Jeroen Onrust. Photo by Margje de Jong

Crawling through meadows

The secret world of worms

Jeroen Onrust. Photo by Margje de Jong
Biologist Jeroen Onrust recently managed to inspire six hundred people to dig around in the earth looking for worms. Even though they’re all around us, there’s still much to be learned about these useful creepy crawlies. ‘There’s thousands of them crawling around a meadow at night.’
28 April om 12:15 uur.
Laatst gewijzigd op 28 April 2022
om 12:15 uur.
April 28 at 12:15 PM.
Last modified on April 28, 2022
at 12:15 PM.

Door Rob van der Wal

28 April om 12:15 uur.
Laatst gewijzigd op 28 April 2022
om 12:15 uur.

By Rob van der Wal

April 28 at 12:15 PM.
Last modified on April 28, 2022
at 12:15 PM.

Rob van der Wal

He was trying to be so careful. But when UG biologist Jeroen Onrust sticks his spade into the ground next to the Naardermeer to pry out a twenty-by-twenty centimetre piece of dirt, he notices that he’s cut a long, fat worm in two. ‘Oh look, this worm is missing a piece. That’s always a risk when you stick a spade into the ground.’

According to popular belief, cutting a worm in two results in two new ones. ‘But that’s not true’, says Onrust. ‘One half dies off, if you’re lucky. Some species just die altogether.’

Anecic worms

He crouches down over the lump of dirt and starts going through the grass blade by blade. ‘Some worms like to hide in between the roots. You have to be gentle so as to not damage them.’ 

In the end, he finds twelve worms. ‘Not bad at all.’ He puts each of them in a little container, and then pulls out a little printed chart. Each type of worm has two sections, one for young and one for old, and Onrust patiently categorises each worm. ‘The old worms have this thick ring, called the clitellum. They use it to reproduce’, he says. 

Why do some species come to the surface at night while others don’t?

The best news is that four of his twelve worms are anecic worms, which are hard to find. Anecic worms ‘commute’ between the surface and the deeper soil layers. ‘This burrowing makes the soil extra fertile.’

Onrust has been fascinated by worms for ten years. Every year, he learns more about how important these little creatures are. Not just for the soil, but also as a source of food.

Jeroen Onrust carefully combs through the grass. Photo by Lars Soerink


He started out studying the behaviour and feeding habits of birds, he says. ‘These rain worms at the soil surface are an important source of food for meadow birds. But we didn’t have enough research on how these worms behaved. Like why some of them do come to the surface at night while others don’t. It’s kind of strange that we don’t know that.’

And so he decided to dig deeper. He’d spend hours in the fields at night, lying face-down on a home-made cart, carefully combing through the grass. He’d wear a torch on his head to help him see. Anyone seeing him would have surely thought he was crazy. But Onrust took his task very seriously: he counted all the rain worms that were coming up out of the ground as part of his PhD research.

It opened up a whole new world

‘I couldn’t disturb the worms, or they’d retreat back into the ground. Crawling around would also do a number on my knees.’ In short, he had no choice but to move around as slowly as possible on his little cart.

Because of this research, Onrust, who’d originally trained as an ecologist and a bird researcher, slowly became an expert on worms. ‘It opened up a whole new world’, he says. ‘People probably don’t realise it, but thousands of worms are crawling around meadows at night. You don’t see them during the daytime.’


What people also fail to realise is just how useful worms are, Onrust says. ‘Not only do they serve as food for various animals such as birds, hedgehogs, badgers, and even moles, but they also work the soil.’ 

In addition to anecic worms, epigeic worms are also essential. ‘Epigeic worms look for leaves and take them back into the soil, where they become plant food.’ 

The most important part of Onrust’s work takes place in meadows on farmland. ‘Looking only at the number of worms that occur on farmland you might think that there are plenty of rain worms’, he says. ‘But if you look at individual species, it’s a completely different picture. Most of the worms I find are endogeic, rather than anecic or epigeic.’

These worms are disappearing because of soil fertilisation. ‘Today’s farmers have basically taken on the role of worms by tilling the soil and using artificial fertiliser. Worms don’t like that.’

But how worms in the Netherlands are actually doing and what type of soil is best for them, we don’t even know, says Onrust. ‘That’s strange, because worms are so useful.’


This lack of knowledge inspired him to create the National Worm Count, which took place two weeks ago. Together with CurioUs, an initiative set up by Forum Groningen, Science LinX, and the Aletta Jacobs School of Public Health, he asked people at home to dig up a patch of dirt and carefully go through it. Participants were not only asked to count the worms they found, but also fill out a short questionnaire about the location they’d found them. 

It was a resounding success, he says. More than six hundred people from all over the country participated. He even received information from places like Maastricht, Amsterdam, and Doetinchem. ‘I definitely didn’t expect that’, says Onrust. ‘We initially focused on the north of the country.’

I might have underestimated how much work it will be

The results of the count will hopefully enable researchers to draw conclusions about where anecic worms occur most. ‘Such as what kind of environment or type of soil, under which plants. Things like that. We’ll ultimately be able to use that to improve the locations that are currently lacking in worms. That way we can make sure as many worms as possible are having a nice time.’

There are many things people can do for worms, says Onrust. ‘Right there in your own garden. It’s best to not clean up fallen leaves, because worms love those. Removing tiles from the garden helps, too.’


After the count and all the media attention the worm weekend got, Onrust will now have to go through all the worm pictures by hand and double-check the count. ‘I’m slightly addicted to data, so I’m really looking forward to going through the photos. Although I might have underestimated how much work it will be’, he says. ‘Perhaps I’ll need some help. But going through photos is a great thing to do on a rainy day. And I hope we’ll be able to organise this even more often, so we’ll be able to track the differences over the years.’

In the meadow next to the Naardermeer, one worm is tired of the piece of paper Onrust put him on and is trying to escape. Onrust deftly puts it back, putting a little beetle to the side at the same time. ‘You have to be quick when you’re counting worms. Anecic and epigeic worms especially are fast little guys.’

After taking a picture of the chart with worms on it so he can check which ones he found later, he puts the clump of dirt back where it was. ‘The worms can go on top. They don’t like sunlight, so they’ll crawl back into the soil quick enough.’