Changes in δ 13C stable isotopes in multiple tissues of insect predators fed isotopically distinct prey

Abstract

Traditionally, researchers have used measurements of carbon stable isotopes to infer the composition of consumers' diets. However, since consumer's tissues may process carbon isotopes differently, particularly following a diet shift, it is possible to use measurements of carbon isotopes in multiple tissues to determine not only the composition of an individual's diet, but also the temporal dynamics thereof. This study examined how stable isotopes of carbon (13C/12C, expressed as delta 13C) changed in different adult tissues of two predacious beetles, Harmonia axyridis and Coccinella septempunctata (Coleoptera: Coccinellidae). In the laboratory, we switched ladybeetles from a C3-based diet (soybean aphids, Aphis glycines) to a C4-based one (corn leaf aphids, Rhopalosiphum maidis). The delta 13C of metabolically active tissues such as the body fat and reproductive organs changed rapidly (< or =5 days) following the diet shift. Tissues expected to be more metabolically inert, such as wings, changed more slowly over the same period. Although these general patterns were largely similar between males and females, females had more rapid changes in delta 13C in fat and reproductive tissues. However, females showed a significant depletion in delta 13C after 10 days, while males' delta 13C continued to increase. Given the results of this experiment, it is now possible to distinguish between ladybeetles eating a mixed diet (beetles with multiple tissues at similar, intermediate, equilibrial delta 13C signatures) from those that have shifted diets (beetles with different tissues at distinctly different delta 13C values). Thus, this approach can be used broadly to infer not only what constitutes the diet of a consumer, but also the temporal history of dietary intake.

Movement of three aphidophagous ladybird species between alfalfa and maize revealed by carbon and nitrogen stable isotope analysis

“…Winter cereals are extensively cultivated in both locations, but most of the fields had already been harvested in La Seu and most of those in Almenar had reached grain hard dough or ripening growth stage before the study started. This implies that, although coccinellids could also move from winter cereals to maize searching for aphids, the migration would have occurred two-three weeks before our first sampling date, when aphid population peaks Pons 1999, 2001), and coccinellid individuals would have had enough time to change their isotopic signatures Gratton and Forbes 2006;Ouyang et al 2012). The size and proximity of alfalfa and maize fields and the characteristics of the surrounding landscape at both locations (see also Madeira et al 2014), together with the low abundance of weeds within the fields, due to herbicide treatments in maize and periodical cuttings in alfalfa, reduced very much the potential effect of other plant sources inside or around the fields on the isotopic signatures of coccinellids at both locations.…”

“…High proportions of migrants occurred on maize after alfalfa cutting, suggesting that such crop management practices may force the dispersal from alfalfa to maize. Even though the turnover rate of d 13 C in coccinellid tissues is known to be relatively fast, more than two weeks are needed for completely reaching an isotopic equilibrium with the new diet Gratton and Forbes 2006;Ouyang et al 2012). This suggests that most individuals of C. septempunctata collected on maize after alfalfa cutting had recently arrived from alfalfa, as samplings were done only three days after cutting.…”

Tracing prey origins, proportions and feeding periods for predatory beetles from agricultural systems using carbon and nitrogen stable isotope analyses

“…In the dietary shift experiment, the d 13 C values in the wing and abdomen of P. japonica adults indicated that individual beetles shifting from a C 3 -to a C 4 -based diet of aphids fed on maize or cotton, respectively, would start to reflect the carbon stable isotope ratios of their new C 4 substrates within 7 days. Following a 14 day interval after the dietary shift, our results show the d 13 C values in the abdomen of the P. japonica adult were significantly higher than those in the wing, implying that the metabolic rate for carbohydrate in the abdomen occurred faster than that in the wing (Gratton and Forbes, 2006). Analogously, studies of two predacious beetles, Harmonia axyridis (Pallas) and Coccinella septempunctata L. (Coleoptera: Coccinellidae), found that the carbohydrate signature in their skeletal wing tissue changed more slowly over the same period as well (Gratton and Forbes, 2006).…”

“…Following a 14 day interval after the dietary shift, our results show the d 13 C values in the abdomen of the P. japonica adult were significantly higher than those in the wing, implying that the metabolic rate for carbohydrate in the abdomen occurred faster than that in the wing (Gratton and Forbes, 2006). Analogously, studies of two predacious beetles, Harmonia axyridis (Pallas) and Coccinella septempunctata L. (Coleoptera: Coccinellidae), found that the carbohydrate signature in their skeletal wing tissue changed more slowly over the same period as well (Gratton and Forbes, 2006). After 21 days there were still differences of the d 13 C values between the wing and abdomen of P. japonica adults and their diet, their differences may be related to sampling or preservation methods that the use of ethanol for cleaning specimens could probably influence sampling (Ponsard and Amlou, 1999;Tillberg et al, 2006).…”

“…Determinations of d 13 C and d 15 N values in animals and their diet substrates are usually used as a mark to ascertain their position in the food webs in aquatic and terrestrial systems (Angerbjorn et al, 1994;Colborne and Robinson, 2013;Gratton and Forbes, 2006;Schoeninger and Deniro, 1984). For example, Ostrom et al (1997) determined carbon and nitrogen stable isotope ratios in organisms of a predatory ladybird beetle, Hippodamia variegata (Goeze), and quantified pathways of energy flow within agroecosystems.…”