Ostrom, P.H., M. Colunga, and S.H. Gage
Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )
Quantifying pathways of energy transfer between plants, pests, and beneficial insects, is a necessary step toward maintaining pest stable agroecosystems in the absence of chemicalsubsidies. A diet switching experiment utilizing a predatory ladybird beetle, Hippodamia variegata (Goeze) was performed to evaluate the use of naturally occurring stable C and N isotopes as an economically feasible and safe method for quantifying pathways of energy flow within agroecosystems. Stable isotope values of the ladybird 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-switching experiment carbon and nitrogen isotope values of H. variegata (-12.0o/oo and 6.3o/oo, respectively) differ by -0.2o/oo and 2.9o/oo from the aphids that were provided exclusively, as their diet. These data are consistent with previous estimates of trophic level isotope effects. After switching the diet of H. variegata to an alternate food, isotope values of H. variegata gradually shifted toward expected values for individuals fed this diet (-22.9o/oo and 8.8o/oo for carbon and nitrogen isotope values, respectively).Isotope values of another ladybird beetle, C. maculata, collected from the field indicated that in May, alfalfa and maize (pollen) obtained in the previous year contributed 32% and 68% of the C or N to the diets of these individuals and in August, 52%, 6%, and 42% of the C or N assimilated by these insects was derived from alfalfa, wheat, and maize, respectively.These data are consistent with expectations based on the relative abundance of C. maculata in variouscrops during the season. The field and laboratory data are a clear indication that isotope values are sensitive to dietary changes on a relatively short time scale (days) and provide a strong basis for the use stable C and N isotope to trace energy flow patterns of these beneficial organisms within agroecosystems.
Back to meeting | Show |