Identification and quantification of predation: novel use of gas chromatography–mass spectrometric analysis of prey alkaloid markers

Abstract

Summary 1 Predation can be difficult to measure in the field and immunological and DNA-based gut analyses are now routinely used to identify and quantify prey items consumed by predators. Alternative methods have largely fallen into disuse. 2 Chromatography has been largely ignored as a method of studying predation on the grounds of low specificity and an inability to provide quantitative results. We demonstrate here that this is not so, using gas chromatography–mass spectrometry of alkaloids of prey ladybird beetles. 3 The alkaloid hippodamine from a single egg of the ladybird Hippodamia convergens was detected in all ten third instar larvae of another ladybird, Harmonia axyridis, for 12 h after they had consumed a H. convergens egg; in one case of ten, hippodamine was still detected 36 h after egg consumption. Hippodamine was detectable in all 10 second instar larvae of the lacewing Chrysoperla rufilabris sampled 12 h after consuming a H. convergens egg. 4 The amount of hippodamine in H. axyridis larvae was quantifiable with an internal standard. Larvae that had consumed more eggs exhibited higher levels of hippodamine. The method therefore can be used to estimate the amount of prey biomass consumed. 5 A comparison of the alkaloids of five ladybird species that co-occur in Kentucky field crops found that, in general, the alkaloids were sufficiently distinct to allow species identification of ladybirds that had been consumed by predators, although there was some overlap between species in alkaloid content. 6 Especially when combined with mass spectrometry, chromatography is thus a method that potentially can be used to identify multiple prey simultaneously, while also obtaining quantitative information on the prey biomass consumed. This has not been achieved by the commonly-used molecular gut-analyses. We suggest the method is suitable for a wide diversity of prey types possessing endogenous taxon-specific chemicals such as defence compounds or pheromones. 7 As a secondary consequence of this study, several alkaloids have been identified from new ladybird species. We also have shown that H. axyridis, an invasive intraguild predator of other ladybirds, does not sequester alkaloids from allospecific ladybird prey.