–by Dr. Jeff Whitworth — Field Crops
Fields planted to corn rootworm susceptible varieties for 3+ years should probably be monitored for corn rootworm larval feeding. Several fields were sampled this week (13-16 June) that have been planted for at least 3 consecutive years without regard for corn rootworms, i.e. not varieties that had a Bt event to help control rootworms. All varieties, however, had commercially applied fungicide+insecticide seed treatments. All fields were planted this year from 15 Apr-3 May, but regardless, were all in the 8-10 leaf stage of development. Insecticide seed treatments seem to always work well but the toxins are only active for up to 21-28 days from the day of planting. Even though rootworm larvae were found in each field, no obvious signs of lodging or goosenecking were evident in any of the fields, which could be indicators of root pruning by rootworm larvae. However, northcentral Kansas has not had a great deal of wind over the last couple of weeks and the majority of the root feeding will probably occur over the next 10-14 days as the rootworm larvae get larger (see fig. 1). If there is not much wind or wind events over the next couple of weeks, the plants may compensate for the root damage and the lodging may be inconsequential. Most larval development in northcentral Kansas, thus seems to be on track to be completed prior to 4 July,–as it has been every year for at least the last 25 years.
Figure 1. Corn rootworm larvae (14 Jun 2021) (pic by Cody Wyckoff)