Effect of Herbicides for Leafy Spurge Control on the Western Prairie Fringed Orchid

Abstract

Leafy spurge has invaded the habitat of the western prairie fringed orchid, a federally listed threatened species. Imazapic ([6]-2- [4,5-dihydro-4-methyl-4-f1-methylethylg-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid) and quinclorac (3,7- dichloroquinoline-8-carboxylic acid) are relatively new herbicides that control leafy spurge and can be used in the mesic areas where the orchid is found. Research was initiated to evaluate the effects of imazapic and quinclorac on the survival and fecundity of the western prairie fringed orchid. Herbicides were applied at the commonly used and maximum labeled rates in mid-September. This timing is optimum for leafy spurge control with quinclorac and imazapic and the orchid generally has senesced by mid-September, making injury less likely. Quinclorac applied at 840 and 1 120 g ha-1 did not affect regrowth or fecundity of the western prairie fringed orchid 1 or 2 years after treatment. Orchids treated with quinclorac at 840 or 1 120 g ha-1 generally were as tall, had racemes as long as, and produced as many flowers and seed capsules as untreated orchids. In contrast, orchids treated with imazapic at 140 or 210 g ha-1 tended to regrow as vegetative plants and were shorter, had shorter racemes, and produced fewer flowers and seed capsules than untreated orchids. Orchids treated with imazapic averaged 21 cm in height and produced an average of 1 flower per plant 10 months after treatment, whereas untreated orchids and orchids treated with quinclorac were approximately 36 cm in height and produced 5 flowers per plant. Quinclorac could be a valuable tool to control leafy spurge in the habitat of the orchid because orchids treated with quinclorac regrew as vigorously and were as fecund as untreated orchids. However, the current quinclorac label prohibits grazing or haying for 309 days after application.