Biotic Stress and Yield Loss

Costa Rica, and there are several genotypes conferring resistance to different herbivorousspecies. 118 Thus, one genotype would have resistance to herbivore-A but notherbivore-B, while another genotype would have resistance to herbivore-B butnot herbivore-A. Clones that experienced greater degrees of herbivory injury grewless than clones that received less damage, which should result in fitness consequencesto individual plants. The species of herbivore causing the most injury to P.arieianum changed over time, so intensity and quality of selection pressure from herbivorychanged over time as well. 118Injury that is concentrated spatially on certain tissues (such as reproductivebranches) of P. arieianum can result in up to an 80% reduction in seed production,and the plant has certain times when it is most vulnerable to concentrated injury. Thesame amount of injury spread throughout a plant rather than concentrated on vulnerabletissues resulted in plants that could not be distinguished from control plants. 119A similar result occurred with wild radish, R. sativus, where flower number, reproductivebiomass, and total biomass were higher in no-injury control plants and plantsthat received 25% simulated herbivory injury on four leaves, than on leaves with 50%injury on two leaves, or 100% injury on a new or mature single leaf. 120 Both pattern(concentrated or diffuse) and timing of herbivory injury can be critical to the degree(if any) of resulting plant damage. On a side note, it has been suggested that inducedplant defenses do not always reduce herbivore densities and hence herbivory pressure.Instead, induced defenses may sometimes help spread out future injury on planttissues, where diffuse herbivory should result in less harm on a plant compared toconcentrated herbivory.121, 122Removal of herbivores was performed with annual morning glory, Ipomoea purpurea,to assess the selection pressure imposed by four different insect herbivores. 55Seed number increased by 20% when insect herbivores were removed, and geneticvariation for seed number was eliminated. Thus, these herbivores seem to imposeselection pressure on some traits of I. purpurea by having a negative impact on individualplant fitness based on seed number. Herbivory on flowers and seeds can alsoimpose selection pressure on flower timing in wild sunflower, Helianthus annuus. 123Five insects were studied that feed on developing sunflower seeds. Primary injuryfrom some herbivores (the head-clipping weevil, Haplorhynchites aeneus, the sunflowermoth, Homoeosoma electellum, and the sunflower bud moth, Suleimahelianthana) occurred early, injury was low early and increased over a season froma seed fly, Gymnocarena diffusa, and injury was constant from two seed weevils,Smivronyx fulvus and S. sordidus. The impact of seed herbivory generally had negativeeffects on plant reproductive fitness, but selection for late flowering by H.annuus seemed to be driven from two of the herbivores, S. helianthana and H.electellum. This finding is based on two phenotypic selection analyses, one analysisexamining effects from all herbivores, and a second analysis examining effects of allherbivores but the two sunflower moths, S. helianthana and H. electellum. 123Although herbivory injury can impose selection pressure on plant size and resistanceto particular herbivores, without genetic variation no evolutionary plantresponses can occur. This was observed in a study examining the selective pressures