Résumé :
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Greenbugs, Schizaphis graminum (Rondani), have considerable genetic polymorphism, involving life history and host utilization traits. Part of this variation has been described as host utilizing biotypes B, C, E, F, G, H, I, and J. Single clones of each biotype were reared in growth chambers on two wheat entries, Triticum aestivum L., 'Triumph 64' (TR64) and GRS1201, and sorghum entries, Sorghum bicolor (L.) Moench, 'Pioneer 8300' (P8300) and PI 550610. The number of aphids produced per plant, damage ratings, and time to plant death were compared. Greenbug biotypes C and I killed TR64 after 14 days and produced about 200 greenbugs per plant. Biotype B also killed TR64 after two weeks, but produced only 105 aphids per plant. Biotypes E and G produced about 190 aphids per plant, but most peaked on the 15 th day and killed the host by the 18 th day. Biotypes E H, and J had maximum counts of 233, 160, and 156 greenbugs respectively, and killed plants only after three weeks or more of infestation. Only biotypes F and H killed GRS1201; only biotypes C, E, and I killed P8300; and only biotype B killed PI 550610. Using the mean maximal number of aphids produced per plant and time of survival on the host as criteria of efficiency in host utilization for aphid population increase, greenbugs of biotypes F, H, and J are considered the most efficient host users. Biotypes C, E, and I have rapid numerical growth and are currently the most frequently recorded greenbugs in wheat and sorghum fields of the United States. They are probably better adapted to the specific environment of agricultural fields, where host plants are abundant, and the time for reproduction may be limited. Biotypes F, Il, and J use host plants like K-strategists, while biotypes B, C, and I use hosts like r-strategists.
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