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Title: Beneficial or not? Decoding carnivore roles in plant protection

item Steffan, Shawn
item CHIKARAISHI, YOSHITO - Japan Agency For Marine-Earth Science And Technology (JAMSTEC)
item Horton, David
item Miliczky, Eugene
item Zalapa, Juan
item JONES, VINCENT - Washington State University
item OHKOUCHI, NAOHIKO - Japan Agency For Marine-Earth Science And Technology (JAMSTEC)

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2015
Publication Date: 12/1/2015
Citation: Steffan, S.A., Chikaraishi, Y., Horton, D.R., Miliczky, E., Zalapa, J.E., Jones, V.P., Ohkouchi, N. 2015. Beneficial or not? Decoding carnivore roles in plant protection. Biological Control. 91:34-41. doi: 10.1016/j.biocontrol.2015.07.002.

Interpretive Summary: Food webs are known to have countless trophic links between resource and consumer species. To better understand the trophic links within complex networks (i.e., who tends to eat whom), analysis of the stable nitrogen isotopic composition of amino acids can be used to reveal animal trophic position. In the present study, we use these isotopic methods to characterize the trophic tendencies of arthropods. The trophic tendencies (or trophic spectra) reveal the high degree to which virtually all consumer species are opportunistic omnivores, sometimes blurring the lines between beneficial, pestiferous, and neutral species. Biological control is a fundamental pillar of integrated pest management (IPM), and is predicated on the concept that carnivores protect plants by eating herbivores. However, sometimes carnivores eat other carnivores (i.e., intraduild predation). Despite wide acknowledgement of intraguild predation, there previously had been no study measuring empirically the degree to which wild, free-roaming carnivores engage in such predation. Our findings illuminate the trophic function of natural enemy species, and establish a method that will allow bio-control researchers and practitioners to make more informed pest management decisions.

Technical Abstract: Recent advances in compound specific isotopic ratio analysis (CSIRA) have allowed researchers to measure trophic fractionation of 15N in specific amino acids, namely glutamic acid and phenylalanine. These amino acids have proven useful in food web studies because of the wide and consistent disparity in their respective fractionation tendencies. In controlled feeding trials, we used CSIRA to confirm the accuracy of the Chikaraishi C3-plant equation for trophic levels 1 through 4. We then collected wild arthropods from an old-field site, and each specimen was analyzed using both the Chikaraishi method and the conventional bulk-15N method. Regression analysis characterized the relationship between these two methods, allowing us to convert bulk-15N values into trophic level estimates (among the many specimens not analyzed via CSIRA). This approach facilitated greater sample sizes and thereby permitted investigations of trophic spectra and functional diversity. We show that species often thought to be strict herbivores were true omnivores, and certain carnivore species were actually pests of crop production. Thus, our study is the first to reveal which species are likely to be beneficial for crop protection, and which are not.