Establishing forbs for pollinators in agricultural landscapes of the Great Plains, USA

Pollinator insects are declining, partly because grasslands containing forbs they feed on have been converted to cropland. This conversion is prevalent in the Great Plains, home to several imperiled pollinators and 40% of U.S. honeybees. Over 1.0 million hectares of former cropland have been seeded with forbs that could benefit Great Plains pollinators, but success of these seeding efforts is unclear. We quantified forb abundances and factors regulating these abundances in 120 crop fields seeded to forbs and grasses by managers in the Great Plains (Colorado and Montana). Our data indicated a need to improve forb establishment. Two to five growing seasons after seeding, seeded forb cover was < 10% in most fields, and no seeded forbs were observed in 23% of fields. Our data also indicated ways to benefit forbs. High grass seed rates and high weed densities shortly after seeding reduced forb cover at the end of the study. Past studies indicate certain post-emergence herbicides sometimes benefit seeded forbs, and we recommend further research in this domain. Instead of beneficial herbicides applied after seeding, managers sometimes applied risky herbicides before seeding that appeared to persist in soil and reduce forb establishment. Seed rates were too low to maximize forb abundances, and much money was wasted buying seeds of species that did not establish. We identified several species with relatively high establishment probabilities that will support most pollinators. For now, these species should be seeded at high rates. Lower rates could become sufficient if effective weed control is implemented.