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Selection Indexes for Resistance to Corn Earworm Based on Realized Gains in Corn¹

Corn earworm [Heliothis zea (Boddie)] injury, husk tightness, husk extension, and days to 50% pollen shed were evaluated among 250 selfed progenies from a random-mated population of Zea mays L. The standard procedure of using relative economic values in conjunction with genotypic and phenotypic variances and covariances was employed to generate a selection index (RS) for identifying progenies with superior resistance to earworm injury. A second index (RI) was generated by substituting direct and correlated realized gains for genotypic variances and covariances, respectively, in the normal equations. A realized selection index (RSI) was derived using weighted earworm-injury responses from selection for each character as the coefficients (b₁) of the index. Comparison of indexes revealed that RSI selections were in closer agreement with RS selections than with selections obtained using the RI index. The RSI index is expected to be fully as effective in providing genetic progress as the RS index, depending on changes that may occur in the interrelationships among characters as selection proceeds.

Key Words: Heliothis zea (Boddie) • Zea mays L. • Earworm injury

² Research Geneticist, Southern Grain Insects Research Laboratory, ARS, USDA, Tifton, GA 31794.

Received for publication October 13, 1973.