Direct and Indirect Effects of Host-Plant Fertilization on an Insect Community

Artemisia ludoviciana plants that received ammonium nitrate fertilizer were visually censused for all insects at ° 11 -d intervals from 30 June to 1 September 1982. Community composition and species densities in fertilized plots were compared with those in control plots. Fertilization resulted in greater numbers of phloem- and seed-feeding insects and a concurrent increase in tending and patrolling by ants in fertilized plots. Chewing-insect densities did not respond uniformly to fertilization, and no chewing species exhibited the marked increase found in all of the sucking insects. Chrysomelid beetles and beetle damage decreased significantly in fertilized plots. A choice test between fertilized and control plants offered to beetles indicated that factors other than plant quality were responsible for the decrease in damage in fertilized plots. I suggest that increased densities of aggressive ants associated with honeydew-producing membracids and aphids caused this decline. Beetle damage was inversely correlated with aphid and membracid densities. Plots with ant nest had fewer plants with beetles and beetle damage than did those without ant nests. The indirect effect of increased ant densities from host-plant fertilization resulted in reduced levels of beetle damage to A. ludoviciana in fertilized plots. Since beetles removed large amounts of leaf area from A. ludoviciana in control plots, the indirect effect of higher ant densities should not be overlooked in assessing the general impact of fertilization on host plants.