Gall wasp larvae spend nine to 10 months developing within live plant stems that protect and nourish them. The presence of the wasps gives the plants a signature scent. John F. Tooker of the University of Illinois at Urbana-Champaign and his colleagues found that adult males, which emerge first, rely on olfaction to locate potential partners still encased in plant stems. Specifically, the males sniff out telltale differences in the ratio of two forms of so-called alpha pinenes and beta pinenes emanating from the plant. "If males find a stem with a 50-50 ratio they will move on," Tooker says. "If they find a stem with a 70-30 or a 100-0 ratio, they likely will stay and find females emerging from it." The wasps also demonstrated a preference for the same species of plant in which they matured. According to study co-author Lawrence Hanks, the findings show "that insects can influence plants for their own needs, using a substitute for sex pheromones."