Four experiments were conducted in Nicaragua over a 3-yr period to determine the effect of whorl-stage infestations of Spodoptera frugiperda (J. E. Smith) and Diatraea lineolata (Walker) on maize grain yields. The experiments separated the effects of period of infestation from level of infestation. Results from all experiments demonstrated that maize is more tolerant of lepidopteran infestation during early growth stages than at later stages. Infestation during mid-through late whorl caused yield losses of 15-73% when 55-100% of the plants were infested with S. frugiperda. D. lineolata infestations were highly correlated with S. frugiperda. D. lineolata infestations were highly correlated with S. frugiperda infestation. Economic injury levels ranged from 23 to 63% of the plants infested with S. frugiperda and D. lineolata over a range of potential yields, using mean yield reduction from the 4 experiments, and an average 75% insecticide effectiveness.
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Hruska et al. (1997) studied this question.