Madison Reporter

In a study that resembles a scene from the popular sci-fi horror film Alien, researchers have discovered that stability in the parasitic relationship between aphids and wasps relies on dispersal. The research focused on the wasp Aphidius ervi, which injects its eggs into pea aphids. When the egg hatches, the larva begins to eat the aphid from the inside out, unless the aphid has inherited resistance in the form of a venom-producing bacterium.

The bacterium Hamiltonella defensa lives inside some pea aphids and produces a venom that kills the wasp egg before it can develop and kill the insect. However, this resistance comes at a cost for the aphids: lower reproductive output.

Researchers from the University of Wisconsin–Madison combined experiments with mathematical modeling to learn that dispersal of organisms involved in parasitic relationships through space can play an important role in balancing both ecology and evolution's effects on those relationships. Lucas Nell, a PhD student in Professor Anthony Ives’ lab, led this research recently published in Science.

Aphidius ervi is often used by farmers as a biological control for pea aphid colonies that feed on crops. Therefore, Nell and Ives’ findings could help guide future pest management strategies.

Nell explains, “There’s a ton of research studying how there’s biodiversity maintained in ecology and lots of research on how genetic variation is maintained in evolution. But looking at both simultaneously is what’s pretty new and exciting about this paper.” He further elaborates that because evolution is so rapid, one cannot untangle ecology from evolution without understanding them simultaneously.

Pea aphids' life cycles are relatively short, and they reproduce by creating offspring genetically identical to themselves. Their populations can double every two to three days, allowing beneficial genetic variations to pass quickly to subsequent generations.

However, hosting venom-producing bacteria also lowers aphids' reproductive rate. Therefore, they are only beneficial when enough parasites threaten the aphids. Otherwise, the aphids have venom-producing bacteria that lower their birth rates and no wasp eggs to kill. This situation leads to a genetically diverse population.

Nell and Ives also considered the parasitoid relationship in its ecological context. They found that what happens to aphids depends on a complex matrix of costs and benefits. The push and pull depend on where in space the wasps are and whether resistant aphids have an advantage over non-resistant but more reproductive aphids.

When aphids disperse beyond a certain tipping point between fields, all fields' aphids will likely evolve to be either entirely resistant or not resistant to the wasps. However, if aphids disperse at moderate rates across areas populated by various levels of wasp density, they maintain genetic diversity that keeps both species coexisting.

This research provides valuable insight into pest management practices and could help inform future best practices farmers can employ to protect their crops. The study was funded by grants from the National Science Foundation NSF DEB- 1240804 (Anthony Ives) and NSF DEB-1240892 (Kerry M.Oliver).