The interactive effects of protein quality and macronutrient imbalance on nutrient balancing in an insect herbivore.

The present study evaluates the interactive effects of protein quality and the ratio of protein to digestible carbohydrate on herbivore performance, various aspects of nutritional homeostasis (feeding and post-ingestive regulation) and food choice. In the first experiment, final-instar caterpillars of generalist-feeding African cotton leafworm Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) were confined to one of 20 diets varying in protein:carbohydrate (P:C) ratio (35:7, 28:14, 21:21, 14:28 and 7:35) and in the quality of dietary protein (0%, 20%, 50% and 80% replacement of casein-based protein mix with the low-quality plant protein, zein). Results indicated that the negative impact of low-quality protein on survival, development and growth was amplified as the P:C ratio of the diet fell. Consumption differences were the main underlying cause of this response. The effect of low protein quality was to reduce the efficiency of nitrogen utilization and to increase the conversion rate of ingested nutrient to lipid growth, irrespective of P:C ratio. In the second experiment, caterpillars were allowed to self-compose their preferred diet, selecting between one of four high-protein diets (all 35:7, but containing 0%, 20%, 50% or 80% zein) and one of four equal-P:C-ratio diets (21:21, again containing 0%, 20%, 50% or 80% zein). Caterpillars showed a compensatory increase in their self-selected P:C intake in response to the moderate decline in protein quality of the 35:7 food. No such response was demonstrated for the insects presented with 35:7 food with the lowest protein quality. The significance of these findings is discussed within the context of herbivore food selection.