Dietary protein and sodium co‐limit cockroach growth and reproduction

Abstract

Abstract Both dietary protein (N) and sodium (Na) are essential nutrients for invertebrate growth and reproduction. Field studies show that insects aggregate in areas with both high protein and sodium and their abundance is co‐limited by these two essential nutrients, but the mechanism behind this aggregation is not well known. We examined a possible mechanism that dietary protein and sodium co‐limit insect herbivore growth and reproduction. To test this mechanism, we conducted a feeding experiment examining the effect of these two nutrients by using artificial diets with two levels of protein and three levels of sodium on a generalist herbivorous cockroach Blaptica dubia. Elevated levels of dietary sodium interacted with low protein level to increase cockroach nymph relative growth rate during earlier instars. Female cockroaches always weighed more and had a higher relative growth rate than males regardless of dietary treatment. When compared between different dietary treatments, female cockroaches' relative growth rates were higher when fed with a high protein diet, while male cockroaches reached the final instar faster when fed with a high sodium diet. Cockroach reproduction was not affected by protein or sodium alone, but the combination of a high protein and high sodium diet resulted in the heaviest offspring. Our results suggest that both protein and sodium may be important for both the growth and reproduction of insect herbivores. The interaction of these two nutrients to improve cockroach growth and reproduction indicate a level of co‐limitation.