Seasonal variations in roadside conditions and the performance of a gall-forming insect and its food plant

A transplant experiment using potted plants was performed over two years in a field located along a heavily used highway to test for the effects of seasonal variations in roadside conditions on the performance of a gall-forming insect, Eurosta solidaginis Fitch, and its perennial host plant, Solidago altissima L. The experiment was designed to separate temporally the two major classes of road pollutants (air pollutants versus de-icing salt). The population density and survivorship of E. solidaginis were not affected by road-stressed goldenrods. However, gall-forming larvae had a greater biomass when they were grown on plants exposed to road air pollutants, although these effects were tempered by a simultaneous exposure to de-icing salt. The shoot growth of S. altissima was severely affected by road stress during each growing season but after two years the biomass of roots and rhizomes combined did not differ between the treatments. This experiment showed that the effects of air pollutants and de-icing salt on a gall-forming insect via stressed host plants are less than additive.