Do the stoichiometric constraints of diet promote omnivory in a marsh katydid?

R.E. Goeriz and R.F. Denno

The mismatch in C:N stoichiometry between insect herbivores and their host plants strongly suggests that herbivores are nitrogen limited. To compensate for this inherent nutritional mismatch, “herbivores” often diversify their diets by feeding on more nutrient-rich plants or plant parts (seeds) or by consuming animal prey, a strategy with tremendous potential fitness benefits. To determine the effect of food resource quality (N content) on the diet choice and spatial distribution omnivorous insect, we conducted experiments in the laboratory and field. We used the salt marsh katydid, Conocephalus spartinae, which feeds on both Spartina cordgrass and other arthropods. By offering katydids a choice in the laboratory among four different diets, we tested the hypothesis that katydids prefer diets with a higher nitrogen content than that of plants. We also hypothesized that katydids would occur in greater abundance in areas of the marsh that offer high-nitrogen resources, namely areas with abundant prey and high plant quality. To do this we manipulated plots in a 2x2 factorial design with two levels of herbivore prey abundance and two levels of plant quality and then measured the abundance of colonizing katydids. When given a choice, katydids chose to eat significantly more of a diet that mimicked the C:N stoichiometry of herbivores. However, in the field, katydid abundance was not affected by elevated prey density but was positively affected by plant quality. These data suggest that despite their preference for herbivorous prey and associated fitness benefits, katydids more effectively track plants of high nutrient quality.