TOXICITY OF ORGANOPHOSPHORUS INSECTICIDES TO 4TH-INSTAR LARVAE OF CHIRONOMUS-TEPPERI SKUSE (DIPTERA, CHIRONOMIDAE)

Commercial formulations of chlorpyrifos, trichlorfon, malathion, diazinon, temephos, fenthion, chlorfenvinphos and chlorpyrifos-methyl were bioassayed at 25 +/- 1-degrees-C against fourth-instar larvae of the rice bloodworm, Chironomus tepperi. Chlorpyrifos was the most toxic, with an LC50 of 1.3 ppb. The relative order of LC50s in ppb was (LC50 values in brackets); chlorpyrifos (1.3) < chlorpyrifos-methyl (2.5) < temephos (2.8) < chlorfenvinphos (6.0) < malathion (8.4) < diazinon (35.5) < trichlorfon (67.4) < fenthion (159.4). All compounds are sufficiently toxic to be effective against C. tepperi in the field, but the results suggest that some insecticides, particularly chlorpyrifos, are being applied at excessive rates.