Effect of Sublethal Concentrations of Imidacloprid and Precocene on Green Peach Aphid, Myzus Persicae (Sulzer) (Hemiptera: Aphididae): A Study of Hormesis at the Gene, Individual and Population Level

Abstract

Threshold and non-threshold linear models that govern toxicology are challenged by an alternative model, hormesis. It is defined as low-dose stimulation and high-dose inhibition from a stressor. Insecticide-induced hormesis has been studied in a plethora of insect-insecticide models at biochemical, individual and population levels. This research focuses on the effects of sublethal concentrations of insecticides on reproductive responses of green peach aphid, Myzus persicae (Sulzer), at individual and population level besides regulation of stress, dispersal and developmental genes during hormesis. In laboratory studies, irrespective of the duration and route of exposure, sublethal concentrations of imidacloprid induced stimulations in fecundity of M. persicae but the nature of response differed intra- and trans-generationally. Fitness tradeoffs could be rendered due to declined fecundity in successive generations. However, continuous exposure to sublethal concentrations does not compromise overall fitness trans-generationally, considering recovered levels of fecundity as controls in successive generations and the total reproduction after four generations. Greenhouse experiments affirmed uncompromised fitness where reproductive stimulations were noted in aphids exposed to imidacloprid treated potato plants. Up- and down- regulation of stress, dispersal and developmental genes was noted during imidacloprid-induced hormesis in M. persicae that mirrored the reproductive responses in few instances. Global DNA methylation results emphasized the heritability of adapted traits trans-generationally via hypermethylation. Dispersal related genes (OSD, TOL and ANT) that are predominantly expressed in alates (about 2- to 5-fold) were affected in apterous aphids continuously exposed to sublethal concentrations of imidacloprid. No direct relation with the previously noted fecundity was established implying adaptive cellular stress response pathways might be triggered rather than normal regulatory processes due to low-dose imidacloprid exposure. At a biochemical level, a study noted that imidacloprid-induced hormesis concurrently stimulated juvenile hormone III (JH) production and fecundity in M. persicae. Precocene, an anti-JH, at sublethal concentrations induced reproductive stimulations in M. persicae. Gene regulation during precocene-induced hormesis mirrored imidacloprid results for few genes including FPPS, a JH precursor gene, with a higher magnitude of regulation. Considering these stimulatory effects that insecticide-induced hormesis at various biological hierarchies, causes for pest resurgence, hormesis could have ramifications from declines in natural enemy population.