Seasonality shapes the amplitude of vole population dynamics rather than generalist predators

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Sublethal concentration of insecticide amplifies interference competition in a tortrix moth

Insecticides are extensively used worldwide to kill insect pests, yet organisms are most often exposed to insecticides at sublethal concentrations. Our understanding of sublethal effects on life histories is needed to predict the impact of insecticides on population dynamics and improve insecticide use and pest control. Sublethal concentrations can impact life histories directly and indirectly through changes in the intraspecific competition. Yet, few studies have evaluated the sublethal effects on intraspecific competition and these do not disentangle the insecticide effects on interference competition versus exploitative competition. As such, sublethal effects on the relative contribution of each pathways in shaping life histories are largely unknown, despite the fact that this can impact population dynamics. In this study, we focused on the neurotoxic insecticide spinosad and investigated its sublethal effects on interference among the aggressive larvae of the tortrix moth Adoxophyes honmai and the consequences for life histories. We conducted a set of paired experiments to disentangle the insecticide effects on interference from the ones on exploitation. Spinosad was found to amplify interference with most effects on mortality which lets us suggest that the insecticide likely increases the level of aggressive interactions resulting in more conspecific killings (e.g. cannibalism). Spinosad exposure was found to impair movement ability. Less movements may increase susceptibility to conspecific attacks and or increase aggresivity for better defence, two plausible mechanisms that could explain the increase in interference with insecticide. This study shows that insecticide at sublethal concentration can impact life histories by altering the strength of interference competition. Many organisms (pest and non-target species) compete through interference and theory predicts that a change in interference can substantially change dynamics. Our finding therefore reveals the importance of assessing the effect of insecticides on the mechanisms of competition when predicting their impact on populations

Experiment1

Data on stage, survival, body length, head width, and pupa mass by age for all individuals in experiment 1

Experiment2

Data on stage, survival, body length, head width, and pupa mass by age for all individuals in experiment 2

Asymmetric interactions and their consequences for vital rates and dynamics: the smaller tea tortrix as a model system

Asymmetric interactions among conspecifics can have diverse effects on population dynamics including stabilization, generation of cycles and induction of chaotic fluctuations. A difficult challenge, however, is establishing the link between the impact of asymmetric interactions on life history and the consequences for population dynamics. The smaller tea tortrix, Adoxophyes honmai, is a good example. Larval instars differ dramatically in size and have a tendency for cannibalism, which suggests the potential for strong asymmetric interactions among instars. Yet whether these asymmetries have any role in generating the distinct single-generation cycles observed in the field and laboratory is unclear. Here we report on the development of a new experimental approach to characterize the impact of asymmetric interactions on life history that can be directly embedded into stage-structured population models. The experiments use donor-replacement protocols in which focal individuals are challenged to complete their life-cycles in competitive environments where the instar and density of the competitors is held constant. The experimentally-derived interaction surface contains all the information about stage-specific interactions and provides a straightforward framework for evaluating alternative ways of abstracting the interactions into traditional models of asymmetric competition. Working with the smaller tea tortrix, we found strong evidence of asymmetric interactions and identified critical ‘tipping points’ in the competitive environment that strongly affected survival but not development. We incorporated the experimentally-derived interaction surface into a stage-structured population model and found that despite the strong impact that asymmetric interactions have on tea tortrix life history, they do not scale-up to impact the predicted asymptotic population dynamics. Comparing these dynamics with two abstracted models of stage-structured interactions revealed that while the quantitative details of the emergent dynamics depends on the shape of the interaction surface, the qualitative features — such as the emergence of single-generation cycles and rapid synchronization of development among individuals — are pleasingly robust