Nematodes — the friendly roundworms for your garden

Do you have a pest infestation attacking your lawn? Maybe you have round dry spots or dying grass in clumps around your lawn? Chances are, you have an infestation of ants, leatherjackets, chafer grubs, or similar insect larvae. But the answer isn’t to nuke your lawn with chemicals; the answer is nematodes — little friendly roundworms that benefit your garden. 

There are over 20,000 species of nematode that we know of. However, scientists estimate there are likely as many as 10 million species. 

This article is about how you can attack garden pests biologically with nematodes — avoiding harmful chemicals that can cause nutritional imbalances in your soil and kill your beneficial wildlife. 

What are nematodes?

Not all nematodes are beneficial; indeed, some can harm your plants and even become parasites for you and your pets. 

So, it’s essential to choose the right nematodes for your garden. Typically, the nematodes available online and in garden centres are safe for domestic use. 

Nematodes are microscopic roundworms. Some species are aquatic, while others thrive in soil — these are the ones you want for your garden. 

Most nematode species feed on microbes, while some prefer to eat living matter, such as larvae and grubs, that can damage your lawn. This is what makes them popular in the war against lawn pests. 

Use these critters in your garden

The types of nematodes you can buy online or in garden centres are suited for everyday garden use and can be super-effective at attacking lawn pests. 

You might be concerned that you’re releasing an unwelcome species into your garden, but the right kind of nematode is a boon for your lawn and dies off once the food source (grubs and larvae) is gone. 

HM & HB nematodes — a weapon against grubs

Both HM & HB nematode species are effective against the grubs of:

• Weevils
• Garden leaf beetle
• June beetle
• May beetle

HM stands for “heterorhabditis megides”, which sounds a bit like a skin condition, but these nematodes are effective in the battle against the beetles and grubs listed above. 

HB stands for “Heterohabdtius bacteriophora”. These are generally cheaper to buy, but they don’t multiply as readily as the HM variety, so they won’t last as long.

These nematodes find their food source, then invade the body of the larvae or bug while secreting a bacteria that eventually kills the infected beast. Both varieties only multiply once they’ve found their host, and once they’ve destroyed the hosts, the nematode population dwindles and dies off. 

For best results, HM & HB nematodes require:

• Soil temperature of at least 12℃
• Moist soil, so water it if it’s looking dry. 

SC nematodes — a weapon against meadow gnat larvae

If your garden is invaded by cricket varieties (it does happen in the UK occasionally) or gnats/mosquitoes, Steinernema carpocapsae (or SC) nematodes can help.

These minute nematodes are just 0.1mm in size, so they’re difficult to see, but they can be particularly effective against mosquitoes, gnats, and crickets. They work best in September for meadow gnats. 

SF nematodes — a weapon against fungus gnats

Sternermena feltiae (SF) nematodes are effective in the battle against fungus gnats, which feed on dead organic matter. Fungus gnats can be helpful in the garden because they clear dead organic materials, but they can also attack plant roots if there’s a shortage of food sources. 

However, fungus gnats are more problematic indoors – they’re commonly found in greenhouses, where the atmosphere is hot and damp. 

SF nematode will kill the fungus gnat larvae, stunting the reproductive cycle of the gnat. 

They work best in soil of 8℃ and over and can be used all year round indoors. 

SK nematodes — a valuable weapon against weevils in low temperatures

Steinernema kraussei (SK) nematodes are harmless to crops, humans, and beneficial insects, but they swim in the thin film of water of soil particles to seek out the vine weevil. 

The SK nematode is effective at temperatures as low as 5℃ and up to 12℃. They’re ideal for use in early spring before the temperatures have risen significantly and in late autumn when the temperature begins to drop. 

PH nematodes — battling against snails

Snails are generally harmless little creatures until they attack your veg patch. They can eat their way through your lettuce in a matter of hours. 

Of course, there are plenty of approaches to battling snails and slugs, from beer traps to poison. However, remember that the birds tend to eat slugs and snails, so if you’ve killed them with poison, it can also make our feathered friends sick. 

So, Phasmarhabditis hermaphrodita (PH) nematodes are the answer to more safely killing off the snails that are chomping through your veg. 

