Conservation of Predatory Carabid Beetles (Coleoptera: Carabidae) in agroecosystems of the Southern Great Plains
Stable carbon isotope ratios (SCIR) can provide evidence of carabid dispersal powers within and among cropping systems of the Southern Great Plains. Isotope data from crop samples were within expected values for C3 (-20 to -35 0/00) and C4 (-9 to -19 0/00) plants. Aphid samples reflected the isotopic composition of their host plants. Plant and aphid SCIR data demonstrate a predictable carbon isotope enrichment or depletion rate of 13C values between primary producers and herbivores. The differences in isotope ratios between carabids, aphids, and host plants will be found and used to elucidate carabid dispersal.
It has been noted that carabids are an important component of natural enemy assemblages consuming agricultural pests. Through the use of pitfall trapping and stable carbon isotope analyses the relationship of carabid dispersal and prey consumption can be better understood (Teeri and Schoeller 1979, Peterson and Fry 1987, Ostrom and Fry 1993).
The first objective has been completed:
1. Quantify carabid colonization of annual crops (sorghum/winter wheat) from a semi-permanent habitat (alfalfa) as it relates to disturbance (tillage);
The second objective is in process.
2. Elucidate carabid dispersal powers through prey selection, diet changes within and among habitats, and larval habitat utilization;
The third objective will begin after data analysis.
3. Provide results of this research to producers and IPM professionals.
Objective 1: I have evaluated colonization of sorghum (conventional tillage vs. no-till) by carabids from a semi-permanent refugia habitat (alfalfa). Using standard pitfall traps spaced at specific distances within sorghum strips, we were able to calculate distances traveled over time for targeted carabid species.
Objective 2: Pending funding, I will elucidate the dispersal power of carabids between a semi-permanent refugia habitat (alfalfa) and an annual crop (sorghum) using stable carbon isotope analyses. This data coupled with results from objective one will be used to estimate carabid dispersal power among habitats and will be used to determine natal origins and therefore source of refuge.
Objective 3: Results from this study will be presented to producers and IPM professionals via extension publications, research journals, field days, and presentations at professional meetings.
Examination of carabid dispersal based on carbon isotope ratios can only be done within systems with distinct 13C sources. Alfalfa, a C3 plant and sorghum, a C4 plant, have distinctly different carbon isotope ratios that provide a predictive relationship as 13C depletion or enrichment occurs. Samples from pre-study wheat, 2006 alfalfa and sorghum were prepared and shipped for stable carbon isotope processing. Data has been returned for all 84 plant samples. Pre-study wheat, alfalfa, and sorghum isotope ratios were within expected values for C3 (-20 to -35 0/00) and C4 (-9 to -19 0/00) plants (See Graph 1).
Alfalfa and sorghum have host-plant specific aphid species. These aphid species exhibit isotopic signatures of the crop type (C3 or C4) they have consumed. In 2006, aphids were collected from each crop and sorted to species. Seventeen aphid samples were prepared and shipped for stable carbon isotope processing. Data has been returned for all 17 samples. Aphid isotope ratios reflected the isotopic composition of the plant type they were collected on (See Graph 2).
Carabid samples from the 2006 and 2007 sorghum growing season have been cleaned and sorted. This entails washing each sample with water to remove dirt and killing fluid. Then the sample is placed in lysis buffer for storage. Each field sample is sorted into two different vials, one containing carabids and the other containing non-carabid arthropods. Identification of carabids to species has been started for the 2006 sample set. Approximately 90% of the 2007 carabids have been identified to genus. Once a beetle is identified to species it is dissected into two subsamples (See Figure 1). One containing carabid flight muscles and soft organs (Recent, R) which are metabolically active and can reflect recent dietary intake over short periods of time. The other subsample contains carabid elytra, wings, and pronotal exoskeleton (Past, P) which are basically metabolically inactive. These inactive tissues retain carbon isotope compositions from the beetle’s past dietary intake and larval compositions indicating natal origins. This process has been completed for 198 carabids from 2006. Data has been returned for 144 carabid samples. This data indicates that the carabids trapped in sorghum retained the isotopic composition more similar to the pre-study wheat and alfalfa. These carabids moved from a C3 habitat into the C4 sorghum (See Graph 3). Samples were shipped to the University of Arkansas Stable Isotope Facility, Fayetteville, Arkansas, for stable carbon isotope processing.
There were a total of 21 sampling days in 2006 and 19 in 2007. Currently, dissections and isotope processing has been completed for five sampling days in 2006. Once 2007 carabids are identified, dissecting and isotope processing will begin for the 19 sampling days in 2007. Alfalfa and sorghum samples from the 2007 sorghum growing season still need to be prepared and shipped.
Impacts and Contributions/Outcomes
Results from this study will be communicated to producers and IPM professionals through extension fact sheets and publication in a scientific journal (e.g. Journal of Economic Entomology, Journal of Ecological Entomology). These data will be presented at county field days when complete. A poster presentation will be made at the Entomological Society of America (ESA) regional meeting and an oral presentation at the ESA national meeting. This research project will be available as a dissection publication when completed.
A poster was presented on the early data trends for adult and larval carabids at the National Entomological Society meeting Reno, Nevada in November 2008.
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