Biotic Stress and Yield Loss

Yield losses also can vary from injury caused by different herbivores, in degreeof yield loss, duration of plant damage following herbivory, and timing of yieldloss. For example, on alfalfa, Medicago sativa, different trends in yield losseswere found from injury by several different herbivores including: a weevil, Hyperabrunneipennis, sucking aphids, Acyrthosiphon pisum and A. condoi, and severalLepidopteran species (alfalfa caterpillar, Colias eurytheme, beet armyworm,Spodoptera exigua, and western yellowstriped armyworm, S. praefica). 90 The weevilcaused yield losses for several cuttings following injury, the aphids caused yieldlosses only while at damaging densities, and yields increased following low injurylevels from the lepidopteran species in this study. Also, an interaction between weeviland aphid feeding resulted in yield loss 30 to 60 days following peak numbers.Although feeding injury resulted in yield losses, there were no reductions in eitheralfalfa stand densities or in individual alfalfa plant mortality rates.Control of weevils and slugs via insecticides allowed white clover, Trifoliumrepens, to have higher yields than when these two herbivores were not controlled. 91Simulated defoliation on sugarbeets resulted in a linear decrease in 0.5% sucroseyield for each 1% defoliation applied. 92 Thus, it is important to observe the intensityof the attack by herbivores, each plant part attacked, and timing of the attack.21, 93With soybean, Glycine max, plants can tolerate 40% pre-bloom but only 25% postbloomleaf defoliation by herbivores without suffering yield loss. Thus, age of plantcan affect its susceptibility to tolerating herbivory injury. In one study, deciduous foresttrees tolerated a rate of 30% annual leaf defoliation without having any effectsdetected on survival or growth rates during the three years of observations. 70Herbivory can affect crop plant architecture, such as leaf canopy structure. Forexample, with soybean, herbivory appears to cause yield losses only if canopy leafarea falls below a leaf area index (LAI) level of 3.5. 94 This is because photosyntheticefficiencies are about 95% at LAIs above 3.5, but efficiencies drop below LAIs of3.5. 44 Although soybeans tend to have a high tolerance for herbivory injury, severalfactors like soil moisture, nutrient levels, and soybean genotypes affect actual yieldlosses observed. Even 50 to 70% leaf defoliation may not result in yield losses underoptimal water and soil nutrient conditions, while under suboptimal conditions soybeantends to experience yield losses following defoliation. 44, 95 Also, canopy architecturechanges when cotton experiences simulated reproductive organ injury,resulting in reduced cotton photosynthetic capacity. 96The attack of herbivores can be avoided, in some instances, by an interestingstrategy where a plant lives in association with other plant species. For example,Rish 97 analyzed the densities of five herbivorous beetle species on monoculturalsquash and a polyculture of squash, maize, and beans. He found that beetles were lessabundant in maize–bean–squash polycultures, partially because the beetles avoidhost plants shaded by corn. Bach 98 focused on the response of one specialist herbivore,the striped cucumber beetle, Acalymma vittata, to cucumber monocultures vs.cucumber–broccoli–maize polycultures. By controlling total plant density, host-plantdensity, and plant diversity, Bach was able to distinguish the effect of these three confoundingvariables. He reported a significant effect of both plant density and diversityon A. vittata abundance, where fewer cucumber beetles per cucumber plant were