Stable carbon and nitrogen isotope signatures to determine predator dispersal between alfalfa and maize

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

“…The proportion of C3 and C4 prey source in each coccinellid individual was estimated inside the 99 % credibility interval. We then classified each coccinellid individual into three categories, for each of the two resources, following Forbes and Gratton (2011) and Madeira et al (2014): (a) 90-100 % assimilation (local, L) = an individual that fed completely on one resource (C3 or C4-feeding prey), with no movement between habitats with C3 and C4 plants; (b)10-90 % assimilation (switching, S) = an individual with intermediate assimilation of the two resources due to its movement between habitats with C3 and C4 plants;…”

“…The d 13 C and d 15 N of source herbivore prey was then obtained from the consumerresource (i.e. herbivore-plant) fractionation by adding 0 ± 1 % for d 13 C and 3.4 ± 1 % for d 15 N (DeNiro and Epstein 1978;Post 2002) to the maize signature, as it was performed in Madeira et al (2014). Moreover, when aphids were not present due to crop management (after alfalfa cutting and maize harvesting or drying) we used the signature of aphids from the previous sampling period.…”

“…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.…”

“…Stable isotope ratios have been proven to be suitable for tracking the plant origin of herbivorous and predatory insects Vialatte et al 2006;Traugott et al 2008;Schallhart et al 2009Schallhart et al , 2011Forbes and Gratton 2011;Ouyang et al 2012). Recently, this method has been used to determine the dispersal of pest predators between alfalfa and maize in the northeast of the Iberian Peninsula (Madeira et al 2014). In this study, the authors observed different patterns of dispersal of the pirate bug Orius majusculus Reuter (Hemiptera: Anthocoridae) and the ladybird Coccinella septempunctata L. (Coleoptera: Coccinellidae) and the different roles of alfalfa and maize as source or receptor habitats of predators.…”

Stable isotope signatures (δ2H‐, δ13C‐, δ15N‐values) of walnuts (Juglans regia L.) from different regions in Germany

“…50,51 This value is similar to those measured in our organic walnuts. 52,53 As decomposed plant material is known to be important for the build-up of soil organic matter and the nitrogen-supply of other crops, 23 in our case Alfalfa plant material as well as metabolic products of the nitrogen-fixing bacteria delivered into the soil could likely have influenced the isotopic composition of the soil. Thus, its stable nitrogen ratio probably would have approached a 15 N-value of ∼0‰ which could explain the low 15 N-values (‰) of the present organic samples.…”