Contrasting effects of nitrogen fertiliser application on the performance of closely related grasshoppers through changes in plant nutrient concentrations

Global environmental changes mediated by anthropogenic processes can affect the nutrient status of plants, with important consequences for the performance and dynamics of insect herbivores that feed on them. While it is well documented that insects from different feeding guilds (e.g., sap-feeders and leaf-chewers) can respond differently to altered food resources due to their distinct physiological and ecological characteristics, little is known about how ecologically similar insect species from the same feeding guild respond to changes in food nutrient status. Using nitrogen (N) fertiliser, the authors examined the effects of N inputs on two sympatric grasshopper species, Euchorthippus cheui and E. unicolor, that share the same host food plant, Leymus chinensis grass. The authors examined the effects of fertilisation on the individual feeding behaviour, performance and abundance of the two grasshopper species. The nutrient (protein) content of L. chinensis leaves was enhanced by fertilisation during the entire season. However, E. cheui and E. unicolor exhibited differing growth rates, development and body size responses to fertilisation. E. cheui preferred L. chinensis leaves from high-N fertilised treatments, while E. unicolor preferred leaves from low-N fertilised treatments. Moreover, fertilisation increased the abundance of E. cheui but had no significant effect on the abundance of E. unicolor in the field. The findings imply that effective management and conservation strategies for insects should target the needs of individual species rather than species groups or communities as a whole because nutritional and environmental requirements are often species-specific.