Vine weevils

EVIL LITTLE VINE WEEVILS

Vine weevil
An adult vine weevil. Picture; RHS

I hate vine weevils. They’re underhand and sneaky. My experiences with them are worst in my Mam’s back garden, which is full of container-grown plants – the perfect place for adults to lay their eggs (they like non-gritty compost).

The adults are bad enough, but it’s the grubs that really get me. Creamy, fat, C-shaped legless grubs have light brown heads and are up to 10mm (about 3/8in) long.

Vine weevil
Vine weevil larvae. Picture; RHS

During autumn and winter, they feed on plant roots, causing wilting, and often plant death.

In winter 2015, a Heuchera Silver Blush came away in my hand as I was weeding round it – roots severed, with grubs underneath.

Plants grown in open ground are less susceptible, although grubs are partial to strawberries, Primula, Polyanthus, Sedum, Heuchera and young yew.

Adult beetles feed on the foliage of herbaceous plants and shrubs, especially Rhododendron, evergreen Euonymus, Hydrangea, Epimedium, Bergenia, Primula and strawberries, causing irregular notches on leaves.

Vine weevils
Heuchera completely severed at the roots by vine weevil grubs

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 hard to see.

Larger yellowish-brown spherical objects seen in potting composts are likely to be fertiliser pellets – similar whitish objects are usually gastropod eggs.


WHAT’S THE DAMAGE?

Vine weevil
Plantlets rescued from damage by vine weevil grubs

Adult weevils cause irregular-shaped notches in leaf margins during summer.

Grubs are found among the roots. Plants wilt and die during autumn to spring as they eat roots.

My Heucheras have been hammered in the past- this one (Ebony – or it could be Obsidian – still has the grub in the centre.

Nothing for it but splitting it into plantlets and growing it on. They did survive despite having hardly any roots left and I ended up with six bonus plants.


TOP 10 ORGANIC CONTROLS

1. On mild spring or summer evenings, 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.

Vine weevil
Tell-tale notched leaves on this Shasta daisy point to adult vine weevil damage

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 (5-20ºC/41-68ºF).

10. Another nematode, Heterorhabditis megidis, is also available but is more temperature-dependent (12-20ºC/ 54-68ºF). 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.