Field Studies on the Toxicity of Insecticides to the Mosquito Fish, Gambusia affinis1

The toxicity of 16 insecticides against the mosquito fish, Gambusia affinis (Baird and Girard), was studied in the field. All the toxicants were applied at two to four dosages. The lower dosages approximated the probable maximum rates of toxicants used currently or which might be used in the future as mosquito larvicides. the higher dosages were four times the lower dosages, and these higher rates were considered to approximate the probable maximum dosages to be administered for the control of pests of agricultural crops. Bayer 29493 (Bayter®) (O, O-dimethyl O-[4-methylthio)-m tolyl] phosphorothioate), Bayer 25198 (O, O-dimethyl O-p-methylsulfinylphenyl) phosphorothioate), Doweo 139 or Zectran® (4-dimethylamino-3,5-xylyl methylcarhamate), AC-5727 (m-isopropylphenyl N-methylcarbamate), and Dow K-6882 (O-ethyl O-(2,4,5-triehlorophenyl) methyl phosphoromidothioate) at lower and higher dosages resulted in slight or no mortality of fish confined for 48 hours or longer in the treated plots. These materials are considered safe against the fish at the indicated dosages. Malathion, DDT and Methyl Trithion® [O, O-dimethyl S(p-clorophenyl) thiomethyl phosphorodithioate] at high and low rates resulted in appreciable mortality of the fish within a period of 48 hours of exposure. Parathion at 0.1 pound per acre caused 30% mortality of the fish 48 hours after exposure, Beyond this period no mortality was produced by this dosage. At 0.4 pound per acre parathion produced a high kill of the fish initially and appreciable mortality was obtained at this dosage up to 6 days after treatment. At the lower dosages DDT and parathion are somewhat safer than malathion and Methyl Trithion. Bayer 25141 (O, O-diethyl O-p-(methylsulfinylphenyl)phosphorothioate) (except at the lower dosage), Bayer 38156 (O-ethyl S-p-methylphenyl ethylphosphonodithioate), American Cyanamid E.N. 18133 (O, O-diethyl O-2-pyrazinyl phosphorothioate), GC-3583 (diethyl-l-(2,5-diehlorophenyl)-2-chlorovinyl phosphate), GC-4072 (diethyl-1-(2,4-dichlorophenyl)-2-chloro-vinyl phosphate), and SD-4402(1,3,4,.5,6,7,8,8-octachloro 3a,4,-7,7a-tetrahydro-4,7-methanophthalan), proved highly toxic to the fish initially at all dosage tested. The initial toxicity of Bayer 25141 at the lower dosage (0.2 pound/acre) was moderate and comparable to that of malathion; the toxicity of both declined sharply after 48 hours. All these materials at the higher dosages had long residual toxicity against the fish, but GC-3583, E.N. 18133 and Bayer 25141 had the most residual toxicity at the higher dosages. Dekafos® (3-pentadecylphenol O-O-diethylthiono phosphate) manifested only moderate initial toxicity.