Quantifying pathways of energy transfer between plants, pests, and beneficial insects is a necessary step toward maintaining pest stable agroecosystems in the absence of chemical subsidies. A diet switching experiment utilizing a predatory ladybird beetle, Hippodamia variegata (Goeze), evaluated 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 δC and δN values of H. variegata (-12.0‰ and 6.3‰, respectively) differed by -0.2‰ and 2.9‰ 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 alternative food, isotope values of H. variegata gradually shifted toward expected values for individuals fed this diet (-22.9‰ and 8.8‰ for δC and δN 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 various crops 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.
A monitoring system established within an array of cultivated and uncultivated habitats was used to characterize the first 5 yr of establishment of the exotic multicolored Asian lady beetle, Harrrwnia axyridis (Pallas), in an agricultural landscape of southwestern Michigan. Population trends over time were summarized for II species of resident cocci nell ids before ,md after the arrival of H. axyridis. In addition, annual population increase, habitat utilization patterns, and within-season population fluctuation for H. axyridis were analyzed. H. axyridis became a dominant coccinellid species in the landscape 4 yr after its arrival. Adults of this species were found in all habitats monitored, including early secondary succession, poplar plantation, alfalfa, soybean, corn, and winter wheat. Significant population peaks were observed early and late in the season, depending on the habitat. Resident species that appear affected by the establishment of H. axyridis are Brachiacantha ursina (F.), Cycloneda /nunda (Say), and Chi/acarus stigma (Say). However, to attribute the decline on the numbers of those species to H. axyridis will require further assessment.
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