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Response to Index Selection in Maize for Resistance to Ear Damage by the Corn Earworm¹

Three selection indexes, 1) derived by standard procedures (RS), 2) derived from direct and correlated responses of the index traits (RI), 3) from direct and correlated responses of the primary trait (RSI), were compared with selection for reduced corn earworm [Heliothis zea (Boddie)] injury per se (CEW) through four cycles of selection in the maize (Zea mays L.) population RFC. Responses for reduced corn earworm injury occurred for all methods of selection, but the significant rates of response of RSI and RI selection were larger than RS or CEW. CEW and RI selections resulted in tighter husks, shorter husk extension, and slightly less yield reduction than in RS and RSI selections. None of the index procedures were superior to conventional selection for resistance to corn earworm, but the RI index was judged to compare most favorably when considering all trait responses.

Phenotypic correlations obtained from S₁ progeny data within each cycle indicated that the interrelationships among index traits, other than days to anthesis, had changed substantially during the course of four cycles of selection. The significant relationships of corn earworm injury with husk tightness and husk extension were greatly reduced and significant reversals of the relationship between husk tightness and extension occurred among the RS and RSI selections. The RI index will be revised for further comparison with conventional selection in this maize population for reduced injury by the corn earworm.

Key Words: Zea mays L. • Heliothis zea (Boddie) • Host plant resistance • Recurrent selection • Population improvement

² Research geneticist and research entomologist, USDA-ARS, Tifton, GA 31793.

Received for publication September 21, 1981.