Nematodes for vine weevils: application & tips

Once you have pests like vine weevils in your garden, it is often difficult to get rid of them. Nematodes are a simple and natural way of tackling a vine weevil infestation.

Vine weevils are nocturnal creatures that leave characteristic feeding marks on plants such as rhododendrons, roses, clematis and strawberries. More problematic than the unattractive feeding marks, however, is the extensive damage the beetle’s larvae can cause to plant roots. Read on to find out how to use beneficial nematodes for tackling vine weevils.

Recognising a vine weevil infestation

It is usually relatively easy to recognise when a garden is infested with vine weevils by the irregular, semi-circular shaped notches that these pests leave behind in the leaves of ornamental plants, such as rhododendrons. However, it is the larvae that do the real damage. They live in the soil and feed on the plants’ roots, often even peeling off the bark at the root crown. This cuts off the plant’s nutrient and water supply and can cause it to die. It is therefore important to start combatting vine weevils as soon as possible once you find them.

When to use nematodes for vine weevils

There are two times of year during which it is recommended to use nematodes to control vine weevils, namely in spring from April to May or in late summer between August and September. The temperature of the soil should stay above 12 °C for at least a few hours every day. These recommendations are based on both the life cycle of vine weevils and the temperature requirements of the nematodes. In order to completely eliminate vine weevils, we suggest using the nematodes once in spring and once again in late summer. Nematodes that tackle vine weevils are only effective against the vine weevil larvae and pupae, not the beetle itself or the eggs. The first nematode treatment will kill the vine weevil pupae and the second round of treatment in autumn will take care of the newly hatched larvae. After applying nematodes, the first of the vine weevil larvae will die within seven days. As the nematodes continue to multiply among the insects, this treatment gets more effective over time. As a result, the nematodes may be an effective long-term solution depending on the severity of the infestation.

Which nematodes for vine weevils?

HB nematodes are effective against both the pupae and the larvae of the vine weevil. HB nematodes are of the Heterorhabditis bacteriophora species and are tiny, measuring just 0.6 mm. These beneficial insects seek out vine weevil larvae and pupae in the soil and parasitise them. They then multiply inside the infested larvae or pupae, ultimately killing them. Finally, the nematodes leave the now-dead hosts and look for further larvae to enter.

Tip: In addition to controlling vine weevils, HB nematodes can also help to combat grubs in lawns which can cause a fair bit of damage.

Since HB nematodes only target a select few pests, they pose no danger to warm-blooded animals, humans or plants. So, there is no need to worry if the nematodes spread out in your garden. Once the beneficial nematodes have done their job and there are no more vine weevil pupae or larvae left, the nematode population will also decline. However, HB nematodes can sometimes survive for longer, in which case a degree of long-term protection remains.

Note: There are also other nematodes that are suitable for controlling the vine weevil. Nematodes of the species Steinernema kraussei mainly attack larvae, whereas Steinernema carpocapsae attack the adult beetles. However, HB nematodes are most commonly used because of how effective they are.

Using nematodes for controlling vine weevils

Applying HB nematodes for vine weevils and grubs is simple and does not require any special equipment. That said, there are some things you need to keep in mind when using these beneficial insects. As nematodes are sensitive to UV light, only apply them in the morning, evening or when the sky is overcast. To ensure the treatment is successful, it is also important to apply the nematodes when the soil is around 12 °C as they require such temperatures to reproduce. When it comes to storing nematode preparations, bear in mind that they are living organisms. It is best to apply them as soon as possible after receiving them. However, you can also store them in the fridge where they will keep for up to six weeks.

How to use nematodes for vine weevil infestations:

1. Moisten the soil.
2. Stir the contents of the package into 10 litres of water.
3. Further dilute the solution according to how many you bought and the area being treated. See the booklet provided with your nematodes for further information on dosing. If necessary, divide into several watering cans or mix several separate batches one after the other.
4. Keep stirring the mixture well to prevent the nematodes from settling at the bottom of the watering can.
5. Pour the finished mixture into a watering can (or other watering tool) or a nematode sprayer and distribute evenly over the area.
6. Immediately after application, follow up by watering with at least 1 litre of water per m² to flush any nematodes from plants and help them penetrate deeper into the soil.
7. Do not let the soil dry out for at least the following 4 weeks, as the nematodes need moisture to survive.

If you are struggling with more than just vine weevils in your garden, you can read more about nematodes as beneficial insects in our other article.