A novel mechanism for insecticide tolerance: The role of phenotypic plasticity in amphibian tolerance and cross-tolerance to insecticides

Background/Question/Methods

With increasing human impacts on the environment, an important challenge for ecologist is to consider how organisms respond to anthropogenic stressors. The global use of insecticides is critical to disease control and efficient crop production, but they may also pose unintended consequences to ecological systems. Phenotypic plasticity can allow organisms to rapidly induce adaptive phenotypes to novel environments. Surprisingly, despite growing interest in plasticity and the prevalence of insecticide use, only one other study has considered the possibility of induced insecticide tolerance. Secondly, natural systems are exposed to many pesticides that vary in mode-of-action but no studies have considered the role of plasticity in allowing organisms to induce cross-tolerance to different insecticides that share and differ in mode-of-action. We exposed four populations of wood frogs (Lithobates sylvaticus) located close-to and far-from agricultural fields to sublethal concentration of the AChE-insecticide, carbaryl, at the embryonic and hatchling stage. Then, we used time-to-death assays to assess whether this early exposure to carbaryl induced increased tolerance to lethal concentrations of carbaryl later in life. Using similar methods, we also investigated whether sublethal exposure to carbaryl could induce cross-tolerance to two insecticides that share (chlorpyrifos and malathion) and differ in their mode-of-action (cypermethrin and permethrin).

Results/Conclusions

We discovered that exposing some populations of embryos and hatchlings to sublethal concentrations of the insecticide carbaryl induced higher tolerance to a subsequent lethal concentration later in life. Consistent with predictions of genetic assimilation, inducible populations were located far from agricultural areas (>800 m) whereas noninducible populations were located close to agricultural areas. Populations induced to have higher tolerance were often induced to also have higher AChE concentrations. Next, we found that embryonic exposure to sublethal concentrations of carbaryl induced higher cross-tolerance to malathion and cypermethrin. These results are the first to demonstrate inducible tolerance in a vertebrate species and the pattern of inducible and constitutive tolerance among populations suggests the process of genetic assimilation. Additionally, these results demonstrate that that the phenomenon of induced tolerance also provides induced cross-tolerance that is not restricted to families of pesticides that have the same mode of action. The ability to rapidly induce tolerance to insecticides has important conservation implications potentially allowing organisms a rapid way to persist even in the face of insecticide contaminants.