Insecticide Performance for Black Cutworm Larvae

April 16, 1999
Knowledge of scouting techniques and economic thresholds, along with the use of rescue treatments as needed, continues to be the "backbone" of our recommended pest management approach for black cutworms. A rescue treatment may be warranted in commercial corn when at least 3 percent of the plants are cut and larvae are still present. In seed-production fields, rescue treatments may be needed if 3 to 5 percent of the plants have leaf feeding and two or more cutworms are found per 100 plants. This threshold is based on the premise that for each 1 percent of plants with leaf feeding, 3 to 5 percent of the plants may be subject to cutting at a later date.

In general, rescue treatments have performed better than insecticides applied before or at planting. Table 1 provides some efficacy data for our 1998 black cutworm trial. For this experiment, black cutworm larvae were introduced into barriers. Barriers were made of 14-gauge steel with sides that were 5 inches tall and 6 feet in length. The barriers were placed in the field on May 21 and 22, 1998, after corn plants had reached the first true leaf stage. Barriers were kept free of weeds (hand weeding) throughout the duration of the experiment. On average, 15 plants could be found within a barrier. One laboratory-reared black cutworm larva (third or fourth instar) per plant was placed in each barrier arena on May 26.

Overall, the level of cutting in our trial last year was low. This was most likely due to the excessive precipitation that occurred during our experiment. In general, most insecticide treatments offered good protection against cutting. This was true even for some products applied well before planting. Keep in mind that our recommended approach is to "wait and see" before treating a field for black cutworms. Look for early signs of leaf feeding in corn prior to the 4-leaf stage. This is a good indicator of potential cutting. We'll continue to provide updates on black cutworms throughout the spring.--Mike Gray

Author: Mike Gray