So, what is a nematode? Nematodes are animals, worms to be precise, and numerous ones at that. Experts estimate there are more than a million different species of nematodes. In fact, 4 out of every 5 animals are nematodes, making them the most numerically abundant animals on the planet, by a lot. They can be found absolutely everywhere from the deepest part of the ocean to the top of the highest mountain. They can be so tiny you need a microscope to see them, while a few others grow up to 40 feet long.

While all nematodes have the typical worm-shaped body at the beginning of their life cycles, their appearance can be very different when they mature into adults.  Also, while some look like plain worms, others are really quite stunning when viewed under a microscope.

But what do nematodes do? Dr. Thomas Baum tells us about different nematode life styles and some fascinating nematodes that have been making the news.

Nematodes are what we call roundworms, no different than other worms that you might dig up in the soil. They’re very diverse. They’re everywhere. And, of course, they have different mechanisms of survival. They feed on different substrates, they have different mechanisms of how they obtain their food. There’s one big group of nematodes that we call free-living and they feed as scavengers, they feed on bacteria, they feed on fungi. There are even some that are predatory that feed on other animals, like other nematodes, so they literally attack other nematodes and eat them. There are many, many different species of these nematodes that fall into that group and as such they are incredibly important for food nets and for the cycling of nutrients in their habitat.

In terms of well-known nematodes among those free-living nematodes is one in particular that is a downright or outright star that is a nematode that we call caenorhabditis elegans. So, in short we just refer to it as C. elegans. This nematode has been chosen as a model system for biological research. For example, we know the complete wiring diagram of all the nerves in that nematode. That is very unique for any animal. We know the cell fate of every cell in that nematode, when each cell is generated through cell division where it migrates, where it dies. We know all this, so as such C. elegans is a tremendous resource for developmental biology. On top of that, C. elegans was the first multicellular animal that has its genome completely sequenced. So, that was a tremendous breakthrough and that has been done with a nematode.

In addition to the free-living worms, the free-living nematodes, there is a whole group of nematodes that are parasitic and they are parasitic on animals as well as plants and humans. One example would be the filarial nematodes that are transmitted by insects, by mosquitoes for example. There is one disease that is referred to as elephantiasis that leads to a gross deformation of the extremities of the poor victims, extremely crippling and devastating. Another disease, for example, caused by filarial nematodes is so-called river blindness that the patients will end up going blind because of the infection by the nematodes.

And just like humans and other animals, plants are subject to severe nematode infection.  In fact, some of the biggest obstacles to sustainable agricultural plant production are parasitic nematodes living in the soil. Dr. Greg Tylka is an expert in plant nematodes at Iowa State University.

So, the first thing to understand is not all nematodes that live in the soil are bad or feed on plants. In fact, most of them are beneficial nematodes. They feed on bacteria and fungi and even other nematodes and they’re very involved in cycling nutrients in the soil. So, a thing to keep in mind is that nematodes are very important in the soil to maintain soil health. Now, as plant health professionals, we’re interested in managing or controlling the nematodes that feed on plants. And those are called plant parasitic nematodes. And they’re a tremendously diverse group of nematodes. Some of them, most of them feed from outside of the root and they kind of poke at the root with their pointed mouth spear, it is called a stylet, and they basically harm the roots by killing plant cells from the surface of the root. But the most damaging plant parasitic nematodes are ones that burrow into the root and attach to the vascular tissue in the center of the root. And there, they’re able to set up a large feeding site, obtain a lot of nutrition and produce hundreds and hundreds of offspring. And two nematodes that fall into that category are the root-knot nematode, which is probably the most economically damaging nematode worldwide and the cyst nematode, including soybean cyst nematode, which is probably the most economically damaging nematode in the United States.

These soil-dwelling, plant parasitic nematodes are very small, one-sixty-fourth to one-one-hundredth of an inch long. So, they’re capable of existing in tiny soil particles. And so they don’t really move a lot on their own power, maybe at most an inch or so. So, they’re not going to crawl from one end of a field to another end of the field on their own power. But the way they move long distances is through movement of soil. And there’s lots of ways soil can be moved in an agricultural setting. Probably the most obvious one to think about is on the wheels of combines and tractors and planters. Then sometimes those pieces of machinery are tillage implements, which dig into the soil to kind of break up the soil and that moves larger quantities of soil. But we also think about nematodes being moved in soil that is eroded by the wind and also by heavy rains through surface water. So, there’s many different ways that nematodes are spread from field to field and farm to farm and state to state. And really the way to think of it is anything that moves soil has the capability of moving plant parasitic nematodes.

Another interesting thing to understand about plant parasitic nematodes is that they’re part of nature. And we would expect to find plant-feeding or plant parasitic nematodes in virtually every natural soil. So, a soil core collected from any person’s front yard probably will have some plant parasitic nematodes. They are a native part of nature. Now, that is very different than soybean cyst nematode, which is an introduced pest. So, it came, we believe, from China where soybeans originated and it was first found in the United States in 1954 in an area of North Carolina that received shipments of ornamental plants from China. So, we believe it came over on soil associated with those ornamental plants. And in a few years it was discovered not only in North Carolina but all the way over in southern Missouri. And then as the years went by and the decades it spread through Missouri and throughout the southern United States. And then in the late 1980’s it was found for the first time in southern Minnesota and in northern Iowa. And it is believed that it might have made that jump on soil attached to farm equipment. And so now we have seen it spread into Minnesota, in throughout Iowa and it is spreading westward into North Dakota, South Dakota and Nebraska. And, again, we’re following this spread because it’s important. It’s a very damaging pathogen and it is an introduced pest that we would never find in a natural soybean field.

Nematodes really are everywhere and are a much bigger part of our world than we realize.  Even though they have a bad reputation, nematodes as a whole are critical and beneficial components of ecosystems. This idea is summed up well Nathan Augustus Cobb.

In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes.