A genetic analysis of corn earworm (Heliothis zea Boddie) resistance in maize (Zea mays L.)

Resistance to the corn earworm (Heliothis zea Boddie) in corn (Zea mays L.) was studied to determine the inheritance of several characters thought to contribute to this complex trait.

Genetic effects among the inbred lines: T115, T220, T222, T224, T232, Mo18W, Ga209, Ky226, and the Mexican flint corn 'Zapalote Chico' (PI217413) were obtained using combining ability and generation means analyses for depth of earworm penetration, husk extension, and depth of blank tip. Silk maysin content was included in the combining ability analysis. Laboratory reared corn earworm larvae were applied to ensure a uniform level of infestation for all genotypes. Problems with the supply of corn earworm larvae resulted in variation for the number of days between infestation and mid-silk. However, covariate analysis showed that, within the interval studied (zero to five days), this source of variation did not affect damage ratings.

Parent-offspring regression gave a heritability estimate of 0.01 for depth of earworm penetration and 0.59 for husk extension. A preponderance of the genetic effects for depth of earworm penetration, husk extension, and depth of blank tip were additive in nature, although significant dominance and epistatic effects were also found in several crosses. Since additive genetic effects provided the most consistent source of variation (significant in 67% of the crosses for depth of penetration) selection methods which utilize this type of variation should increase the frequency of alleles for resistance. The genotypes which had the highest level of resistance and would be useful in a breeding program were T232, Ky226, and PI217413.

The analysis for maysin content gave results which are inconsistent with those found by others. In particular, PI217413 had a low maysin content among the genotypes studied.

The relationship between depth of penetration and husk extension appeared to be somewhat dependent on the level of resistance. In resistant crosses, such as T232 x PI217413, this correlation did not differ from zero, while in some susceptible crosses, such as T220 X T224 (r = -0.37) this relationship appeared to be stronger.