A Praon pequodorum wasp drags a pea aphid (Acyrthosiphon pisum) onto its ovipositor. P. pequodorum is the one native North American parasitoid wasp that managed to outcompete the wasp species Aphidius ervi when A. ervi was introduced from the Old World as a predator for the invasive pea aphid. P. pequodorum earns the tag “underdog” for its immunity to the pea aphid’s bacterial defenses, which are effective against A. ervi. (Photo credit: Laura Kraft)

By Laura Kraft

North America in the 19th century was like the Wild West for insect pests and plant diseases. The young United States had yet to write laws to reject infested or infected plants at its border. Insect pests could land on the new continent, free of predators and parasites from their homeland, and grow without any checks to balance them. A pest called the pea aphid, likely hidden on plants brought from Europe, landed in North America during that time. It sucked the life out of crops such as clover and alfalfa, reducing what American farmers could feed their livestock.

Laura Kraft

At some point, North American parasitoid wasps began to try to take advantage of this new food source. A parasitoid wasp has a special life cycle in which it lays an egg inside a specific host, typically another insect or arthropod. The egg develops into a wormlike juvenile that feeds on the host, and the young wasp finishes development by killing the host, at which point it emerges as an adult to start the cycle again. Surveys of early pea aphids (Acyrthosiphon pisum) show that a handful of different North American wasp species successfully preyed on them; however, it did little to reduce the aphid population. The aphids persisted, causing damage year after year to clover and alfalfa crops.

In the 1950s, someone had the bright idea of introducing a robust parasitoid from the Old World: the Aphidius ervi wasp. After some years of acclimating to the new land, the A. ervi wasp suddenly surged and outcompeted native wasps—all except one underdog North American wasp, Praon pequodorum. What was allowing the underdog P. pequodorum wasp to persist?

The answer lies in pea aphids’ natural defenses. These aphids weren’t lying around, waiting to give themselves up as wasp baby food. Some aphids have specialized bacteria inside their bodies that neutralize the venom the A. ervi wasp injects with its egg, thus allowing their immune system to kill the egg inside. This bacteria coevolved in the Old World as part of a steady arms race between the A. ervi wasp and its pea aphid hosts. But the bacteria was entirely impotent to the New World underdog wasp.

My colleagues and I in Dr. Kerry Oliver’s Lab at the University of Georgia figured that the underdog wasp, known from previous field surveys to be an inferior competitor, was able to scratch out a living by attacking aphids that had defenses against the A. ervi wasp and that it would lose out to the stronger European wasp if it tried to attack pea aphids that had no bacterial defenses.

In the fall of 2015, we set up cages in our laboratory to test our idea. We put in an equal proportion of each wasp species with populations of either bacteria-defended aphids or uninfected, defenseless ones. In the cages with only uninfected aphids, the A. ervi wasp won, as expected; however, in cages with entirely bacteria-defended aphids, the underdog P. pequodorum wasps outcompeted the A. ervi wasps after only four generations.

After seeing such a strong effect, we seeded the cages with varying levels of bacteria-defended aphids, which better simulated the mix of aphid populations found in the wild. We expected cages with more than 50 percent of bacteria-defended aphids to allow the underdog to compete. To our surprise, P. pequodorum wasps outcompeted A. ervi wasps in cages with only 10 percent of bacteria-defended aphids. The bacterial defense appears to be so effective that even a small foothold against the A. ervi wasp is enough to give the underdog a leg up in the competition.

In the wild, the underdog wasps don’t attack just the imported pea aphids but also several other species, including native ones. An increase in the population of the underdog wasps where the bacteria-defended aphids live may overflow into neighboring areas. For example, higher numbers of underdog wasps in an alfalfa field may leak over into native grasslands and to the native aphids on them. The underdog wasp might then compete in these areas with other native parasitoid wasps.

Such small competitive interactions can cause a cascade of downstream effects in the ecosystem, which can be difficult to tease out or predict ahead of time. This one example shows the care that must be taken when we consider introducing exotic predators to control invasive pests. It’s no longer the Wild West, and more than ever we need to see the bigger picture when looking at natural systems.

Laura Kraft is a Ph.D. student at North Carolina State University and a National Science Foundation Graduate Research Fellow. When she isn’t traveling the world, she spends her time making science more accessible through science writing and outreach. Email: ljkraft@ncsu.edu