Tackling slugs and weevils with organic nematodes

Are hi-tech nematodes an effective, organic way to tackle difficult pests such as vine weevil and slugs? Hannah Stephenson investigates

Slugs and vine weevils are vulnerable to nematodes  just mix and apply correctly. Picture: PA
Slugs and vine weevils are vulnerable to nematodes  just mix and apply correctly. Picture: PA

Those of us who yearn to ditch the blue pellets and quick-fix chemical sprays in favour of more eco-friendly garden pest controls should perhaps be taking a biological approach. Many of us incorporate a variety of plants in our garden to attract beneficial predators such as ladybirds which will gobble up aphid eggs, or hedgehogs, song thrushes and frogs to feast on slugs and snails. They all assist the balance of nature – but sometimes that balance needs a helping hand.

Many organic gardeners have been using nematodes, or microscopic “eelworms”, for years. Nematodes are tiny micro-organisms which occur naturally in the soil where they hunt down and attack pests, with each different nematode having their own specific prey species.

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Chemical giant BASF has the largest specialist nematode production facility in the EU and the only one in the UK, breeding trillions of up to five different species of beneficial nematode to create its Nemasys range, and now has products which will treat many common soil-dwelling pests including vine weevil, chafer grubs, slugs and ant nests.

Nematodes generally come in sealed packs. The contents are mixed with water and applied via a watering can at particular times of year depending on the pest being targeted. Once applied, they will seek out their prey, whether it be slugs, vine weevil or other pests, entering the target’s body and releasing bacteria that will kill it. There is no need to keep children or pets away from treated areas. The nematodes will then reproduce inside the corpse and create a new generation which will hunt for further prey.

The company has also developed an all-round treatment aimed at fruit and veg growers which tackles the underground larval stage of common veg pests such as carrot fly, sciarid fly, cabbage root fly, sawflies, onion flies and codling moths.

“They each work on the very simple format that the nematodes deliberately seek out their particular prey pests and kill them – simple as that,” says BASF’s UK sales manager Gavin Wood. “Treatment can continue throughout the growing season and when the pests have all been wiped out, the level of nematodes in the soil will immediately drop back to the normal concentrations. When temperatures drop at the end of the season, the nematodes naturally reduce in quantities until soil warms up again.”

Guy Barter, RHS chief horticultural adviser, says: “They are an excellent idea because they have no effect on non-targeted organisms, are safe for the user and are organic. The only downside is that they are rather expensive.”

However, Paul Hetherington, of the charity BugLife, points to research that has shown incidence of certain nematodes crossing species, infecting bees as well, so would recommend using them only as a last resort.

For best results

When applying, use a soil thermometer as nematodes require certain soil temperatures to be most effective, depending on which pest you are targeting.

Apply the nematode solution to soil which is already moist. It’s best to water them in in the evening as soil rarely dries out overnight.

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Keep sealed packs in the fridge so the nematodes remain inactive, ensuring their good health until you need them. Use the entire packet at one time.

When applying nematodes to grass or flowerbeds, water well afterwards so they’re washed into the soil.

See the advice on when it is best to apply each product to target a pest – there can be an eight-week difference in the seasons between the north and south of the UK.


Hydrangea anomala subsp. petiolaris, the climbing hydrangea, is a self-clinger with lacecap heads of creamy-white flowers in summer which will lighten up a shady wall, while H. paniculata ‘Grandiflora’, produces conical heads of white flowers that flush pink with age by early autumn. Hydrangeas prefer fertile, humus-rich, moisture-retentive soil in partial shade, sheltered from cold, dry winds. Leave the flower heads on them over the winter, which will protect emerging new shoots. Then, in late winter, cut back all sideshoots to within 5cm-8cm of the main stems.


Once a rampant grower with vicious thorns, you can now buy more compact varieties and even a couple which are thornless. Plant bare-rooted canes in November or December. Cultivate a 60cm x 60cm area at each planting site, digging a trench about 30cm-45cm wide and 7.5cm deep and remove weeds before planting. Add 7.5cm of garden compost and scatter some general fertiliser over the ground before planting. You will need a support system to start it off – the most popular is post-and-wire. Fruits appear between late July and September. Grow Oregon Thornless if you don’t want to get scratched, or Fantasia, which bears heavy yields of huge fruits.