Flies Associated with Floral Canopies of the New Oilseed Crop, Pennycress, in the Midwestern U.S.A.

Flies are frequent visitors to flowers of many species of plants within the mustard family (Brassicaceae). They derive nutrition from these flowers, and some fly species are pollinators. Field pennycress (Thlaspi arvense L.) is a mustard species that is being developed as a new “cash cover crop,” i.e., an autumn-sown cover crop whose oil-rich seeds can be harvested profitably in spring. Although pennycress is largely wind-and self-pollinat-ed, its flowers also attract insect visitors. However, the extent of visitation to pennycress flowers by flies remains largely unknown, especially the identities of those flies. Thus, we examined flies associated with pennycress flowering canopies at five site-years in Illinois and Minnesota. The number of fly species averaged 16 per site-year. Hover flies (Syrphidae) were common visitors to pennycress flowering canopies, representing 24% of all Diptera collected. Toxomerus marginatus Say (margined calligrapher), whose larvae are aphid predators, was especially abundant within this family. However, the most common flies detected were Delia spp. (Anthomyiidae, root maggot flies), which averaged 51% of all flies collected. Adults of these flies are known pollinators, but their larvae also are pests that can damage seedlings of common summer crops. Although seedling damage to plants that are double-or relay-cropped (i.e., inter-seeded in spring) with pennycress has not been observed yet, close observation of this insect group and its effects may be needed if pennycress is widely sown in the future as a cash cover crop.