Tannic Acid, Protein, and Digestible Carbohydrate: Dietary Imbalance and Nutritional Compensation in Locusts

The combined effects of dietary imbalance and the allelochemical tannic acid on fifth stadium Locusta migratoria were investigated. In a factorial—design experiment, insects were fed artificial diets containing digestible carbohydrates and proteins in equal proportions (14 or 28%), or in a 1:2 or 2:1 ratio, with or without 10% tannic acid. Growth, consumption, and utilization efficiencies were measured over the course of the fifth stadium, and histological analyses of the midguts of newly molted adults were undertaken. While the consumption of diet was higher on the 14%—protein diets, consumption of protein remained lower than on the 28%—protein diets, indicating that compensatory feeding for protein was incomplete. Similarly, compensation for low dietary carbohydrates occurred, but was incomplete. Low levels of either nutrient resulted in increased intake of the other, but the effect of protein on carbohydrate intake was stronger than vice versa. There was a statistically interactive effect of carbohydrate and protein on the amounts of carbohydrate consumed, resulting from the fact that particularly high levels of carbohydrate were consumed when the diets simultaneously contained 28% carbohydrate and 14% protein. The terms "incidental augmentation" and "incidental restriction" of intake are introduced to describe the ways that certain nutrient groups may affect the intake of others in foods that are nutritionally imbalance. Tannic acid resulted in reduced efficiency of conversion of ingested nitrogen, but the efficiency of conversion of total ingested nutrients was higher on the diets containing tannic acid. A higher frequency of lesions was observed in the midgut epithelia of tannin—fed insects than controls, and it is suggested that the sloughing of necrotic epithelial tissues into the lumen and their subsequent egestion with the feces may account for the reduced efficiency of nitrogen conversion in these insects. A statistical interaction between protein levels and tannic acid demonstrated that tannic acid reduced consumption, but only of the low protein diets. Therefore, the effect of tannic acid was to restrict compensatory feeding for protein. A consequence of this was reduced growth in insects fed low—protein diets containing tannic acid. Total growth (dry mass) was lower on the 28%—protein diets and higher on the 28%—carbohydrate diets, and therefore corresponded with levels of carbohydrate intake. Nitrogen accumulation increased with dietary protein levels, but remained constant across carbohydrate levels despite increased intake of nitrogen on low—carbohydrate diets. This was accounted for by lower efficiency of conversion of nitrogen on the 14%—carbohydrate diets. Approximate digestibility was higher for the 14%—protein diets, possibly reflecting increased intake of easily digested carbohydrates. There were no statistically significant interactive effects between the two nutrients, or between either nutrient and tannic acid, on utilization efficiencies.