Imidacloprid is a long-term systemic insecticide that is currently labeled under the trade name Marathon (imidacloprid, 1-[(6-chloro-3-pyridyl)methyl-4,5-dihydro-N-nitro-1-H-imidazol-2-amine, 1% granular on fritted clay, Bayer Corp., Kansas City, Mo.) for ornamental crops grown in greenhouses. The company that markets Marathon is seeking to expand its label to greenhouse-grown vegetable crops, although the rates they plan to label have not yet been divulged. Marathon was applied to cucumber (Cucumis sativus L. `Turbo') and tomato (Lycopersicon esculentum Mill. `Rutgers') at 0, 1/8, 1/4, 1/2, 3/4, and 1 tsp (0, 5, 10, 20, 30, and 40 mg a.i.) per 4.5-inch (550-mL) pot. Both species developed phytotoxicity symptoms of leaf chlorosis of the oldest leaves and distorted growth and marginal necrosis of newer leaves within 1 week after application. By the end of the experiment, even the lowest rate caused phytotoxicity symptoms. The symptoms were similar in appearance to Ca deficiency but cucumber foliar analysis revealed no difference in Ca, Zn, Fe, or Co across imidacloprid rates, however, Mg and B decreased whereas K and Mn increased linearly across imidacloprid rates. P, Cu, and Mo varied quadratically with 1/2 tsp (20 mg a.i.) per pot having the lowest P and Mo, and Cu increasing at the higher rates. These data indicate that imidacloprid can alter plant nutrition. The rates of imidacloprid applied here are not recommended for use on greenhouse-grown cucumber and tomato under similar growing conditions as in this study.
