In any population of European corn borers, a few of the borers will have two copies of genes for resistance (rr), some will have one copy of the gene (rs) and most will have none (ss). Resistance genes are believed to be rare. In ECB Bt corn, European corn borer with one or more copies of resistance genes (rr or rs) could perhaps survive and produce more offspring. Improved survival or reproductive success results in a "selective advantage." As the Bt corn acreage increases, and with it the proportion of the European corn borer population exposed to Bt corn, more larvae carrying resistance genes could survive to adulthood. The overall population of Bt-resistant individuals could increase with each generation. At some point, control failure could occur with resistant larvae reaching infestation levels in Bt corn fields similar to levels found in non-Bt corn fields, hence the necessity of implementing IRM on your farm and keeping good records.
This diagram illustrates how resistance could emerge (view full size):
Managing ECB Resistance Through the “High Dose/Refuge Strategy”
The North American corn industry has adopted a high dose/refuge strategy to manage corn borer resistance to Bt technology. The strategy involves exposing one portion of the pest population to Bt plants with an extremely high concentration (dose) of the Bt protein, while maintaining another part of the population as a refuge where the pests do not encounter any Bt protein.
Plant geneticists designed Bt corn to produce very high levels of Bt Cry proteins, much higher than levels found on corn treated with Bt insecticides. The intent is to kill all European corn borer (ECB) larvae with no genes for resistance (ss), plus those with one copy of a resistance gene (rs).