EVALUATION OF REPRODUCTIVE STAGE MAIZE FOR RESISTANCE TO THE SOUTHWESTERN CORN-BORER (LEPIDOPTERA, PYRALIDAE) USING VISUAL RATING SCORES OF LEAF SHEATH AND HUSK DAMAGE

A better technique for evaluating damage to maize, Zea mays L., in the reproductive stage of growth from southwestern corn borer, Diatraea grandiosella Dyar, is needed to screen maize for resistance to this pest. The old technique of splitting stalks of infested plants and measuring the tunneling 35 to 45 d after infestation is inefficient. We developed a new technique that uses rating scales to estimate visually the degree of damage to the leaf sheath and ear husks, the preferred feeding sites by larvae, within 14 d of infestation. The rating scales range from 0 to 9 with scores based on types and numbers of lesions made by the larvae. A rating of 0 indicates no damage and ratings of 7-9 indicate substantial damage. Experiments were conducted to test the usefulness of the rating scales in detecting differences in larval growth and numbers. Our results show that the rating scores were significantly correlated with larval growth and numbers. Significant differences in larval growth and numbers were found among larvae reared on some of the test hybrids. When this occurred, differences in rating scores among hybrids also were found to be significant. Significant differences were found in larval numbers and growth when maize hybrids were infested on the same day but at different physiological stages.