Comparison of the toxicity of a synergized and non-synergized insecticide to young trout

The toxicities of 2 formulations of resmethrin, one containing the synergist, piperonyl butoxide (PBO), were measured in 96 h and short exposure tests with brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta). Both formulations were equally toxic to brook trout with 96 h LC50 values being 1.69-1.83 ug per litre. Ageing the formulation before testing increased these values 2-3 fold. PBO was non-toxic up to 500 ug per litre. Older brown trout were significantly more sensitive to the formulations than younger brook trout. Intoxication from 6 h exposure was greater with the synergized formulation and recovery rates were lower. Standard 24 h and 96 h tests did not reveal such differences. PBO probably interfered with detoxification processes; this suggested that short-term toxicity tests should be carried out with synergized formulations to obtain realistic results.