The effect of dietary protein on the growth and digestive physiology of larval Heliothis zea and Spodoptera exigua

We compared the ability of larval H. zea (Boddie) and S. exigua (Hubner) to digest and utilize dietary protein by: (a) determining the ability of different concentrations of dietary protein to support larval growth, and (b) determining the effect of different concentrations of dietary protein on the digestive physiology of the organisms, as measured by in vivo digestion of protein and proteolytic activity. Using an artificial diet containing casein as the primary source of protein, we found that H. zea was able to grow at very low levels of casein (≤0.6%), while optimal growth occurred at 1.2% casein. For S. exigua, dietary casein levels of >0.6% were required for growth, and optimal growth occurred at ≥1.2% casein. However, optimal growth in both species was not correlated with the degree of in vivo digestion of protein. The level of in vivo digestion of protein and tryptic activity in S. exigua was proportional to the concentration of dietary protein (under both acute and chronic exposure), and not the amount of food in the gut, suggesting that enzyme synthesis and/or secretion is controlled by a secretagogue mechanism. H. zea only demonstrated a secretagogue mechanism of control of tryptic activity while under acute exposure to different concentrations of casein; under chronic exposure, tryptic activity was uniform regardless of the concentration of dietary casein. When comparing the two species of noctuid, H. zea, which is the larger of the two species, produced less tryptic activity on a unit weight basis, and also digested less of the available dietary protein than S. exigua. Hence, these closely related organisms are processing dietary protein at different efficiencies.