Generational variation in nutrient regulation for an outbreaking herbivore

Abstract

Multivoltine insects can produce multiple generations in one year. Favorable conditions support more generations, leading to serious outbreaks. For herbivores, plant nutrient availability is a major environmental factor affecting fitness and it can shift substantially throughout seasons. In a stochastic environment, organisms can adopt several strategies to regulate their nutrient intake and maximize performance. However, data regarding nutrient regulation of wild herbivores are scarce, and even more so regarding potential intergenerational plasticity. To bridge this gap, we measured nutritional regulation and performance of an outbreaking multivoltine herbivore -one of the most serious agricultural pests in the Sahel: Oedaleus senegalensis. We surveyed a field population in Senegal and measured its nutritional preference and regulation across two generations (G1 and G3) using artificial diets and plant choice experiments. In the field, G1 locusts were five to ten times more abundant than G3 locusts. We found that G1 and G3 locusts selected different protein:carbohydrate ratios but also that the strength of regulation was different. G1 locusts regulated their nutrient target more tightly than G3 locusts. In contrast, studies with laboratory populations demonstrate strong regulation for grasshoppers, appearing less plastic than field populations. Both generations selected a carbohydrate-biased nutrient ratio, although it was more carbohydrate-biased for G3 locusts. In both cases, plant nutrient contents in the field were more protein biased than their preferred diet. Therefore, choices by locusts were likely influenced by other ecological variables such as leaf toughness or plant defenses. G1females were heavier and laid more eggs than G3 females. However, G3 locusts survived longer during the experiment than G1 locusts, suggesting a potential generational tradeoff between reproduction and survival. Our data highlight the importance of studying nutritional regulation in situ and incorporating field and lab data to better understand foraging decisions and nutritional tradeoffs.

Special issue: Questioning the roles of resources nutritional quality in ecology

“…Terrestrial and aquatic ecosystems are almost equally represented in our analysis of existing scientific literature, which contrasts with the articles published in the special issue, largely dominated by aquatic studies (only two articles considering terrestrial consumers: Le Gall et al 2022, Mathieu‐Resuge et al 2022). Interestingly, parameters used for describing resources quality significantly differ between ecosystems.…”

“…As expected, most studies dealing with resources quality consider the consequences of quality parameters on the fitness and/or on the foraging behavior of consumers. Most authors thus investigate the consequences of resource quality on the growth, on the survival and on the reproduction of the consumers (Le Gall et al 2022), ultimately permitting to question secondary production and population/community dynamics in ecosystems (Rosen and Trites 2000) – even if such studies remain generally limited in the literature. Other studies (Le Gall et al 2022, van Deurs et al 2022), question food choices of consumers when exposed to resources of different quality, in more or less close relationship to the so‐called optimal foraging theory (Pyke et al 1977).…”

“…In comparison to our literature review, the articles published in this special issue of Oikos constitute a small but relatively well representative subset of studies dealing with resources quality‐related issues. Based on these studies and on the literature review, we identify a non‐exhaustive list of challenges and perspectives of research that we consider of the highest priority in the large topic of trophic ecology.‐ Both resource quantity and quality matter, but they might not intervene within the same processes, life‐stages or even generations of consumers (Le Gall et al 2022, Leal et al 2022, van Deurs et al 2022). There is a crucial need to merge those two nutritional aspects within the study of ecological processes.…”

“…A lot of published articles thus focuses on ‘potential' resource quality parameters and never measures their effective consequences on consumers. Yet, consumer requirements may vary widely depending between taxa (Frost et al 2006) but also between individuals of a similar taxon (Leal et al 2017, Le Gall et al 2022). Evaluation of consumers' requirements might also greatly depend on the purpose of the study.…”

“…‐ Both resource quantity and quality matter, but they might not intervene within the same processes, life‐stages or even generations of consumers (Le Gall et al 2022, Leal et al 2022, van Deurs et al 2022). There is a crucial need to merge those two nutritional aspects within the study of ecological processes.…”