Chromogen-Ratio Method for Determining Digestibility of Plants by Grasshoppers

The concentration of plant pigments in excreta vs. food provided a measure of digestibility of most plants by grasshoppers. Digestibility values obtained with the chromogen-ratio method and consumption-excretion ratios were highly correlated (r = 0.789**). Greatly disparate results were obtained in 4 cases. The chromogen method, when applicable, gave better duplicability and eliminated many of the errors and much labor associated with consumption-excretion ratios. The chromogen-ratio method is based on the assumption that nonabsorbable substances in a forage may be used as markers and that the concentration of these substances in the excreta will be proportional to the digestibility of the forage. Inert substances such as chromic oxide have been successfully utilized in insect-nutrition studies (McGinnis and Kasting 1964). Most plant pigments would seem applicable to such a technique and are naturally incorporated into insect diets in uniform quantities. Reid et al. (1950) first reported a method for using plant pigments as markers in digestibility studies with dairy cows. Smart et al. (1954) modified Reid’s method by introducing copper into the porphyrin ring of chlorophyll and pheophytin, thus stabilizing these pigments to light, acid, and alkali, and at the same time largely eliminating the carotenoids. This paper reports results obtained with the chromogen-ratio method in digestibility studies involving insects. H E I N R I C H S A N D PR U E S S , J O U R N A L O F E C O N O M I C E N T O M O L O G Y 5 9 (1 9 6 6 )