A tale of two metrics: The EPA risk quotient approach versus the delay in population growth index for determination of pesticide risk to aquatic species

The risk that two closely related insecticides, spinetoram and spinosad, posed to three Cladoceran

species, Ceriodaphnia dubia, Daphnia pulex, and D. magna was determined using two approaches, the

USEPA Risk Quotient method and the Delay in Population Growth Index (DPGI). Results of the RQ method

showed that spinetoram posed a risk to all three species, but spinosad posed a risk only to C. dubia. The

DPGI analysis showed that exposure to spinetoram resulted in populations of all three species being

delayed > 3 generation times. Exposure to the LC50 and the lower 95% CL resulted in delayed populations

while exposure to the upper 95% CL concentration of spinetoram resulted in no recovery of any of the

three species over the course of the modeling exercise (88 d). Exposure to the lower and upper 95% Cl and

the LC50 of spinosad resulted in C. dubia populations being delayed > 3 generations. D. pulex populations

were not negatively affected after exposure to spinosad. D. magna populations were delayed > 3

generations, but only after exposure to the upper 95% Cl of spinosad. These results illustrate that

although the EPA risk quotient method indicated that spinetoram posed a risk to all three species and that

spinosad only posed a risk to C. dubia, the DPGI showed that D. magna would be negatively affected by

spinosad and none of the three species would recover after exposure to the upper 95% CL of spinetoram.

Because the DPGI uses the 95% Cl as well as the LC50 in its calculation and produces a measure of

population growth and recovery or lack thereof, it provides more detailed information in terms of the

potential risk of pesticides to populations than the RQ method.