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Study Finds California’s Central Valley Milkweed Contaminated by Pesticides

A study published in Frontiers in Ecology and Evolution found that milkweed samples from California’s Central Valley were all contaminated with pesticides. This contamination was at high enough levels to harm monarch butterflies that rely on the milkweed as a sole food source, as well as other insects. Although data on toxicity to monarchs is available for just four of the 64 pesticides identified in testing, 32% of the samples collected in this study contained pesticide levels known to be lethal to monarchs. Samples contained an average of nine different pesticides, with a high of 25 different pesticides on a single sample. University of Nevada researchers participating in the study say the findings indicate that key breeding grounds for western monarchs are contaminated with pesticides at harmful levels.