YIELD REDUCTION IN A MAIZE DIALLEL UNDER INFESTATION WITH SOUTHWESTERN CORN-BORER

The southwestern corn borer (SWCB), Diatraea grandiosella Dyar, is an important pest of maize (Zea mays L.) in some parts of the Americas. Germplasm with resistance to leaf feeding by this insect pest was developed at the International Maize and Wheat Improvement Center (CIMMYT) in Mexico, but little is known about the inheritance of resistance or combining ability for yield in this germplasm. A 10-parent maize diallel cross of eight CIMMYT inbreds and two susceptible public inbreds was evaluated for leaf feeding resistance to SWCB and for yield under both infestation and protection from SWCB. Trials were grown at Tlaltizapan, Morelos, Mexico, in the dry season of 1989-1990 and rainy season of 1990. General combining ability (GCA) was the most important source of variation among F(1)s for leaf feeding damage rating and for yield. The relationship between leaf feeding damage and yield reduction was linear but variable. The most insect resistant lines in this study were selected under infestation with larvae of several species of stalk boring insects but were never directly selected for combining ability for yield, These lines improved GCA for yield only under heavy SWCB infestation. An inbred that was susceptible to leaf feeding had better combining ability for yield than all other lines in the study in both seasons under infestation and protection. The results of this study demonstrate that selecting directly for combining ability for yield across environments may be more useful than selecting directly for insect resistance without selecting for yield.