However, this isn’t an instant battle — it takes a little time as the PH nematode penetrates the snails’ bodies, slowly decomposing them from the inside. And while that sounds totally gross, it’s nature’s way of dealing with pests — so you don’t need to feel quite so guilty.

You may have to apply nematodes several times to get rid of your snail invasion, and it can take a few weeks to clear them. So, I recommend combining nematode warfare with the typical slug tracks and creep barriers for the best results. 

PH nematodes are most effective when the soil temperature is between 10-25℃.

Lawn doctor Louis says:

Before you take action against your garden’s grubs, ensure they are not the larvae of rose or rhinoceros beetles. These species are protected and won’t feed on your plants.”

How many nematodes do I need to kill off my garden pests?

Calculating the number of nematodes you need depends on the treatment area. Generally speaking, we recommend 500,000 nematodes per square metre, which might sound like a massive population, but remember: they’re teeny tiny. 

Use this table to help you calculate according to the surface area:

Number of nematodes	Square meterage
6 million	12
12 million	24
50 million	100

How to apply nematodes in your garden

Once your nematodes have arrived, use them as quickly as possible — remember, they’re living things and need to eat! If you’re not going to use them for a while, stick them in your fridge. 

Avoid applying nematodes if it’s too hot or sunny — they might dry out and die in these conditions. Overcast weather is ideal.

They’ll arrive in a sachet containing a nematode paste. 

How to water them in

1. Open the sachet and mix the paste with a small volume of water, turning it into a slurry. Then dilute the slurry with 2-5 litres of water — follow the instructions on the packet.
2. Pour the nematode solution into a watering can, and sprinkle the mix wherever you’re experiencing problems. It’s wise to treat AROUND problem areas, not just the direct spot — this way, you’ll catch more pests. 
3. Keep the soil moist over the next few weeks to ensure the nematodes remain active.

And that’s it. You should see results in around 3-7 days, although you may have to wait between two and four weeks for the full effect. 

Nematodes attack pests’ bodies from the inside out, so you won’t find dead insect bodies — they will decompose naturally. 

What are the pros of using them?

These are the advantages of using nematodes rather than chemical pesticides:

• Odourless after use
• Harmless
• Comparatively cheap
• Suitable for large and small areas
• Can be used outdoors and in greenhouses
• Invisible (compared to traps, etc.). 

Signs of harmful nematodes

You won’t see any adverse effects of nematode use if you buy them from a reputable source. However, as mentioned earlier, not all nematodes fight grubs and larvae; some invade your plants’ roots. 

The harmful species invade the root system and change the plant’s metabolism via their saliva. For example, Beet cyst nematodes (Heterodera schachtii) can damage beetroot, spinach, and rapeseed. 

Other types prey on potato plants, while other harmful species can attack your bedding plants. 

Symptoms of harmful nematode invasions

The typical signs of harmful species invasions are:

• Irregular plant growth
• Deformed roots and leaves
• Root rot
• Plant discolouration
• Wilted leaves and branches
• Reduced number of flowers

The following plants are susceptible to harmful nematodes:

• Apple trees
• Strawberries
• Cucumbers
• Hydrangea
• Begonia 
• Potato
• Spinach
• Turnips
• Celery
• Tomatoes
• Aubergines

Fighting a harmful species invasion

The best approach to killing off harmful species is to starve them out. So, if you can, lay your soil fallow for a few months — remove the entire root system of your plants, and dispose of the waste in an organic waste bin (not your compost heap). 

This approach is most successful during a long hot summer when the soil is likely to dry out. Keep the fallow soil weed free, or the nematodes will eat those instead. 

Marigolds and Tagetes — nature’s natural remedy

Some plants have developed natural defences against nematodes; marigolds and tagetes are prime examples. Their roots grow terthiophenes, which are toxic to nematodes. 

Marigolds make great bedding partners for carrots, celery, and onions to help keep invading species at bay. Alternatively, plant marigolds in your fallow soil — that will also keep the nematodes away. 

Any questions

I hope you’ve got everything you need regarding the use of nematodes to attack lawn and garden pests. But if you have questions, don’t hesitate to get in touch!

We love hearing from you and will get back to you as soon as we possibly can. 

Thanks for reading.