Biotic Stress and Yield Loss

FIGURE 5.2 Generalized alfalfa growth response from leaf-mass consumers and assimilateremovers. Reduced phenological rates affect nutrient yields harvested on a calendar date basis(from Hutchins et al. 14 ).of specific needs of the animal consumer. 17 Specific examples for establishingEILs, including details on each of the variables, have been reviewed 1 and recentlyupdated. 18 In addition, specific examples and considerations for pests affecting qualityand on-farm EILs are available for review.16, 17Although much of this review has focused on the special case of addressing phenologicaldelay in forage crops because of the many complex factors affected, phenologicaldelay induced by pest injury also is a significant issue for annual crops witha single harvest event. Specifically, delayed development opens the window of hostsusceptibility and eventual yield loss from pests because they may be able to extendtheir life cycles longer or continue through additional generations. Indeed, much ofcurrent theory on avoiding pest-induced injury is focused on preventive agronomictactics to “de-synchronize” the host:pest phenologies. The general management principlesinclude establishing crops as early as possible, promoting their rapid growthand maturation, minimizing loss of metabolic energy focused on grain production,and harvesting as soon as possible.In addition, crop development that is unnaturally delayed may extend the harvestevent into a seasonal period that makes harvest risky from a weather standpoint, especiallyin more northern latitudes. In situations where double or continuous croppingis practiced, final harvests may be placed at risk as a result of the growth delays (similarto the multiple harvest of forages example).Delayed development and excessive stress on crops may lead to the invasion andbiotic release of secondary or tertiary pests from all classes, further affecting the rateof development and yield potential. Early delays in canopy development, for example,have been documented as leading to increased weed pressure 19 and eventual loss