Radioisotope Measurement of Food Consumption by a Leaf Beetle Species, Chrysomela Knabi Brown

Abstract

A field using radioisotopes to estimate consumption of vegetation by insects was evaluated by comparisons with feeding rates measured in the laboratory. The uptake of 137Cs was investigated for beetle larvae (Chrysomela knabi Brown) feeding on willows in the White Oak Lake bed, an area contaminated with radioactive waste products. Beetle larvae and adults accumulate 137Cs from willow leaves and reach steady—state equilibrium concentrations. Information on biological half—lives (elimination rates) for cesium was used to convert the 137Cs concentrations attained by the beetle larvae into estimates of 137Cs intake rates. Food intake rates were estimated from radiocesium intake rates, by measuring 137Cs concentrations in willow leaves. Food intake rates estimated by this method for field areas were 7 to 16 mg dry wt of plant per larva per day. Laboratory measurements of food consumption rates, by comparison, were about 9 to 10 mg/larva per day for larvae of similar size. Influences of temperature on biological half—lives were also studied, but no correction was applied to the field estimates since mean field temperatures were close to those used in laboratory experiments. The good agreement between laboratory and field measurements supports the validity of previous applications to entire communities of plant—feeding insects. The method would be equally useful with predaceous animals as with herbivores. Measurements of food eaten can be converted to energy units by bomb calorimetry.