INSECT HERBIVORY ABOVE- AND BELOWGROUND: INDIVIDUAL AND JOINT EFFECTS ON PLANT FITNESS

Abstract

Understanding of the selective effects of insect herbivory on plants comes primarily from studies of herbivory aboveground. The impact of belowground herbivory, either in isolation or in concert with herbivory aboveground, on plant fitness is only beginning to be understood. I reduced the densities of root-boring ghost moth (Hepialus californicus) larvae and/or flower- and seed-feeding insects of bush lupine (Lupinus arboreus), a woody shrub, and followed fecundity and survival of plants for three years. In year one, suppression of aboveground herbivores increased mean seed output by 31%, but suppression of belowground herbivores had no significant effect on plant fecundity. In year two, suppression of aboveground herbivores increased mean seed production by 123%, and belowground herbivores again had no significant effect on plant fecundity. In year three, suppression of aboveground herbivores had little effect on lupine fecundity; plants protected from root borers, however, produced, on average, 85% more pods and 83% more seeds compared with unprotected plants. In each of the three years, there was no statistical interaction between herbivory above- and belowground; effects of individual herbivores on plant fitness were additive. Both modes of herbivory had significant cumulative effects on lupine fitness. Protection from chronic aboveground herbivory increased mean cumulative seed output over 3 yr by 78%; suppression of belowground herbivores increased mean cumulative seed production by 31%. Cumulative average mortality across all three years was 18% greater for plants exposed to root herbivory than for plants protected from root herbivory. Taken together, results show that both above- and belowground herbivores can potentially impose strong selection on bush lupine.