Ecologist and pest managers have long been concerned with why some plants are more heavily infested with insects than others, but no strong consensus has emerged to explain these variations. A major focus in understanding the distribution of insects has been the role of plant quality. Two of the most influential hypotheses used to explain variations in herbivore loads among plants that stress the role of plant quality are the plant vigor and plant stress hypotheses. Insects prefer physiologically stressed plants in the plant stress hypothesis and fast growing, vigorous plants in the plant vigor hypothesis. This study examines the role of plant quality on insect distributions using an entire assemblage of herbivores (the sum total of herbivores that feed on the same plant species).
In experimental plots of Rosa arkansana, the prairie rose, various amounts of salt (NaCl) and nitrogen (NH4NO3) were added to stress and invigorate plants respectively. A large assemblage of herbivores feed on prairie rose; some of the more common herbivores include: five cynipid wasps, two aphid species, one thrip species, one fly leaf galler, one leaf tier, and two hip pests. Soil salinity and various plant physiological attributes including plant growth, xylem water potential, plant nitrogen levels, and plant fecundity were analyzed to ascertain that treatments altered soil and plant physiology. Plots were then examined throughout the field season to check for presence of each herbivore species.
Salt additions increased soil electrical conductance, caused plants to turn yellow, but did not significantly affect xylem water potential or plant growth rate. The application of nitrogen increased both plant nitrogen levels and plant growth rate. In general, all herbivores responded in a similar manner to both stressed and vigorous roses; herbivore probability of occurrence and density declined with nitrogen and salt additions. The only herbivore that appeared to show a different distribution pattern were the aphids. The probability of aphid occurrence increased from control plots to low nitrogen additions but dropped off sharply in high nitrogen treated plots and also showed a trend for increased probability of occurrence in salt treated plots.
Overall, I show that nitrogen treatments invigorated plants, salt treatments stressed plants, and herbivores responded to changes in plant quality by distributing themselves on neither physiologically invigorated or stressed plants. I found that herbivore distributions in this experiment could not be explained by either the plant vigor or plant stress hypotheses. One possible explanation for this consistent pattern is that treated roses might have highly unbalanced amino acid profiles and large concentrations of organic acids or defensive compounds, which are common for both stressed and fertilized plants. Along with defensive compounds, diets of unbalanced amino acids have been shown to be detrimental or even toxic to insects. Because both nitrogen and salt are potential contaminants in natural systems, it is important to understand how insects respond to changes induced by theses environmental factors.