Biotic Stress and Yield Loss

of the tobacco flea beetle, Epitrix parvula, and a grasshopper, Sphenarium purpurascens,on an annual weed, Datura stramonium. 124 Both herbivores imposedselection pressures on plant size and/or plant resistance to herbivores. Yet, there wasno detectable genetic variation on any reproductive traits that might affect plant fitnessas assessed with paternal half-sibling family analysis. Thus, information on theheritability of the plant traits is necessary to determine where herbivores can havepresent evolutionary selection pressure on a plant host.9.7 HERBIVORE INJURY: INDIRECT EFFECTS ONPLANTS9.7.1 HERBIVORY AND PLANT COMPETITIVE ABILITYMuch of the debate around the evolutionary importance of herbivory to plants hasfocused on evidence for direct impact of herbivores on individual plant performanceor on plant population dynamics. However, herbivory can influence plant fitnessthrough various indirect mechanisms, including altering plant competitive ability relativeto conspecific or heterospecific plants, and how plants interact with other bioticstresses.Louda et al. 125 reviewed data on the influence of herbivory on plant performanceand competitive relationships. From their review, they concluded that herbivory caninfluence competition in two ways. First, competition was altered by indirectlychanging plant access to resources through changes in plant growth and morphologyas a result of herbivory. Second, competition was altered because herbivory canchange the distribution and abundance of some plant species, which again can changeresources available to competing plants. The review and theoretical work by Loudaet al. 125 led to a concluding observation that “herbivory was particularly importantwhere constraints in resources, growing season, or growth strategies limited plantcompensation for losses, and diminished the species capacity to maintain itselfagainst competitors.”9.7.2 LEAF QUALITYDefoliation by herbivores can alter the quality of replacement leaves. On a scrub oak,Quercus ilicifolia, that was completely defoliated by gypsy moth caterpillars,Lymantria dispar, plants produced a second set of leaves each summer after defoliationoccurred. 126 These secondary leaves had lower nitrogen, copper, and zinc levelsthan primary leaves of control plants, so it is possible that leaf quality changed. Yet,how the change in leaf nutrient content might affect leaf functioning or attractivenessto herbivores was not studied, so a connection to plant performance or fitness couldnot be estimated.9.7.3 FLORAL ATTRACTIVENESS TO POLLINATORSLeaf herbivory also can result in reduced flower attractiveness to pollinators, and differentiallyaffect male and female fitness parameters. 127, 128 For example, leaf her-