Biologists have long raised concerns about the ecological impact of the world’s most popular weed killer, RoundUp.RoundUp’s active ingredient, glyphosate, kills milkweed plants in and around farm fields and on roadsides, depriving monarch butterfly caterpillars of their sole source of food. This has contributed to a 90 percent decline in monarch butterflies over the two decades RoundUp has been widely sprayed on genetically-modified corn and soybean crops. But the herbicide has also unleashed a second trend: a rash of lawsuits. People suffering from a form of cancer allegedly linked to RoundUp have filed more than 9,000 lawsuits against the weed-killer’s manufacturer, Monsanto, and its German parent company, Bayer pharmaceuticals.Environmental attorney Robert F. Kennedy Jr. represented a California man dying of lymphoma who in August won a $289 million jury verdict against Monsanto and Bayer.