Nectar sugar production in alfalfa fields treated with several herbicides

Field studies were conducted at three sites in northeastern Saskatchewan to determine the effects of several herbicides applied in late fall or early spring on nectar sugar production in alfalfa (Medicago sativa L.) grown for seed. In general, sugar production was not affected detrimentally by the herbicides, but at one site alfalfa treated annually with metribuzin for 4 yr at the break of winter dormancy produced flowers in early July containing significantly less nectar sugar than flowers of plants not treated with herbicide. Increases in nectar sugar production sometimes occurred. At one site, chlorsulfuron at 0.11 kg ha −1 or more, hexazinone at 1.0 kg ha −1 and dichlobenil at 2.4 kg ha −1 increased nectar sugar production in early August. Herbicide effects were not consistent among sites, and it seems unlikely that leafcutter bee (Megachile rotundata Fabr.) cocoon production will be affected significantly by these weed control practices. Key words: Metribuzin, hexazinone, chlorsulfuron, terbacil, dichlobenil, imazethapyr