Compared with uninfected whiteflies, infected insects develop faster, are more likely to survive to adulthood and lay more eggs. Moreover, the bacterium induces the insects to produce a larger number of daughters, advantageous for a bacterium that is passed to the next generation only by the females.

Infected sweet potato whiteflies now flourish across the Southwest. As of 2008, 94 percent of whiteflies were infected, compared with 1 percent in 2000.

“This study shows that acquiring a bacterial symbiont can fundamentally change the biology of an insect, really instantaneously,” Dr. Hunter said. “And what results is really a different animal than what was there before. This is something we want to know about for pest management. We want to know whether a bacteria can make an insect a worse pest.”