Beneficial nematodes for pest control in the garden


As a child, I played every day in the garden in front of my house in my home town of Rheinlandpflanz. There, my interest in nature grew, as did my aspirations to become a natural scientist. I now study horticultural phytotechnology and am currently writing my bachelor’s thesis on the topic of crop protection in orchards. Since living Berlin, I have become particularly interested in improving the quality of life in cities with the help of plants.

Favourite fruit: figs, passion fruit, berries, limes and oranges.
Favourite vegetables: potatoes, garlic, tomatoes, pickles, lamb’s lettuce and rocket.

Nematodes are beneficial insects that can help to combat many plant pests. But what exactly are nematodes and how do you use them correctly?

Nematodes under a microscope.
Nematodes can be used in various ways as beneficial insects [Photo: D. Kucharski K. Kucharska/]

Nematodes are used as a biological pest control method to tackle a wide range of pests. When you use nematodes, however, it is important to keep in mind that they are living organisms. Read on to find out which pests you can use nematodes against and how to apply these beneficial insects correctly.

What are nematodes?

Nematodes, also known as roundworms or eelworms, are worm-like organisms that are transparent to white in colour and are barely visible to the naked eye. So far, there are more than 20,000 known species of nematodes. Depending on the species, the nematodes have very different characteristics and feed on different things, such as plants, bacteria, algae, fungi, predatory prey or dead animals. Many types of nematodes are parasites, like threadworms, whipworms and Trichinella, for instance.

The nematodes (Nematoda) relevant to crop production are small roundworms which are less than one millimetre in size. Depending on what they feed on, they can either be harmful or beneficial to plants. Some species infest vegetables and other plants and are considered pests. See our article on nematodes as pests if you are dealing with nematodes of the harmful sort.

Nematode under a microscope.
Most nematodes are microscopic [Photo: F.Neidl/]

Nematodes as beneficial organisms

The great thing about using beneficial nematodes is that there are no negative side effects on your plants or on the environment. But what makes nematodes such effective beneficial insects? Roundworms have the ability to actively move around in the soil, giving them a greater chance of finding prey in your garden. Their small size enables the nematodes to penetrate the inside of the target pests through bodily openings, killing them in the process. They then reproduce inside of the hosts’ bodies. Nematodes also have some allies of a much smaller size at their disposal to help them in their mission. These are special bacteria that have a symbiotic relationship with the nematodes. When a nematode comes face to face with a target insect, it injects these bacteria into its victim. The bacteria then start to digest the pest from the inside, allowing the nematode to feed on and reproduce inside of it. Once the target insect is dead, the nematode leaves in search of a new host. This means that, under the right conditions, nematodes can survive in the soil for several months.

In almost all cases, the nematodes attack the larvae, not the adult pests themselves. And thanks to the diverse range of nematodes species available to us, we can use them to tackle a whole host of different pests, including:

  • Fungus gnats
  • Vine weevils
  • Chafer grubs
  • Ants
  • Slugs
  • Cherry fruit flies
  • Box tree moths
  • Mole crickets
  • Thrips
  • Crane flies

How to apply beneficial nematodes

In order for the nematodes to complete their mission successfully, you need to keep a few things in mind when using them:

  • Identify which pest you are dealing with before buying the nematodes, as certain nematodes target different pests.
  • The best time to apply nematodes depends on the pest and the weather. If it is too cold or too dry, the nematodes will go dormant and will be ineffective. Since it is only usually possible to control the target insects’ larvae living in the soil, you should apply the nematodes when the pest is in its larval stage. Keep reading to find out the optimal times for controlling some of the most common pests.
  • The ground temperature must be right for the respective nematode species. If it is too cold, the nematodes will go dormant and will not be able to do their job. Most species can be used at a ground temperature between 12 and 28 °C.
  • Nematodes are sensitive to UV light, so avoid applying them during the daytime, especially on bright, sunny days. Apply them early in the morning or wait until the evening or an overcast day.

So where can I buy nematodes? You can buy various nematodes on the internet in specialised shops or via order cards from your local garden centre. Nematodes come in a powder that acts as a carrier for the larvae and are delivered directly to your letterbox. To keep them dormant, store the nematodes in the fridge until you are ready to use them. See the product packaging of your nematodes to see how long you can store them for.

How do I apply beneficial nematodes correctly? First, mix the nematode powder with water. Next, moisten the area to be treated before applying the nematodes. Then, use a watering can (or similar water tool) to spread the nematode solution evenly over the area. See the product packaging for exact mixing ratios and information on how large of an area the nematodes will cover. After applying the solution, water the area again to encourage the nematodes to penetrate deeper into the soil. It is important to keep the soil most for the duration of the treatment, but avoid waterlogging at all costs.

Water can being used to apply nematodes in garden.
Nematodes are applied to the area to be treated with a watering can [Photo: DUSAN ZIDAR/]

Tip: To tackle pests with multi-year development cycles, such as garden chafer beetles or mole crickets, we recommend applying the nematodes several years in a row.

How long do the nematodes remain effective? You will notice the first dead pest larvae in the soil after just a few days. The roundworms may remain in the soil for several months under very good conditions, but their numbers will steadily decline over time. The nematode treatment is most effective in the first four weeks after application.

Did you know? After being parasitised and killed by the nematodes, the larvae of the vine weevil (Otiorhynchus) turn a red-brown. This is a good way of checking whether the treatment is working.

