DAMAGE BY SOUTHWESTERN CORN-BORER (DIATRAEA-GRANDIOSELLA DYAR) ON RESISTANT AND SUSCEPTIBLE MAIZE AT 3 PLANT-GROWTH STAGES IN MEXICO

Two resistant varieties, MBRV‐SWCB and P47R, and one susceptible variety, P47S, were infested with southwestern corn borer (SWCB), Diatraea grandiosella Dyar, larvae at the 4–5, 6–8 and 9–11 fully extended leaf stages. Leaf damage ratings were highest in all varieties when plants were infested at the 4–5 leaf stage and declined with the age of plants at infestation, the decline being less between the 4–5 and 6–8 leaf stages than between the 6–8 and 9–11 leaf stages. There was greater variation in leaf damage when infested at the 4–5 leaf stage than at the 6–8 leaf stage. Variation was lowest at the 9–11 leaf stage, but the ratings did not indicate any real differences between varieties. The MBRV‐SWCB and P47R had significantly less leaf feeding damage than P47S across infestation stages. Yield, compared to that of an uninfested check, was reduced at all infestation stages in all cultivars; the greatest reduction occurred when plants were infested at the 4–5 leaf stage. The MBRV‐SWCB and P47R showed significantly less yield reduction than P47S. Because the resistance levels were detected more accurately and there was more uniform damage, results suggest that the 6–8 fully extended leaf stage would be the best for artificial infestation with first brood SWCB in Mexico. © 1990 Taylor and Francis Ltd.