Some pests make gardeners sigh deeply, swear loudly and tear their hair out because they cause such damage. They wipe out plants overnight and destroy crops. They destroy hard work and devoted coddling of precious plants. These pests include slugs which like salad crops and other soft green growth and vine weevil whose larvae love to eat container plants. Luckily, beneficial nematodes, a form of biological control, applied at the correct time, can help enormously with the problems presented by these two pests.

Slug cruising down the outside of the bedroom window. Slugs and snails. Nematodes

Slug cruising down the outside of the bedroom window, April 2013. M K Stone

How nematodes work

Beneficial nematodes work by parasitising a host such as slugs or vine weevil. Each nematode is adapted to a particular host which they live off by infesting or infecting it in some way, and eventually killing it.

Rest assured that these beneficial nematodes aren’t dangerous. They are quite harmless to people, animals and birds, and do not affect food crops.

How to use nematodes

Order your nematodes online. They arrive freeze-dried. You can store them in the fridge for a few days but you do have to use them fairly soon.

Read the instructions carefully before you start. The idea is to mix the freeze-dried nematodes with water and then apply the soupy mixture to the affected area or plants. Different nematodes work at different temperatures so make sure you take this into consideration, otherwise you’ll be pouring your money away.

Controlling slugs

Use the slug nematode, Phasmarhabditis hermaphrodita, from mid-spring when the temperature is above 5C and the soil is warm and moist. It is very effective in most gardens although it does not work so well in cold clay soil.

Phasmarhabditis hermaphrodita occurs naturally in the wild so when you apply the nematodes to your garden, you increase the likelihood of slugs meeting their parasite nemesis. This will happen underground as that’s where the nematodes live.

When their paths do cross, the nematode infects the slug with bacteria. Within a few days the slug stops eating and it dies within a week. The nematodes feed off the decomposing slug and reproduce. That means more nematodes, infecting more slugs as they go on their way. Some people find that their garden can be clear of slugs for up to a year after using nematodes.

Unfortunately, although this nematode can kill snails (which are cousins of slugs, after all), in practice this happens rarely. That’s because nematodes live in the soil and snails live on the surface, so the two are less likely to meet.

If slugs are not present when you apply the nematodes, they will die back to their natural levels in the garden.

N.B. It is possible to make your own nematode mixture but this may not be for the faint-hearted.

Controlling vine weevil

vine weevils. nematodes

Vine weevil. Image by Martin Cooper. This file is licensed under the Creative Commons attribution 2.0 generic license

Adult vine weevil nibble the edge of foliage and leave their droppings everywhere. Their larvae are cream in colour and are shaped like a C. These grubs love to eat the roots of plants in pots and containers which then, suddenly and unexpectedly, keel over and die. They can also attack plants in the open ground.

Vine weevil can be controlled with three types of nematode.

  1. Apply the nematode Steinernema kraussei in August or early September before the larvae have grown large enough to cause serious damage. The soil temperature should be between 5C and 20C.
  2. The nematode Heterorhabditis megidis also kills vine weevil larvae but is only effective at higher temperatures, between 12C and 20C.
  3. During the summer, the nematode Steinernema carpocapsae can be used in traps to control adult vine weevil. Place the traps on the ground below the damaged plants. The adults enter the trap during the day and are then infected by the nematodes.

Nematodes can be very effective in controlling these pests. Using them also cuts down on the use of garden chemicals. Good luck!