An invasion of soybean aphids poses a problem for soybean farmers requiring application of pesticides, but a team of Penn State entomologists thinks a careful choice of nitrogen-fixing bacteria may provide protection against the sucking insects. Soybeans are legumes, plants that can have a symbiotic relationship with nitrogen-fixing bacteria -- rhizobia -- and therefore do not need additional nitrogen fertilizer. Each type of legume -- peas, beans, lentils, alfalfa -- have their own rhizobia. "Soybeans are from Asia and so there were originally no nitrogen-fixing bacteria that would colonize soybeans in U.S. soils," said Consuelo De Moraes, associate professor of entomology. "The rhizobia had to be transferred here."