The horned passalus beetle (Odontotaenius disjunctus) is an insect that is commonly found in rotting logs. These beetles do more than just eat wood, they excavate extensive tunnels within those logs where they would mate and raise a whole family of baby beetles. By doing so, they play an important ecological role in breaking down dead wood and making their nutrients accessible to other organisms in the forest such as bacteria and fungi.
|Top: A piece of dead wood with burrows made by the horned passalus beetle. Bottom left: A horned passalus beetle. Bottom right: Chondronema passali nematodes taken from the hemocoel of a beetle.
Photos from Fig 1 of the paper
Living inside these beetles is a species of nematode worm called Chondronema passali. These nematodes are also very common - each beetle harbours dozens to thousands of such worms, which swim freely in the beetle's hemolymph - the insect equivalent of blood. Having this many worms squirming around inside them must be affecting these beetles somehow - but how exactly?
The study featured in this blog post looked at how Chondronema affected the beetle's "freezing" behaviour - this is a defensive response where the beetle tries to hold as still as it can so they won't get noticed by potential predators. The researchers exposed beetles to different sources of stress such as digging them out of their shelter and placing them in a brightly lit room, flipping them on their back so they can't right themselves, putting them on a tray with a vibrating phone underneath, or loudly banging the tray they're sitting on with a metal rod. All these treatments honestly sound pretty stressful even if you're not a beetle.
The researchers observed that after being exposed to one of those distressing stimuli, female beetles tend to hold still for about twice as long as the male beetles. But this trend is flipped among beetles which have a lot of worms. For female beetles, the more worms they have, the less they seem to care about being stressed - they don't hold still for as long and seem to be in a hurry to get on with their day. In contrast, the worms seem to make male beetles more fearful, and they tend to stay still for longer after being stressed out.
It is unclear why these nematodes cause their hosts to change their defensive response, and why the effects differ across the beetle's sexes. Such parasite-induced behavioural changes are sometimes attributed to some form of host manipulation by the parasite, altering host behaviour in such a way that would enhance the parasite's own transmission, such as making the host more vulnerable to predators. But such changes in host behaviour isn't always due to the parasite taking control, sometimes it could just be a side-effect of the infection, and the behavioural change doesn't always benefit the parasite.
Furthermore the life cycle of this nematode does not involve being eaten by a predator. The beetle serves as a safe haven where young Chondronema grow and develop. Once they become adult worms, they leave the beetle to live amidst the tunnels excavated by their hosts. So it wouldn't be beneficial for Chondronema to compromise the beetle's safety, especially when they affect the male and female beetles in such different ways.
Aside from the beetle's freezing behaviour, Chondronema can also affect their host in many other facets, including their fighting abilities, as well as their immune response. Additionally, infected beetles also grow to be bigger and heavier, and they chew through more wood than their uninfected counterparts, possibly to compensate for the energetic cost of the wormy passengers inside their (larger) body. All this indicates infected beetles are physiologically compensating for the presence of the nematodes in multiple ways, so this change in their defensive behaviour might simply be a byproduct of the beetle's coping mechanisms.
The impact that a parasite has on its host can manifest itself in many different ways. In the case of Chondronema, its effects on the host also has far-reaching implications, since these beetles play such important ecological roles. By making its host chew through more wood, this tiny worm can have a major impact on an entire forest.