Residual and sublethal effects of fenpyroximate and pyridaben on the instantaneous rate of increase of Tetranychus urticae

The residual and sublethal effects on Tetranychus urticae Koch adult mortality, and their influence on the reproduction and population growth, were evaluated after exposure to fenpyroximate and pyridaben. The effects on the reproduction were divided into two parts and analyzed separately to determine the contribution to the population growth parameter, the instantaneous rate of increase (ri); (1) reproduction on survivors (RS, number of eggs produced), and (2) survivors of progeny (SP, number of immatures developed). The RS and SP were censused 3 and 6 days after treatment, respectively. The direct toxic effects on female adults on treatment with fenpyroximate were always higher and more persistent than those with pyridaben, whereas the effects on the RS and SP were always higher with pyridaben. When the ri was the evaluated endpoint, the RS ri and SP ri for both acaricides declined as the concentration increased, but the reduction was always greater with exposure to pyridaben. The residual effects on the RS ri and SP ri remained strong over the range of residual days tested for both acaricides. The negative values of the SP ri with all the pyridaben treatments indicated that the pyridaben possessed strong and persistent ovicidal activity against T. urticae. The effective concentration (EC50) that reduced the RS ri or SP ri by 50% was always higher than 50% of the lethal fenpyroximate concentration (LC50), but the opposite was true with pyridaben, suggesting that pyridaben treatment would be more devastating to the T. urticae populations than fenpyroximate. The results of this study indicate that fenpyroximate and pyridaben have different direct acute toxicity and reproduction effects in T. urticae, even though they share the same mode of action: mitochondrial electron transport inhibition.