Changes in herbivore control in arable fields by detrital subsidies depend on predator species and vary in space

Abstract

Prey from the decomposer subsystem may help sustain predator populations in arable fields. Adding organic residues to agricultural systems may therefore enhance pest control. We investigated whether resource addition (maize mulch) strengthens aboveground trophic cascades in winter wheat fields. Evaluating the flux of the maize-borne carbon into the food web after 9 months via stable isotope analysis allowed differentiating between prey in predator diets originating from the above- and belowground subsystems. Furthermore, we recorded aphid populations in predator-reduced and control plots of no-mulch and mulch addition treatments. All analyzed soil dwelling species incorporated maize-borne carbon. In contrast, only 2 out of 13 aboveground predator species incorporated maize carbon, suggesting that these 2 predators forage on prey from the above- and belowground systems. Supporting this conclusion, densities of these two predator species were increased in the mulch addition fields. Nitrogen isotope signatures suggested that these generalist predators in part fed on Collembola thereby benefiting indirectly from detrital resources. Increased density of these two predator species was associated by increased aphid control but the identity of predators responsible for aphid control varied in space. One of the three wheat fields studied even lacked aphid control despite of mulch-mediated increased density of generalist predators. The results suggest that detrital subsidies quickly enter belowground food webs but only a few aboveground predator species include prey out of the decomposer system into their diet. Variation in the identity of predator species benefiting from detrital resources between sites suggest that, depending on locality, different predator species are subsidised by prey out of the decomposer system and that these predators contribute to aphid control. Therefore, by engineering the decomposer subsystem via detrital subsidies, biological control by generalist predators may be strengthened.

Fig. 1

Mean population densities [(numbers + 1) ± SE per 2 m²; note log-scale] of soil surface-dwelling generalist predators (a–f) and epigeic Collembola captured by pitfall traps (g–i) in no-mulch (open circle with lines) and mulch (filled circle with lines) treatments in the three different fields at three consecutive sampling dates (June 9, July 7, August 4). Significant differences between means of no-mulch and mulch treatments are marked by asterisks (*P < 0.05, **P < 0. 01, ***P < 0.001; one-way ANOVAs following RM-ANOVAs, see text for details)

Fig. 2

Mean δ¹⁵N values and fraction of maize-borne carbon in the body tissue (% of total, black pie) of soil dwelling taxa captured by heat extraction which incorporated maize carbon (means with SD given below taxon name). The lower dashed line represents the δ¹⁵N value of maize, the upper dashed line the assumed threshold of the second trophic level

Fig. 3

Mean δ¹⁵N values and SD of soil surface dwelling species captured by pitfall traps in the no-mulch (a) and mulch (b) treatments

Fig. 4

Aphid populations (given as aphid numbers per shoot) as affected by flying (FP) and soil dwelling predators (SP) in the three different fields (a) and in the no-mulch and mulch treatments (b); 0 = SP+SF removal, FP = SP removal, SP = FP removal, FP/SP = control without predator removal. Error bars indicate SE, significant differences between means are marked (*P < 0.05, ***P < 0.001)