Effect of root extracts of Mexican marigold, Tagetes minuta (Asterales: Asteraceae), on six nontarget aquatic macroinvertebrates

Abstract

Water-soluble root extracts of the Mexican marigold, Tagetes minuta L., composed mainly of the compounds alpha-gurjunene, 5-(But-3-ene-1-inyl)-2,2'-bithiophene, palmitic acid, alpha-terthienyl, and 5-(4-Acetoxy-1-butenyl)-2,2'-bithiophene, were assessed to determine their impact on six species of nontarget aquatic invertebrates. Test organisms were collected from a polluted stream - Gammarus lacustris Sars (Amphipoda) - as well as from a local nearly pristine stream - Lepidostoma pluviale (Milne) (Trichoptera), Drunella grandis (McDunnough) (Ephemeroptera), Baetis tricaudatis (Dodds) (Ephemeroptera), Rhithrogena morrisoni (Banks) (Ephemeroptera) and Hydropsyche cockerelli (Banks) (Trichoptera). Root extract of T. minuta, formulated with a commercial combination of petroleum solvents and surfactants, caused mortality to some of the organisms tested. Some mortality was attributable to presence of the inert, surfactant/petroleum-based solvent. For example, with L. pluviale, the LT50 for 4 ppm marigold root extract formulated with 0.01 ppm inert materials was 54.7 h (CI 48.1, 64.2), and for 0.01 ppm inert materials alone was 69.8 h (CI 55.8, 101.2). Of the macroinvertebrates tested, R. morrisoni was the most sensitive to inert materials (LT50 0.01 ppm inert = 4.5 h) and G. lacustris was the least sensitive (LT50 0.01 ppm inert = 400 h; LC50 96 h = 1.2 ppm). Marigold root extracts did have a toxic effect at the highest concentration (4 ppm) used on all macroinvertebrates tested. Although previous studies reported the effects of one compound in the root extract, alpha-terthienyl, our research is the first published report evaluating effects of full-formulation marigold root extract on immature aquatic nontarget insect species and a crustacean. If root extracts of T. minuta are to be used as a pesticide, further research should focus on effects of inert surfactants used in marigold extract formulation on nontarget macroinvertebrates.