Matt Squires carried a tray of plants to the corner of a two-acre plot outside DuPont Pioneer’s Beaver Creek research building in Johnston and began sinking the seedlings into the ground, following a carefully designed plan outlined by a colleague.

But Squires, a senior research associate, wasn’t planting the company’s latest corn or soybean varieties designed to withstand insects, diseases or drought.

Instead, he and about 40 other Pioneer employees were planting three varieties of milkweed in a pollinator habitat...