Those that survive the pesticide reproduce and can result in the selection of a resistant population. Pests that survive in the refuge mate with those whose resistance allowed them to survive in the BT resulting in offspring that are more likely be susceptible to the toxin. This model helps us to understand the dynamics behind the workings of refuges and helps understand the interactions between pest behavior and refuge layout. In the figure below, p atterns a, b, e, and f are relatively easy to plant by putting non-BT crop seed in some or all of the planting hoppers. Pattern c represents a random pattern, here depicted as small patches, but potentially random plants could also be planted.
Orange individuals are heterozygous, and green are homozygous non-resistant.
The next figure depicts the state of the system after time step 1. Caterpillars in BT fields have died, the remaining matured to adults, moved around randomly, mated, laid eggs, and died. This figure shows the locations of eggs in the next generation. In a similar experiment only the size of the refuge is changed. By time step 13 a resistant population has emerged where all individuals are homozygous for the resistant gene.