Introduced to North America from Eurasia in the 1860s, cabbage white butterflies (Lepidoptera: Pieridae: Pieris rapae) are now found throughout most of our continent. The larvae feed on a variety of plants in the mustard/cabbage family (Brassicaceae). Some of these plants are wild like the mustard in these photos, but others are crops. Cabbage white larvae can be serious pests of cabbage, kale, cauliflower, broccoli, and other cultivated mustards. Farmers call the larvae imported cabbage worms.
Adults, in contrast, feed on the nectar from a wider variety of plants. The individual shown here happened to be feeding on a wild mustard, but I’ve also seen them on asters like giant ironweed. With two black spots on each forewing, this cabbage white was a female. Males have only one black spot.