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 yea rs in a field located along a heavily used highway to test for the eff ects 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 sepa rate temporally the two major classes of road pollutants (air pollutan ts 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 plan ts exposed to road air pollutants, although these effects were tempere d by a simultaneous exposure to de-icing salt. The shoot growth of S. altissima was severely affected by road stress during each growing sea son but after two years the biomass of roots and rhizomes combined did not differ between the treatments. This experiment showed that the ef fects of air pollutants and de-icing salt on a gall-forming insect via stressed host plants are less than additive.