Intrinsic Rate of Increase for Greenbug (Homoptera: Aphididae) Biotypes E, F, G, and H on Small Grain and Sorghum Varieties

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Abstract
Development (d), reproduction (Md), and intrinsic rate of increase (rm)for greenbug, Schizaphis graminum (Rondani), biotypes E, F, G, and H were observed on 13 small grain and sorghum entries, including five sources of resistance to biotype E (GBE). The d was expected to increase, whereas Md and rm were expected to decrease for each biotype on resistant plants. However, the d and Md values for each biotype did not correlate as well as rm to the resistance status of the cultivar. In general, the d, Md, and rm values for biotype E were better than comparable values for biotypes F, G, and H. Biotype E also equaled or outperformed the biotypes on most of the biotype E-resistant cultivars except ‘Insave F.A.’ rye. The low rm's for GBF, GBG, and GBH compared with GBE provide some insight as to why these biotypes are rare in wheat and probably would not reach damaging infestation levels. GBE is currently the predominant biotype by virtue of its broader host range and resistance breaking ability, plus the most fit as measured by the superior rm's. Therefore, there does not appear to be a cost for virulence in this greenbug biotype. The displacement of biotype E by the other biotypes does not seem likely, based on our results.