The plant stress hypothesis and variable responses by blue grama grass (Bouteloua gracilis) to water, mineral nitrogen, and insect herbivory

Acting simultaneously or sequentially, plants encounter multiple stresses from combined abiotic and biotic factors that result in decreased growth and internal reallocation of resources. The plant stress hypothesis predicts that environmental stresses on plants decrease plant resistance to insect herbivory by altering biochemical source-sink relationships and foliar chemistry, leading to more palatable food. Such changes in the nutritional landscape for insects may facilitate insect population outbreaks during periods of moderate stress on host plants. We examined the plant stress hypothesis with field experiments in continental grassland (USA) using the C-4 grass Bouteloua gracilis. Water, nitrogen fertilizer, and herbivory from the grass-feeding grasshopper Ageneotettix deorum were manipulated. Combined stresses from water and mineral-N in the soil decreased plant growth and altered foliar percent total N (TN) and percent total nonstructural carbohydrate (TNC) concentrations in an additive fashion. Grasshopper herbivory affected final biomass only in dry years; plants compensated for tissue loss when rainfall was abundant. Foliar TN and TNC concentrations were dynamic with respect to variable climatic conditions and treatment combinations, showing significant interactions. Grasshopper herbivory had its greatest impact on TN or TNC in dry years, interacting with other forms of stress. Herbivory as a single factor had strong effects on TNC in years with normal precipitation, but not in a dry year. Performance (developmental rate and survival) by the grasshoppers Phoetaliotes nebrascensis and A. deorum were not greatly affected by plant stress in a manner consistent with the plant stress hypothesis.