Farmers planting Bt corn seed this spring should consider a pest management strategy to maximize the yield protection provided by these hybrids that kill European corn borers. A successful strategy will be to plant Bt corn and non-Bt corn so that the plantings capitalize on the behavior of the European corn borer moth and minimize the damage potential of the larvae.

European corn borer moths are attracted to early and late planted fields.

Typically, there are two generations of European corn borers in Iowa. The first flight of moths start to lay eggs in mid- to late June while the second flight begins in late July or early August. The first flight usually is attracted to the earliest planted (usually the tallest) corn in an area. However, the second flight later in the summer seeks out the latest planted corn (usually at pollinating and green silk stages). The damage caused by the corn borer larvae also decreases throughout the season so that the first-generation larvae normally cause a greater yield loss than the second-generation larvae (see yield loss article).

By understanding the moth and its attraction to fields in different stages of corn development and the damage potential of first- and second-generation corn borers, Bt corn can be planted to take advantage of this insect behavior and plant response to pest injury. Here are two recommendations based upon the time required for a farmer to plant all acres in an operation:

1. If all corn acres can be planted within several days, plant the Bt hybrids first, followed by the non-Bt hybrids. The reason is that the earliest corn often is planted on the best ground, it grows faster, it should be taller, and therefore is more likely to attract the largest number of first-flight moths when they are laying eggs. Early planted Bt corn works like a "bug magnet," pulling in large numbers of corn borer moths to lay eggs where the larvae will die.
2. If all corn acres require more than a week to be planted, plant 80-90 percent of the Bt hybrids first, followed by the non-Bt hybrids, and finish with the remaining 10­20 percent of the Bt hybrids. Planting a majority of the Bt corn first follows the same reasoning of the first recommendation. However, large acreages that require more than a week to plant often get delayed by spring rains, equipment breakdowns, or other unplanned interruptions and are more likely to have several fields that are planted late (remember last year?). It is in these last or late-planted fields, where second-flight corn borers are strongly attracted to lay their eggs, that the remaining 10-20 percent of the Bt corn will protect yields from late-season damage. By spreading out Bt corn plantings on large acreages, the earliest and latest plantings are protected where most of the yield loss is expected to occur.

Bt corn is a novel tool for managing a serious insect pest of corn. Because of its rapid development and entry into the market, we are evaluating these planting strategies, plus a spectrum of other research topics. We will keep you informed as we learn more about the management of the European corn borer with Bt corn.

Plant 80-90% Bt corn first.	Plant regular hybrids second.	Plant 10-20% Bt corn last.
Most moths fly to early-planted Bt corn.	Fewer moths fly to later-planted non-Bt corn.	Most second-flight moths fly to last-planted corn.