Abstract
Hormesis is a biphasic phenomenon that in toxicology is characterized by low-dose stimulation and high-dose inhibition. It has been observed in a wide range of organisms in response to many chemical stressors, including insects exposed to pesticides, with potential repercussions for agriculture and pest management. To address questions related to the nature of the dose-response and potential consequences on biological fitness, we examined transgenerational hormesis in the green peach aphid, Myzus persicae, when exposed to sublethal concentrations of the insecticide imidacloprid. A hormetic response in the form of increased reproduction was consistently observed and a model previously developed to test for hormesis adequately fit some of our data. However, the nature of the dose-response differed within and across generations depending upon the duration and mode of exposure. Decreased reproduction in intermediate generations confirmed that fitness tradeoffs were a consequence of the hormetic response. However, recovery to levels of reproduction equal to that of controls in subsequent generations and significantly greater total reproduction after four generations suggested that biological fitness was increased by exposure to low concentrations of the insecticide, even when insects were continuously exposed to the stressor. This was especially evident in a greenhouse experiment where the instantaneous rate of population increase almost doubled and total aphid production more than quadrupled when aphids were exposed to potato plants systemically treated with low amounts of imidacloprid. Our results show that although fitness tradeoffs do occur with hormetic responses, this does not necessarily compromise overall biological fitness.
Highlights
Hormesis is a biological phenomenon whereby a stressor can have inhibitory effects at high exposure levels, but at low levels can stimulate biological processes [1,2]
Leaf-dip Exposure Exposure to sublethal concentrations of imidacloprid on leaf discs had a significant effect on M. persicae fecundity (Table 1)
One of the key questions for scientists that study hormesis within the context of environmental toxicology is, what is the consequence of the hormetic response on biological fitness [6,7,8,9,10]? Using the aphid M. persicae and insecticide imidacloprid as a model, we implemented various exposure scenarios over multiple generations as a unique approach to examine the temporal nature and biological consequences of the hormetic dose-response
Summary
Hormesis is a biological phenomenon whereby a stressor can have inhibitory effects at high exposure levels, but at low levels can stimulate biological processes [1,2]. It has been observed in a plethora of organisms responding to a wide range of chemical, physical, and biological stressors. Only through inclusion of multiple time points in experimental designs can hypotheses related to biological fitness be fully examined. Increased energy allocation to certain processes (possibly observed as hormesis) is predicted to result in decreased allocation of energy to other processes or traits, and shifts over time in tradeoff expression are expected. It has been suggested that in certain situations and for certain species hormesis might be less energetically demanding than expected, and may come at no fitness cost, even if a stressor is not encountered again or encountered at low levels [10]
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