Biotic Stress and Yield Loss

injury guilds would center on the first injury guild requirement as promulgated byHutchins et al. 42 Injury guild species must produce a similar type of injury.Consequently, similar types of injury can be determined by assessing homogeneitiesof photosynthetic responses. This approach has been used to support injury guildstatus in soybean and alfalfa.6.3.3.1 Soybean Leaf-mass Consumer Injury GuildBecause several soybean leaf-mass consumers elicit similar physiological impacts onsoybean, they can be placed into the same injury guild. As discussed above, soybeanleaf-mass consumption does not alter photosynthesis in remaining leaf tissue.Canopy gas exchange responses are altered by leaf-mass consumption injury, butthey most likely are similar among pest species. The other requirements for injuryguild membership (discussed above) also are met for soybean leaf-mass consumers,so several species can be placed into the guild, such as soybean looper, Pseudoplusiaincludens, velvetbean caterpillar, Anticarsia gemmatalis, green cloverworm, beetarmyworm, Spodoptera exigua, and bean leaf beetle, Ceratoma trifurcata. 42However, not all species that consume soybean leaves can be members of theleaf-mass consumption injury guild. For example, Mexican bean beetle larvae andadults produce a physiological response (reduced photosynthetic rates) different thanthat produced by members of the injury guild. 26 Therefore, the exclusion of theMexican bean beetle from the injury guild is based on physiological response, notphysical appearance of the injury.6.3.3.2 Alfalfa Stubble Defoliator Injury GuildAlfalfa stubble defoliation occurs after the hay crop is cut and the stubble is regrowing.Several insects may defoliate alfalfa stubble, including alfalfa weevil larvae andadults, Hypera postica, clover leaf weevil larvae and adults, Hypera punctata, andvariegated cutworm larvae, Peridroma saucia. These insects consume dry matter,delay regrowth initiation and subsequent plant maturity, and reduce growth rates afterdefoliation. 46Peterson et al. 44 concluded that alfalfa responses to clover leaf weevils, alfalfaweevils, and variegated cutworms are similar. The three species are leaf-mass consumers.Research on leaf-mass consumption in alfalfa showed no alterations in photosyntheticrates of remaining tissue. 19 Similarities in gas exchange responses inaddition to similarities in consumption patterns and timing of injury meet the requirementsfor placing the three species into a common injury guild in alfalfa. Peterson etal. 44 developed injury equivalencies for the three species, which could then be usedfor multiple-species management guidelines.6.4 FUTURE DIRECTIONSFuture research must emphasize how and why changes in gas exchange rates occurin response to biotic stress. This type of research will do more to advance our under-