Towards Incorporating Insect Isotope Analysis Using Cavity Ring-Down Spectroscopy into Area-Wide Insect Pest Management Programs

Measurement of natural stable isotope signatures of insects has proven to be a useful method in determining their natal origin, feeding strategies and mating behavior, and has spawned a new discipline of insect isotope forensics. However the complexity of measuring isotope signatures using elemental analysis-isotope ratio mass-spectrometry (EA-IRMS) has encountered significant technical barriers, which together with costs, have hindered the advancement and wide spread adoption of this potentially useful approach. In the last decade simpler technologies to measure stable isotope ratios in samples based on laser spectroscopy have been developed. We foresee that these instruments will fuel the advancement and adoption of insect isotope forensics into pest management programs because they are easy to use, more affordable, and complement existing marking tools. However prior to the adoption of these new technologies it is necessary to test their fitness for purpose and explore their limitations. In this paper we set out to test the suitability of using combustion module-cavity ring-down spectroscopy (CM-CRDS) for measuring isotope ratios in an entomological context, covering marking of insects, mating behavior and dietary assessment. We carried out a series of experiments to compare the isotope measurements made with the CM-CRDS with the traditional method of EA-IRMS. Initially we compared measurements of carbon isotope values of a variety of common diet components used in insect rearing, which covered a wide range of carbon isotope values. There was good agreement in the isotope values obtained, using the CM-CRDS and the EA-IRMS with similar accuracy and precision. Secondly we compared the 2 methods to measure carbon isotope values of the common cutworm, Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) samples, and again there was good agreement in the values obtained. Thirdly we tested whether the CM-CRDS could be used to determine the paternity of spermataphores, based on the differential isotope signatures of the paternal and maternal moths, as a result of differences in their rearing diets. The CM-CRDS proved to be fit for the purpose of measuring isotope values in the spermataphores of common cutworm and suggested that the CM-CRDS technology would be suitable for these isotope based mating studies in moths if the spermatophores were of sufficient mass. Finally we explored a number of the issues surrounding CM-CRDS and isotope technologies in general, e.g., logistical considerations, economics of operation, sample size, etc. We conclude that CM-CRDS is a suitable instrument for measuring stable isotope carbon signatures in moths, and most probably other sufficiently large insects and that CM-CRDS could be easily used in both operational and research contexts.