Originally published on October 20, 2018.

Nematodes, or tiny worms, that ride along on the genitals of dung beetles help raise the beetle's offspring by boosting the good microbes in the beetle's dung-ball nursery nests.

In a serendipitous discovery, scientist Cristina Lédon-Rettig found nematodes wriggling inside the "brood balls" she created in her lab to house dung beetle eggs.

Initially concerned that the worms might harm the beetles she was studying, with further investigation she realized the beetles and the worms actually shared a friendly relationship.

She found that the nematodes can modify the microbial environment of the brood balls that are prepared by the beetle parents from dung to house their offspring. The nematodes help boost beneficial microbes in these brood balls, which help the beetle offspring grow faster and larger.

Nematodes are enormously common and exist in a multitude of different species. Many live on and around a host organism. In insects, that relationship is often parasitic. But that wasn't what Lédon-Rettig discovered.

Sexually transmitted worms

She discovered that the nematodes were commonly discovered tucked into the dung beetles' genitalia. This provided a protected environment and transportation as they could ride on the beetles to different dung patties to feed and reproduce.

"Nematodes can't travel very far. But if you have an insect as a ride, you can get around to different types of resources that you wouldn't otherwise have access to," said Lédon-Rettig.

A Diplogastrellus nematode (Erik Ragsdale)

Both dung beetles and nematodes use animal dung as a food source. Dung beetles eat the plant material in the dung, while nematodes feed on the bacteria and fungi.

The genitalia is also a strategic spot for nematodes to spread themselves. Adult beetles pass on nematodes to their partner during mating. And when the mother lays her egg in the brood ball, the nematodes get transferred to the brood ball as well.

This system is not only found in dung beetles, according to Lédon-Rettig. Crickets also have sexually transmitted nematodes, as do bees and certain termites.

Wormy terraformers

Brood balls provide all the nutrients a young beetle needs as it develops from an egg to adult, but its microbial makeup is extremely important for the beetle's development.

That's where the nematode's help is so valuable to the growing beetles. Somehow the nematodes are able to transform the landscape of the brood ball so that there are more good and less harmful bacteria for the developing beetle.

"The nematodes are actually enriching the membership and abundance of the microbes that are good at breaking down plant material," said Lédon-Rettig.

In essence, these bacteria are pre-digesting the plant material for the developing beetles.

(A) When a beetle deposits an egg in a brood ball, nematodes achieve high population densities in the brood ball as the beetle offspring develops. (B) As the beetle larva nears pupation, nematodes leave the brood ball with their beetle host. This single species of brood ball-associated nematode was found at high densities on O. taurusgenitalia: compare nematode absence and had a presence in C and D, respectively. (from research paper)

Lédon-Rettig's study demonstrated that the overall health of the larvae is improved when nematodes are around.

"They grow faster and larger. And the larger size helps them survive better and reproduce more."

It's still unclear what mechanisms are involved in transforming the microbial community, but Lédon-Rettig thinks nematodes could be eating certain bacteria while allowing other bacteria to grow, or emitting chemicals which influence which bacteria grow and which ones don't.

Once the beetle larvae develops into an adult and is ready to leave the brood ball, the nematodes migrate to the beetle's genitalia to await their next destination.