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.