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Pest Patrol

Vine weevil

Introduction

Vine weevil is one insect pest that most gardeners live in fear of. And it is a pest to worry about. Suddenly, otherwise healthy-looking plants collapse and die through the root-eating, soil-borne grubs. You should suspect a weevil attack if watering does not revive the plant. In severe cases the plant may be completely severed from its roots at ground level.

In the past, their favourite food was fuchsias, cyclamen, begonias and primulas. But in recent years they have become less fussy and will make a meal of a much wider range of plants - especially any that are growing in containers. This is possibly due to the increasingly widespread use of peat-based and peat-free composts - being less gritty than John Innes composts, the adults are more likely to use them for laying eggs.

So what do you do if you notice a vine weevil attack?

Don't despair, there are several ways of dealing with this plant-nibbling nasty. It is important to be vigilant and keep a close eye on all your plants - especially those that are known to be favourites on the menu.

Always check the rootballs for grubs before you buy plants. Check when potting up or repotting plants, and destroy any that you see; it usually pays to dispose of affected compost too.

Because the vine weevil is such a destructive pest, every effort must be taken to control both the adults and the soil-borne grubs.

There are three ways of controlling vine weevil: chemical, biological control and physical barriers.

Controlling grubs

The c-shaped, vine weevil grubs

Typical vine weevil grubs, which will eat away at the roots of a large number of different types of plants.

They are up to 10mm (0.5in) long, c-shaped, with a brown head and have no legs.

Often the first signs of damage are only spotted when a plant completely wilts.

Commercial nurseries sometimes add a strong chemical called Suscon Green to the compost, which gives good control. It is worth asking when you are buying particularly susceptible plants if Suscon Green has been used.

Scotts Miracle-Gro introduced a compost in the UK containing a chemical called Intercept (active ingredient imidacloprid), which also kills the grubs. This compost is available from garden centres and is called Levington Container & Hanging Basket Compost with Vine Weevil Control. The results are excellent - providing plants are grown in this compost alone and it is not mixed with other composts.
Not only does it give good results against vine weevil and other soil-borne pests, but it gets into the sap stream of the plant and gives control against aphids and other sap-sucking insects. It remains effective for up to six months.
It can be used with any plants except edible ones.

Bayer Provado Vine Weevil Killer 2 contains a similar insecticide thiacloprid but is more flexible than the compost. Provado can be used for any container-grown plants (apart from edible ones) and comes as granules which are mixed with water and simply watered onto the compost. Provado Ultimate Bug Killer is sprayed onto the foliage of the plant to kill leaf pests, but may also have some control of vine weevil grubs (some edible plants can be treated; check the label first).

Another solution for the gardener is to use a biological control based on nematodes. These microscopic creatures enter the bodies of the grubs and release a specific bacteria that poisons and kills them. They then multiply and go on to attack other grubs. The nematodes are available from several mail order supplies, such as Defenders, Scarletts PlantCare, Just Green and Green Gardener. See the suppliers list for details.

Controlling adults

Signs of typical vine weevil adult damage to a rhododendron leaf

Typical vine weevil adult damage to rhododendron leaves.

Vine weevil adult

And a typical vine weevil adult.

One of the most effective - but time-consuming - methods of controlling the adults is to check your plants at night when the adults feed. They can then be scooped up and disposed of.

Another way is to use a physical barrier as the adults cannot fly from plant to plant. Stand potted plants on upturned pots sat in saucers of water - the adults can't swim. Or surround the pots with Barrier Glue available from Agralan - the adults cannot walk across it. With either of these methods it is important to move plants away from walls or the sides of the greenhouse as the adults can jump down onto them; they cannot fly.

Physical barriers on top of the soil or compost also work very well. Add a 2cm (0.75in) deep layer of sharp grit or gravel over the top of the pot or around the base of the plant to prevent the adults from laying eggs; the egg-laying structure is irritated by the scratchy feel.

Eggs

If you spot golden-brown or clear egg-like objects in the soil or compost then they're NOT vine weevil eggs. These are tiny and you'd need a hand lens to even see them. Clear spheres are usually slug or snail eggs and the golden-brown ones are actually controlled-release fertiliser!

Suppliers

If you want to know more, or there's a pest you're having big problems with, then send an e-mail to: geoff@gardenforum.demon.co.uk

Problems with whitefly? Then turn to my whitefly control page.

 

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