When should insecticide applications be made for southwestern corn borer?

Insecticide applications for southwestern corn borer should ideally be made when 25 percent of plants have eggs and live larvae present. This timing is critical because it helps target the pest when they are most vulnerable and actively feeding, maximizing the effectiveness of the insecticide. Early intervention, at this stage of infestation, can prevent significant damage to the corn plants as the larvae begin to feed and tunnel into the stems, which causes physical harm and can lead to yield loss.

The other options provided suggest either too early or too late an application. If insecticides are applied only at the peak of first-generation feeding, it may be too late for effective control, as significant damage might already have occurred. Waiting until after the second generation has emerged risks additional damage, as the pests would have already established themselves further in the plant. Applying insecticides at the time of planting does not target the larvae effectively because the pest activity typically increases later in the growing season, making it an irrelevant timing for managing southwestern corn borer infestations. Therefore, monitoring and timely action when 25 percent of plants show signs of eggs and live larvae is crucial for pest management success.