Influence of feeding guild on insect response to host plant fertilization

1. The densities of insect herbivores in fertilized and unfertilized field plots of goldenrods, Solidago altissima (Compositae), were monitored over a period of 4 years. 2. A total of seventeen insect taxa occurred on the plots over the course of the study, including sap feeders, leaf chewers, leaf miners, leaf gallers and stem gallers with multiple representatives in each of these feeding guilds. 3. Nine of the seventeen taxa significantly increased in density on fertilized plots in at least one year of the study, two taxa showed marginally significant increases on fertilized plots, two significantly decreased in density on fertilized plots in at least one year, and the remaining taxa were unaffected by the fertilizer treatment. 4. The effects of fertilization on the insects were not strongly related to feeding guild; the group of insects that increased on fertilized plots was functionally diverse, and for the most part members of the same guild did not respond to the fertilizer treatment in consistent ways. 5. Differences between fertilized and unfertilized plots were greatest in the fourth year. The insects that showed delayed responses to fertilizer treatment may have been affected by changes in microclimate that developed slowly over the course of the study, suggesting that long-term studies may be necessary to detect effects of host plant stress on insect herbivores.