Plants growing in pots or other containers, out of doors or under cover, can be severely affected by vine weevil grubs.
Plants growing in the open ground are less likely to be damaged, although the grubs sometimes kill strawberries, primulas, polyanthus, Sedum, Heuchera and young yew plants.
The adult beetles feed on the foliage of many herbaceous plants and shrubs, especially Rhododendron, evergreen Euonymus, Hydrangea, Epimedium, Bergenia, Primula and strawberry. Irregular-shaped notches are eaten in leaf margins by the adult weevils during the summer.
Plants wilt and die during autumn to spring as a result of grubs devouring the roots. The serious damage is caused by the soil-dwelling larvae which are plump, white, legless grubs up to 1cm (0.5in) long with pale brown heads. These feed on roots and also bore into tubers and succulent stem bases, devastating many herbaceous pot plants. They can also kill woody plants by gnawing away the outer tissues of the larger roots and stem bases.
There are two main methods for control: non-chemical control – on mild spring or summer evenings inspect plants and walls by torchlight and pick off the adult weevils. Shake shrubs over an upturned umbrella to dislodge and collect more.
In glasshouses, look under pots or on the underside of staging where the beetles hide during the day. Trap adults with sticky barriers, such as Agralan Insect Barrier Glue, around pots or glasshouse 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 beetles.
A biological control of the larvae is available as a microscopic pathogenic nematode (Steinernema kraussei) it is available from suppliers of biological controls. Apply in August or early September when the soil temperature is warm enough for the nematode to be effective (5-20oC/41-68oF) and before the vine weevil grubs have grown large enough to cause damage.
Chemical control Levington Container & Hanging Basket Compost has been premixed with slow-release granules of the pesticide imidacloprid and will control vine weevil larvae for up to 12 months. It is best used for potting up seedlings, plug plants, cuttings and tubers of cyclamen and begonia.
When repotting older plants, wash the old compost off the roots or it will provide a haven for the grubs. A better option for established pot plants is to apply a liquid drench (Provado Vine Weevil Killer 2). This insecticide gives protection against the grubs for up to four months, but treatment in mid- to late summer will control the young larvae and prevent damage occurring later in the autumn to spring period. Gardeners with vine weevil should keep up their guard because stopping treatment after the apparent disappearance of the pest can allow numbers to build up again.
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