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Abstract

In field studies of plant-insect herbivore interactions it is often difficult to establish which herbivore has fed on a particular plant. We investigated the suitability of three different N-15-labeled nitrogen compounds (ammonium, nitrate, and glycine) for indirect marking of three grasshopper species [Omocestus viridulus (L.), Chorthippus parallelus (Zett.), and Chorthippus biguttulus (L.) (Orthoptera: Acrididae)] through labeling their food plants in the field. In two short-term experiments grassland plots of 1 m(2) were separately labeled with either one of the different nitrogen compounds. Grasshoppers were caged on three food-plant species [Dactylis glomerata L., Holcus lanatus L. (Poaceae), and Trifolium repens L. (Fabaceae)] present in these plots for 72 h. Significantly enriched delta(15)N values in grasshoppers were found in all plant/grasshopper combinations. Enrichment in grasshoppers was positively correlated with the enrichment of plants and labeling with nitrate resulted in highest N-15 enrichment. In a long-term experiment, individuals of C. biguttulus were placed in a cage covering an area of 1 m(2) for 37 days, with sampling of grasshoppers at regular intervals. delta(15)N values of the grasshopper and a common food plant, D. glomerata, increased steadily over time, up to 40-fold by the end of the experiment. Our results demonstrate that N-15-labeling of plants is an appropriate tool for the investigation of insect-plant interactions under natural conditions. [References: 20]