Why is it, entomologists have wondered, that the pea aphids of southern Wisconsin, like those shown here, come in two colors? Both belong to the same species; both feed on the same plants--peas and other legumes-- yet some are red, some green. Researchers have argued that the colors are nothing more than a quirky side effect of evolution, with no real significance for the bugs themselves, but John Losey, an entomologist at Cornell, has found that the two main predators of aphids--ladybugs and a type of parasitic wasp--actually maintain the bicolored aphid population. In a field study, Losey found that the ladybugs eat more red aphids because these stand out against the green pea plant. Parasitic wasps, on the other hand, prefer to lay their eggs inside green aphids, perhaps because evolution has taught them that eggs laid in red aphids get eaten by ladybugs. The wasps that hatch inside aphids eat their way out. The aphid population, Losey found, fluctuates depending on the type of predator in the area. If you have a higher proportion of ladybugs in the field relative to wasps, then in the next week you’ll see a decrease in the reds.