GARDENING: How to stop vine weevils ruining your plants

A vine weevil adult. They have become a real problem in our warmer autumns and winters.
A vine weevil adult. They have become a real problem in our warmer autumns and winters.

Vine weevils have become a real problem in our warmer autumns and winters, especially in potted plants.

You have the double whammy of adults eating notches out of leaves, followed by the real danger - the grubs in the soil, which sever roots.

Notches eaten out of Shasta daisy leaves by vine weevils.

Notches eaten out of Shasta daisy leaves by vine weevils.

Adults like to lay their eggs in non-­gritty potting compost. By the time you see grub damage (a plant falling over and wilting), it's probably too late.

The creamy, fat, C-­shaped legless grubs have light brown heads and are up to 10mm (about 3/8in) long. During autumn and winter, they feed on plant roots.

This time last year, one of my new Heuchera Silver Blush simply came away in my hand as I was weeding round it ­- roots severed, with a couple of grubs underneath.

Plants grown in open ground are less susceptible, although the grubs can kill strawberries, Primula, Polyanthus, Sedum, Heuchera and young yew plants.

The unpleasant root-eating grubs.

The unpleasant root-eating grubs.

The adult beetles feed on the foliage of many herbaceous plants and shrubs, especially Rhododendron, evergreen Euonymus, Hydrangea, Epimedium, Bergenia, Primula and strawberry.

They are about 9mm (about 5/16in) long, dull black beetles with a pear­-shaped body when viewed from above.

All adults are female and each can lay many hundreds of eggs from April to September.

The eggs are brown and less than 1mm (about 1/16in) in diameter, making them very difficult to see.

Heuchera Silver Blush, its roots eaten by vine weevil grubs.

Heuchera Silver Blush, its roots eaten by vine weevil grubs.

Larger yellowish­brown spherical objects seen in potting composts are likely to be controlled-release fertiliser pellets -­ similar whitish objects are usually slug/snail eggs.

10 WAYS TO ORGANICALLY CONTROL VINE WEEVILS

1. Inspect plants and walls by torchlight and pick off adult weevils. Shake shrubs over an upturned umbrella to collect more. In greenhouses, look under pots or on the underside of staging, where the beetles hide during the day.

2. Trap adults with sticky barriers placed around pots or on greenhouse staging.

3. Encourage natural enemies ­- birds, frogs, toads, shrews, hedgehogs and predatory ground beetles.

4. Stand potted plants on upturned pots sat in saucers of water ­- the adults can't swim.

5. Surround pots with Barrier Glue from Agralan -­ the adults cannot walk across it. Move plants away from walls, as the adults can jump down on to them; they cannot fly.

6. Add a 2cm (0.75in) deep layer of sharp grit or gritty gravel on top of the compost or around the base of the plant to prevent the adults from laying eggs.

7. Adult vine weevils hide in debris around the bases of plants, so keep the area free from dead and fallen leaves.

8. Place landscape fabric at the base of plants to prevent newly-­hatched larvae from entering the soil.

9. A biological control of the larvae is available as a microscopic pathogenic nematode (Steinernema kraussei). Apply in August or early September when the soil temperature is warm enough for the nematode to be effective (5­20oC/41­68oF).

10. Another nematode, Heterorhabditis megidis, is also available, but is more temperature-dependent (12­20oC/ 54­68oF). Both nematodes can also be applied to garden soil, but give poor

results in dry or heavy soils. They work best in open potting composts, such as peat or coir. Nematodes can be used safely on all edible and ornamental plants.

* For more gardening tips, news and giveaways, visit my website, www.mandycanudigit.co.uk