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Vine Weevil: Your questions answered

Nurtured in Norfolk’s gardening expert Martyn Davey answers all your questions.
If you would like any horticulture query answered please do e-mail our head grower at 
martyn@nurturedinnorfolk.co.uk to add your thoughts to our horticulture hacks.

Dear Martyn, 

I need your help urgently I have been moving some of the plants in my greenhouse getting ready for spring and have found what I think are vine weevil maggots. They have been eating the roots of my hollyhocks and fuchsias, I am concerned as to what else they may be living in that I have not noticed yet. What can I do to control them and how can I get rid of them quickly. I don’t really want to use strong chemicals but I don’t want to lose my plants either. Please advise?

J. Easton

Dear Reader,

Vine weevil is an insect that can feed on a wide range of ornamental plants and fruits, especially those grown in containers. Adult vine weevils eat leaves and the grubs eat roots.

It is one of the most widespread, common and devastating garden problems. The adult weevils eat leaves during spring and summer, but it is the grubs that can cause the most damage over autumn and winter when they feed on plant roots. This damage can result in wilting and plant death.

Plants growing in pots and containers, outdoors or under cover, are most likely to be severely damaged by vine weevil grubs. Plants growing in the open ground are less likely to be damaged, although heavy infestations of grubs can occur on strawberries, Primula, polyanthus, Sedum, Heuchera and young yew plants.

Vine weevil is a widespread insect in Britain. Gardeners with vine weevil should keep up their guard because stopping control measures after the apparent disappearance of the weevil can allow numbers to build up again.

On mild spring or summer evenings inspect plants and walls by torchlight and pick off the adult weevils. Shake shrubs over an upturned umbrella, newspaper or similar to dislodge and collect more. In greenhouses, look under pots or on the underside of staging benches where the beetles hide during the day.

Trap adults with sticky barriers placed around pots or on greenhouse staging. Encourage natural enemies. Vine weevils and their grubs are eaten by a variety of predators such as birds, frogs, toads, shrews, hedgehogs and predatory ground and rove beetles. Remove as many larvae as possible from compost infested with grubs.

A biological control for the larvae is available as a microscopic insect pathogenic nematode (Steinernema kraussei) this is available from suppliers of biological controls and a sachet formulation is available from some retail outlets, marketed by Neudorff. This is suitable for use in containers and in the open ground. For best results apply in August or early September when the soil temperature or potting media is warm enough for the nematode to be effective (5-20ºC) and before the vine weevil grubs have grown large enough to cause serious damage.

The nematode, Heterorhabditis megidis, is also available for larvae but is more temperature-dependent (12-20ºC)

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

A trap containing nematodes (Steinernema carpocapsae), is available for controlling adult vine weevil. The traps should be placed on the ground below plants damaged by the weevils during the summer. The adults enter the trap during the day and are infected by the nematodes.

Ornamental plants grown in containers can be treated with the systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra Vine Weevil Killer) as a liquid drench applied to the compost. This insecticide gives protection against the grubs for up four months; treatment in mid- to late summer will control the young larvae and prevent damage occurring later in the autumn to spring period. This product cannot be used to treat edible plants or ornamental plants growing in the ground.

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Martyn Davey – Head Grower 

EDP Gardening Expert Columnist 

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