An assessment of the chronic toxicity of fenpyroximate and pyridaben to Tetranychus urticae using a demographic bioassay

Demographic bioassays were carried out in the laboratory to assess the demographic responses of Tetranychus urticae Koch to fenpyroximate and pyridaben at three levels of sublethal concentrations (LC₁₀, LC₃₀ and LC₅₀). Life table data were used to generate an age-classified projection model in each sublethal treatment. The elasticity of population growth rate (λ) to changes in each of the individual life traits was calculated. Exposure to fenpyroximate and pyridaben treatments reduced the net reproduction rate (R_o) in a concentration-dependent manner. The treatment of pyridaben had dramatic effects on egg hatchability. The hatch rate dropped from 0.83 at untreated control to 0.27 at LC₁₀. Two different scales of population-level endpoint were estimated to compare the total effect on T. urticae between the treatments: the first endpoint, λ values were determined based on the net reproductive rate (fecundity λ); the second endpoint incorporated the mean egg infertility rate into the net reproductive rate (vitality λ). The fecundity λ decreased in a concentration-dependent manner in both acaricide treatments, but the vitality λ decreased abruptly after treatment of pyridaben. The relative difference between the two λ values was great in pyridaben treatment. Contributions of survival probability (P_i) and fertility (F _i) to the population growth rate were markedly different between the two population-level endpoints. In fecundity λ, the contribution of F_i to the λ was always higher than that of P_i. These dramatic changes in elasticity patterns reveal that acaricide effects at the population-level are strongly dependent on the life history characteristics of T. urticae and mode of action of the chemicals.