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

John D. Stark, John E. Banks

Research output: Contribution to journalArticlepeer-review


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.

Original languageAmerican English
StatePublished - 2021


  • Life Sciences
  • Biology

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