Mortality Pattern of Poecilus cupreus Beetles after Repeated Topical Exposure to InsecticideStochastic Death or Individual Tolerance?

The mortality of organisms exposed to toxicants has been
attributed to either stochastic processes or individual tolerance
(IT), leading to the stochastic death (SD) and IT models. While the
IT model follows the principles of natural selection, the relevance
of the SD model has been debated. To clarify why the idea of stochastic
mortality has found its way into ecotoxicology, we investigated the
mortality of Poecilus cupreus (Linnaeus,
1758) beetles from pesticide-treated oilseed rape (OSR) fields and
unsprayed meadows, subjected to repeated insecticide treatments. We
analyzed the mortality with the Kaplan–Meier estimator and
general unified threshold model for survival (GUTS), which integrates
SD and IT assumptions. The beetles were exposed three times, ca. monthly,
to the same dose of Proteus 110 OD insecticide containing thiacloprid
and deltamethrin, commonly used in the OSR fields. Kaplan–Meier
analysis showed that the mortality of beetles from meadows was much
higher after the first treatment than after the next two, indicating
the IT model. Beetles from the OSR displayed approximately constant
mortality after the first and second treatments, consistent with the
SD model. GUTS analysis did not conclusively identify the better model,
with the IT being marginally better for beetles from meadows and the
SD better for beetles from OSR fields.