ESTABLISHING PATHWAYS OF ENERGY-FLOW FOR INSECT PREDATORS USING STABLE-ISOTOPE RATIOS - FIELD AND LABORATORY EVIDENCE

Quantifying pathways of energy transfer be tween plants, pests, and be neficial insects is a necessary step toward maintaining pest stable ag roecosystems in the absence of chemical subsidies. A diet switching ex periment utilizing a predatory ladybird beetle, Hippodamia variegata ( Goeze), evaluated the use of naturally occurring stable C and N isotop es as an economically feasible and safe method for quantifying pathway s of energy flow within agroecosystems. Stable isotope values of the l adybird beetle Coleomegilla maculata lengi (Timberlake) collected from an agroecosystem were used to estimate the relative amount of C and N derived from agricultural plants and incorporated by ladybird beetles based on mass balance equations. At the beginning of the diet-switchi ng experiment delta(13)C and delta(15)N values of H. variegata (-12.0 parts per thousand and 6.3 parts per thousand, respectively) differed by -0.2 parts per thousand and 2.9 parts per thousand from the aphids that were provided exclusively as their diet. These data are consisten t with previous estimates of trophic level isotope effects. After swit ching the diet of H. variegata to an alternative food, isotope values of H. variegata gradually shifted toward expected values for individua ls fed this diet (-22.9 parts per thousand and 8.8 parts per thousand for delta(13)C and delta(15)N values, respectively). Isotope values of another ladybird beetle, C. maculata, collected from the field indica ted that in May, alfalfa and maize (pollen) obtained in the previous y ear contributed 32% and 68% of the C or N to the diets of these indivi duals and in August, 52%, 6%, and 42% of the C or N assimilated by the se insects was derived from alfalfa, wheat, and maize, respectively. T hese data are consistent with expectations based on the relative abund ance of C. maculata in various crops during the season. The field and laboratory data are a clear indication that isotope values are sensiti ve to dietary changes on a relatively short time scale (days) and prov ide a strong basis for the use stable C and N isotope to trace energy flow patterns of these beneficial organisms within agroecosystems.