Generational variation in nutrient regulation for an outbreaking herbivore

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. G1 females were heavier and laid more eggs than G3 females. However, G3 locusts survived longer during the experiment than G1 locusts, suggesting a potential generational trade-off 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 trade-offs.