Summary: How to apply beneficial nematodes correctly

  • The time of year you need to apply the nematodes depends on the pest. See the product packaging for further information.
  • Ensure the ground temperature is due to stay between 12 and 28 °C for the duration of the treatment.
  • Apply the roundworms in the morning, evening, or on overcast days.
  • Dilute the nematode powder into water to create your nematode solution.
  • Use a watering can to apply the nematodes to moist soil and then water them in.
  • Keep the soil moist for at least two weeks to help the nematodes penetrate deeper into the ground.
Fungus gnat in soil.
Fungus gnats can cause extensive damage to potted plants [Photo: Tomasz Klejdysz/]

Nematodes for fungus gnats

When it comes to fungus gnats (Sciaridae), it is not the adults, but rather the larvae that live in the soil that cause the real damage. They feed on plant roots which can cause extensive damage to your plants. Houseplants in particular are often attacked by fungus gnats. See our article on how to combat fungus gnats for helpful tips on how to tackle these pesky pests.

The beneficial nematode species Steinernema feltiae (SF nematodes) has proven effective against fungus gnats. Steinernema feltiae can be used on houseplants all year round and also outdoors, provided the ground temperature is 12 °C or above.

In the event of a severe infestation or if you just want to be on the safe side, we recommend using a neem-oil-based biological agent alongside the nematodes.

Nematodes for vine weevils

Vine weevil larvae that live in the soil can also be controlled by nematodes. You can use HB nematodes to save rhododendrons (Rhododendron), ivy (Hedera helix), hydrangeas (Hydrangea) and other plants from the dreaded vine weevil. This nematode species is known as Heterorhabditis bacteriophora and is a reliable and biological method of controlling a vine weevil infestation. They can be applied when the ground temperature is at least 12 °C. We recommend applying HB Nematodes either between April and May or between August and September. For more information on how to control vine weevils with nematodes, see our in-depth article.

Vine weevil eating leaf.
Leaf damage is typical for a vine weevil infestation [Photo: Tomasz Klejdysz/]

Nematodes for chafer grubs

Chafer grubs are the larvae of chafer beetles, including summer chafers (Amphimallon solstitiale), cockchafers (Melolontha melolontha) and flower chafers (Cetoniinae). They can cause extensive damage to lawns if they multiply en masse. It can sometimes take years to completely and permanently eliminate chafer grub larvae using nematodes, especially in the case of the cockchafer. This is due to the fact that chafer grubs have many larval stages, not all of which are susceptible to the effects of sprays.

One biological way of controlling cockchafer, summer chafer and flower chafer larvae is the use of HB Nematodes. As the nematodes can only effectively control the first two young developmental stages of the chafer grubs, timing is very important, as is employing the right nematode species. Heterorhabditis bacteriophora are effective against grubs of summer chafers, garden chafers and Welsh chafers when applied between July and September.

Nematodes for ants

Ants are part of a healthy garden ecosystem and are not actually pests. That said, if they get out of hand on patios or paths, they can become a bit of a nuisance. Should you choose to control them, you can use our SF Nematodes of the species Steinernema feltiae, the same ones you would use to tackle fungus gnats. These beneficial nematodes disturb the ants and make them leave the area along with their brood.

Ants on a terrace.
Ants can be a nuisance on the patio or on paths [Photo: Yashkin Ilya/]

Ants in your house, on the other hand, can become a bit of a problem. We recommend using a biological agent to combat ants in the home.

Nematodes for slugs

As every hobby gardener knows, slugs can cause considerable damage in our gardens. Luckily, you can use beneficial nematodes to control slugs as well. The fact that slugs multiply rapidly in wet weather is of an advantage here as the nematodes also require damp conditions to develop well.

For effective slug control, use the nematode species Phasmarhabditis hermaphrodita. The optimal soil temperature lies between 12 and 25 °C. Another advantage of using these beneficial nematodes is that they do not attack harmless snails like the vineyard snail.

Beneficial nematodes for other garden pests

The Steinernema feltiae nematodes are not only effective against fungus gnats and ants, but also to some degree against the larvae of the cherry fruit fly. They are not quite as effective against these as they are against fungus gnats. It is only possible to control cherry fruit fly larvae the moment they migrate from the infested cherries into the soil. The nematodes must therefore be introduced to the soil in such a way that they are active in the soil at this exact time. The beginning of June is usually a good time to do this.

To be able to use nematodes against box tree moths, you must incorporate them into a specific wetting agent. This is the only way that they will come into direct contact with the box tree moth larvae.

The nematode species Steinernema carpocapsae is suitable for controlling box tree moths. See the product packaging for instructions on how to apply the nematodes correctly. The ideal time to start the nematode treatment is two weeks after you discover the first adult box moths. A box tree moth trap, such as our Plantura Box Tree Moth Trap, can help to determine when this is.

We also recommend using a Bacillus thuringiensis based product, which is likewise a living organism, to control box tree moths in an effective and environmentally friendly way.

Mole cricket in garden soil
Nematodes can also be used to tackle mole crickets [Photo: Soru Epotok/]

Excessive numbers of mole crickets (Gryllotalpidae) can become very bothersome in vegetable patches. You can use Steinerne macarpocapsae nematodes to tackle mole crickets as they kill the adult crickets. The best time to control mole crickets using nematodes is between March and July, at a time when the soil temperature is at least 12 °C.

Crane flies, commonly known as daddy long legs, can be a particular problem in lawns. SC Nematodes are a biological way of combatting crane flies. Steinerne macarpocapsae nematodes kill the daddy long legs larvae, thereby stopping their life cycle. The ideal time to start controlling them is about two weeks after the crane fly’s peak flight period between September and October.

Nematodes can also be used against thrips (Thysanoptera). Western flower thrips (Frankliniella occidentalis) are one of the most common species and are particularly a problem in greenhouses or in living spaces. Steinernema feltiae nematodes, which attack the flower thrip pupae and kill them, can be used all year round.

Besides nematodes, there is a whole range of other beneficial insects that can aid you in keeping pests at bay in your garden. Take a look at our overview of beneficial insects in the garden to discover more of these little helpers.