Why are burying beetles covered in mites?

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Sometimes, when writing for this blog, I learn new things about bugs as well. This is one of those times when I have to admit that I didn’t know the answer, and got curious enough to read up on the topic.

We get a lot of pictures of Nicrophorus burying beetles, which are often covered in mites. I know a little bit about mites, although I haven’t had a lot of training in mites beyond those important for agriculture and medicine. Many can be predatory or parasitic, with complicated lifecycles…and ID of many species isn’t possible without microscopic examination.

Since we’ve had a handful of questions about these mites, I decided to look into what species of mites are commonly found on burying beetles. The story I found was one which is a really neat story about how different animals cooperate to complete a common goal.

Burying beetles, which are the genus Nicrophorus are interesting critters in their own right. They’re decomposers, and feed on corpses. Mom and dad work together to shave, and then bury the corpse of a small animal. It’s formed into a ball, and covered in antibacterial secretions. The mom then stays behind to lay her eggs, and raise babies.

The mites which are on the burying beetles in our pictures are probably in the genus Poecilochirus, which are well known for hanging out on burying beetles. They hitch rides on the beetles, which shuttle them between corpses. Interestingly, the lifecycles of both the mites and beetles are inextricably intertwined.

When talking about dead animals in relation to insects, I have a habit of referring to them as ‘meat islands’. Corpses are incredibly rich resources spread very thin in time and space. They’re good habitats because they’re nutrient dense, but they’re also relatively small and don’t stick around very long. Animals which feed on corpses need to be able to find them quickly, and spread far.

The beetles are great at finding corpses, which is why the mites use them. They can fly, and have a well developed sense of smell. The corpses they find need to be relatively fresh, which means they have competition in the form of botflies.

Botflies are always the first insects on the scene when there’s fresh meat, usually only appearing a few minutes after death. They’ll quickly colonize corpses, and begin feeding. The beetles need to be able to deal with the eggs, and corpses can have high numbers of eggs.

The beetles can take care of some of them, but not all of them. This is where those mites come in. Those mites are specialists on fly eggs, and hungrily devour the eggs and young larvae. This keeps the flies from stealing the food that the beetles need to rear their larvae.

This relationship has another aspect, though. The mites can’t feed on anything bigger than they are, so they need the beetles to raise the mite larvae. The mite larvae feed off the same secretions the mother beetles use to feed their babies.

The Bottom Line

These mites play an important part in the lives of these beetles, and act as bouncers inside their nursery. Fly babies don’t play nicely with their clients, and they need the beetles to complete their lifecycles.

There’s another interesting aspect to this story, one that’s historical. The American Burying Beetle, Nicrophorus americanus, was the first beetle added to the endangered species list…and it’s dependent on these symbionts as well. The mites can hitch-hike with any species of Nicrophorus, but they still play a role in the ongoing recovery of N. americanus populations.

 

Nehring, V., Müller, J. K., & Steinmetz, N. (2017). Phoretic Poecilochirus mites specialize on their burying beetle hosts. Ecology and evolution, 7(24), 10743-10751.

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