Four maize lines, Mp78:518 (resistant), P47S3 (intermediate to susceptible), Tz3 and Ki3 (susceptible), and three experimental hybrids, P47S3 × Mp78:518 (resistant), Tz3 × Ki3 and Ki3 × T×601 (susceptible), were Infested with 15, 30, 45 and 60 southwestern corn borer (SWCB) larvae per plant at the 6–7 fully extended leaf stage. There was a small linear increase in leaf feeding ratings on resistant genotypes (Mp78:518 and P47S3 × Mpρ78:518) as the number of larvae increased. On susceptible genotypes leaf feeding increased sharply at 30 larvae when compared to 15 larvae. Additional larvae resulted in little change as the ratings at 30 larvae on susceptible genotypes were in the upper limit of the scale. Leaf feeding was significantly less on resistant than on susceptible genotypes. Neither plant height nor yield of resistant genotypes was reduced significantly (? = 0.05) at any infestation level. The results indicate that infestation with 15 larvae per plant gives low damage ratings and does not adequately partition the genotypes into resistant or susceptible. Because the components of resistance were identified precisely with 45 larvae per plant, and remained unchanged with additional larvae, the results indicate that artificial infestation with 45 larvae per plant is adequate for first‐generation SWCB resistance screening under field conditions in Mexico.