Well Geraniums and petunias are their favorite host plants. But you can try different kinds. Ivy Geraniums are not as bothered by budworms as are regular bedding Geraniums. I grow a lot of Wave petunias and have not been bothered by budworms, but that might be because the Waves have so many buds and flowers that I just don't notice them. If you are going to use an insecticide Bt won't work. You'll need something that persists like Permethrin. Here's a fact sheet from Kansas State University on these pests.

Steve

Budworm Damage

If you have noticed a small hole in the buds of some of your flowers, you may have tobacco (geranium) budworm (Helicoverpa virescens). Though a number of flowers can serve as hosts, geranium and, to a lesser extent, petunia are most commonly attacked. The larva of this insect damages the buds by boring into them before they open. The caterpillars feed on the flowers for about a month and then drop to the soil to pupate. There are normally two generations per year, with the second causing the most harm. The striped caterpillars vary widely in color with green, red, light brown and dark forms possible. The color of the larva is related to the color of the flowers on which they feed. The adult of this insect is a moth.

Damaged buds often fail to open. Those that do will show evidence of feeding on the petals. Damage normally peaks in late summer because of increased numbers from the second generation.

Control of the budworm is difficult. Handpicking at dusk can be effective on small plantings. For larger plantings, chemical control may be the only practical option. Look for products with synthetic pyrethroid active ingredients such as permethrin, esfenvalerate, cyfluthrin, bifenthrin, lambda-cyhalothrin or related compounds.