Darwin wasps rarely turn back once their host strategies change

Darwin wasps that paralyze their hosts have given up that habit at least 23 times in favor of letting the host keep feeding and growing, and the genetic record shows no return.

The species sample represents every major lineage of Darwin wasps, the family Ichneumonidae, which includes more than 25,000 described species. The wasps lay their eggs on or inside living hosts such as caterpillars, beetles and spiders, and the larvae feed on the host until it dies. Some species permanently paralyze the host first, turning it into an immobile food supply. Others leave the host alive so that it continues to feed and grow while the larvae consume it.

The team traced how these strategies arose across the family tree. The shift from paralyzing hosts to letting them keep developing occurred independently at least 23 times, and the researchers found no evidence of reversals. A second change, larvae developing inside the host rather than outside it, also occurred repeatedly but was only rarely reversed. The work was reported in Current Biology (2026) by Bernardo F. Santos and colleagues, under the title "Directional evolution of parasitoid strategies and contingent host shifts shape the radiation of Darwin wasps (Ichneumonidae)" (DOI: 10.1016/j.cub.2026.10.032). The Natural History Museum, Berlin, provided the account to Phys.org, which describes the paper as peer-reviewed.

Santos, lead author and a researcher at the Museum für Naturkunde Berlin, said a lineage that has evolved a close relationship with a living, developing host finds it extremely difficult to go back. He described the pattern as "something like an evolutionary ratchet: you can move forward, but going backward becomes nearly impossible."

Scientists have long recognized that similar adaptations can arise independently in different organisms. The Darwin wasp result adds the opposite question, which has received less attention: how easily a lineage can turn back once it has taken a direction. The authors' reading is that evolution is shaped not only by chance and natural selection but also by constraints from an organism's own history, so that some paths become increasingly difficult to abandon.

The family takes its common name from Charles Darwin. In an 1860 letter to the American botanist Asa Gray, he questioned how a benevolent God could have deliberately created creatures that feed "within the living bodies of caterpillars."

This is a single study of one insect family. The reported pattern covers the two host strategies the team analyzed, and the source gives no figure for how often the second change, larvae developing inside the host, was reversed beyond calling it rare.