What are nematodes and how can they be used for pest control?

Despite being popularly known as intestinal worms or tapeworms, nematodes are not actually as closely related to worms as we might think. In reality, they are multicellular insects with smooth and non-segmented bodies; and they are commonly known as "round worms" due to the shape of their body in cross section.

Of the 25,000 recorded species, 50% behave as parasites in plants, insects or other animals. They can have different sizes, from 0.2 millimetres to 8 metres, like the Placentonema gigantissima, which lives in the placenta of female sperm whales. However, most tend to be around 2.5 millimetres in length, which makes them difficult to detect by the human eye.

They feed on fungi, bacteria and other organisms found in the soil. And even worse: they also feed on plant cells. And wherever they feed, these little worms can seriously damage crops with their pointed mouths when they pierce cell walls.

The real damage occurs when a nematode injects saliva into a cell from its mouth and then sucks the cellular content. The plant reacts to the parasite with swelling, distorted growth and dead areas. Nematodes can also carry viruses and bacterial diseases that enter plants from the feeding wounds they produce.

"Bad" nematodes in cannabis

There are several species of "bad" nematode that can attack various parts of the plants, such as the bulbs or stems. But the nematodes that should concern us most are those that attack the roots of the marijuana. Of the six species of nematodes that are known to infest cannabis plants, five attack the root system.

If the nematodes feed on the roots, a plant may look yellowish, withered or stunted, and infected crops will generally have a lower yield. Most of the symptoms that appear are vague and resemble those caused by other factors, such as viruses or nutritional deficiencies. This makes many growers think that their plantation has a nutrient or hydration problem, which means that it is more difficult to control than other pests.

If you suspect that the damage is caused by nematodes, carefully lift one of the infected plants and wash the roots for easier inspection. If nematodes are causing the damage, you will see small sores or wounds, injured root tips, root rot or excessive branching.

Although there are several kinds of nematode that attack the roots, there is also another that attacks the stem: Ditylenchus dipsaci. These stems, branches and petioles (the part of the leaf that joins it to the stem) swell and show signs of chlorosis, a disease that leads to the yellowing of the green parts. If not treated in a timely manner, the stems become twisted and distorted with a shortening of the internodes, which results in a delay in growth and a reduction in yields.

How to combat a plague of nematodes

- If you have noticed that your roots already have a plague of nematodes, you will have no choice but to change the substrate and start from scratch with a new one.

- In addition, the irrigation water must be taken into account. Nematodes usually live in the thin films of water that can be found in the soil, so any type of standing water should be avoided by means of a well-drained substrate. Water is also the medium through which they move most easily.

- Similarly, the earth must be turned over: they are very happy in sandy or very porous textures. So, when growing outside, it is better to follow the advice of old farmers and use something similar to a fallow system: in soils where crops rotate or the land is renewed it is less likely that nematodes will appear.

- They are also spread by anything that can carry particles of infested soil, including infected tools, boots, animals and plants.

- If prevention fails (and without a doubt the best advice is to avoid them) there are fungicides and insecticides that can help us to combat them. In general, products with neem oil, mixed with the substrate, can help to eradicate them.

- You can also utilise bacteria, such as Pasteuria penetrans, which impede their growth. The bacteria generate ferments that destroy the eggs or kill the larvae in their early stages of life. We can even use fungi such as those of the genus Verticillium, which can also destroy their eggs or the females.

What about the "good" nematodes?

But there are also beneficial nematodes that can help to improve a marijuana plantation. They are commercially available in products that are sprayed on the ground and are extraordinarily lethal to many insect pests that have larval or pupal stages of life in the soil, such as mosquitoes, aphids, thrips, etc. Although they are deadly to pests, they are completely harmless to humans, pets, ladybirds and other beneficial insects.

The three main types used to fight pests that infest marijuana are Heterorhabditis bacteriophora, Steinernema feltiae and Steinernema carpocapsae. Each of these species attacks different pests. For example, we can use the Steinernema feltiae to combat the fungus gnat, which can be found at the base of the plants and near humid environments.

When these little creatures are released in the soil, they have an attack strategy which is curious to say the least. Juvenile nematodes follow the heat and CO₂ emitted by larvae and pupae. When they find a host, they enter the body cavity through different openings (nose, mouth or anus), or directly through the skin. Once inside, they release a symbiotic bacteria from the intestine of the nematode, which multiplies and causes the death of the insect within 24-48 hours. The nematodes then feed on the liquefied host and during the next few days they mature and reproduce within the host. The life cycle is complete when thousands of new infectious juveniles leave the host in search of new victims.

In summary, nematodes can be very dangerous as a pest, but there are certain classes that can help you to improve your marijuana plantation. Knowing about them, and their good and bad sides, will help you to get the best result