Aerial application of clopyralid demonstrates little drift potential and low toxicity to toads

By Joseph DiTomaso , UC Davis; Jessica R. Miller, UC Davis; Guy B. Kyser, UC Davis; Art W. Hazebrook, Fort Hunter Liggett; Joel Trumbo, Rancho Cordova; David Valcore, Indianapolis; Vanelle F. Carrithers, Indianapolis IN California Agriculture 58(3):154-158. July 01, 2004

The herbicide clopyralid (Transline) is commonly applied by air to control yellow starthistle, a noxious weed, in California. In laboratory studies, clopyralid toxicity in Fowler's toad was low, indicating a wide safety margin when used under field conditions. In addition, monitoring of clopyralid drift following aerial application demonstrated that 98-foot (30-meter) buffers between treatment areas and water sources provided adequate drift protection for an adjacent stream and vernal pools. Nevertheless, to ensure that movement of the herbicide to water sources is minimized, it is important to prevent application error, particularly accidental encroachment into established buffer zones. This study demonstrated that drift potential for clopyralid was minimal even with an aerial application and a slight downwind breeze toward sensitive aquatic sites. It is also the first report demonstrating a high tolerance to clopyralid in larval toads.