the toxic effects of the coprecipitated dispersible formulation and erythrosin B used alone, but suppressed the toxicity of fluorescein.

Eight dispersants were tested in combination with free-acid erythrosin B and fluorescein to determine their toxicity. Solidogen LT-13 and Aquonium C-IV were most effective in combination with erythrosin B. Aquonium C-IV, Emulphor EL-719, Nekal BX-78, and sodium dodecyl sulfate were equally effective in combination with fluorescein. None of the dispersants alone was toxic. Four formulations of erythrosin B (free-acid dispersed with 8% sodium dodecyl sulfate, microencapsulated free-acid, aluminum lake dispersed with 8% sodium dodecyl sulfate, and microencapsulated aluminum lake) were exposed to natural sunlight and environmental conditions for up to 28 days and tested for toxicity to fourth-instar Culex pipiens quinquefasciatus Say. The dispersed free-acid erythrosin B was most toxic of the four formulations tested. The dispersed free-acid was significantly toxic at 0.5 ppm, and all four formulations were toxic at 4.0 ppm. Dispersed and microencapsulated free-acid formulations exhibited residual toxicity up to 7 days. In small-plot field tests, the dispersed free-acid erythrosin B caused significant reductions in larval and adult populations of C. p. quinquefasciatus at concentrations ranging from 0.25 to 8.0 ppm.