The single generation-type corn borer has reached the point where treatment decisions need to be made. There is still significant acres of non-Bt corn in the region, and the single generation corn borer can infest and damage this corn.

In an earlier issue of the newsletter (Issue 5 on 06/21/05), management information was summarized. They was also discussion on the biology of the insect and using Growing Degree Days (GDD) to predict moth emergence. An extension of knowing when the moths are emerging is some insight into what the larvae are doing. GDD models which estimate larval development have been available for some time. By joining the emergence model with larval development models, we can get an estimate of where the larval population is in terms of development.

Figure 1 summarizes the GDD model for larval development. The dashed lines indicate at what GDD accumulation we would expect 50% of the population to have reached that stage. There are five larval stages, or instars. The critical stage for corn borer decisions is the third instar. Third instar larvae are expected to tunnel into stalks or ears and thus protecting them from insecticide applications. This is also the point where feeding injury has the greatest impact on plant health. Accumulated GDD at Sabin, MN (Figure 1— green line) reached 1499 GDD on July 25 and is approximately the point where 50% of the larvae are third instar. As you move northward, GDD accumulation is somewhat less and age of the larvae should reflect this.