Tobacco Hornworm larva on artificial diet
Nutrition can be measured by its effect on growth and quantified by Relative Growth Rate (RGR). RGR is calculated as the Change in Weight per Day divided by the Initial Weight. RGR is measured as a percentage of the initial weight because larger caterpillars will grow much faster than smaller ones but the percentage increase is more similar.
Nutrition is important when developing artificial diets for research insects. Not only must the essential nutrients be present, but the ratios of the diet components must be optimized. For example, diets may be prepared with variable ratios of protein to sugar. If an insect requires more protein than sugar it would have a higher RGR on a diet with 60:40 protein:sugar than a diet with 40:60 protein:sugar. At some ratio of protein:sugar, RGR is optimal and that ratio can be determined experimentally.
Plants can be evaluated for nutritional quality. On two different plants, similar caterpillars may grow at different rates, even if consumption is the same. Further experiments can determine specific difference in nutrients that influence RGR. Some plants contain suitable nutrients in suitable ratios, but may contain toxins that make them less nutritious. The effects of sublethal toxins can be quantified using artificial diets. RGR is often lower on a diet with a toxin than diet without.
RGR is a useful tool for comparing plant quality and understanding why some plants in nature are found and eaten and not other.
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