Abstract
Greenbugs, Schizaphis graminum (Rondani), present a well-documented example of the ability of insects to overcome designed host plant resistance. Four green bug biotypes have been identified on wheat, Triticum aestivum L., and 3 on sorghum, Sorghum bicolor (L.) Moench, since green bugs were identified as pests of small grains. Biotype E has been the predominant biotype since the mid-1980s. Greenbug biotype I is the most recent greenbug biotype identified in field plantings of small grains and may be replacing biotype E. Replacement of biotype E green bug by biotype I will depend on the fitness of biotype I on biotype E-resistant cultivars. We attempted to predict this event by assessing relative fitness of the 2 biotypes, defined by life history characteristics on sorghum, wheat, rye, Secale cereale L., and barley, Hordeum vulgare L. We used several sorghum cultivars or lines with different reported levels of resistance to greenbug biotypes E and I, and 1 cultivar or line each of wheat, barley, and rye with reported resistance to biotype E. Biotype E development time, intrinsic rate of increase ( rm ), and to a lesser extent number of offspring produced were adversely affected by E-resistant and I-resistant sorghum cultivars or lines, whereas biotype I was only adversely affected by I-resistant cultivars or lines. Even on I-resistant cultivars or lines, biotype I outperformed biotype E. Wheat line CI 1795 and barley cultivar 'Post', previously identified as resistant to greenbug biotype E, also showed resistance to green bug biotype I. No differences occurred in the reproductive characteristics of greenbugs between E-resistant and E-susceptible rye cultivars. Given these differences in fitness on both E- and I-resistant sorghum and the importance of sorghum to greenbug population dynamics in the central and northern plains (compared to wheat, barley, and rye), it is likely that as increasing acreage is planted to these sorghum cultivars biotype I will replace biotype E.
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