Advice
RHS Help & Advice
Vine weevil (Otiorhynchus sulcatus)
Plants affected
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.
Symptoms
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.
Cause
Adult vine weevils are responsible for the leaf damage which can be unsightly but rarely affects the plant's growth. The adults are 9mm long, dull black beetles with a pear-shaped body when viewed from above. Adult weevils may be seen on the foliage at night; during the day they hide in dark places. They are slow moving insects that cannot fly but they are very persistent crawlers and climbers. Other related species that have recently become established in Britain, such as Otiorhynchus armadillo and O. salicicola, cause similar damage to foliage.
All Otiorhynchus sulcatus adults are female and each can lay many hundreds of eggs over a period of several months from April-September. The eggs are brown and less than 1mm in diameter, making them very difficult to see in the soil. Larger yellowish-brown spherical objects seen in potting compost are most likely to be controlled-release fertiliser pellets added by the the nursery that raised the plants. Other related species that have recently become established in Britain, such as Otiorhynchus crataegi, O. armadillo and O. salicicola, cause similar damage to foliage.
Far more serious is the damage caused by the soil-dwelling
larvae which are plump, white, legless grubs up to 10mm 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.
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) 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. Another nematode, Heterorhabditis megidis is also available but is more temperature-dependant (12-20ºC/ 54-68ºF). Both nematodes can also be applied to garden soil, but gives poor results in dry or heavy soils. They works best in open potting composts such as peat or coir. Nematodes can be used safely on all edible and ornamental plants.
Chemical control
Ornamental plants that are being grown in pots or other containers can be treated with acetamiprid (Scotts Bug Clear Ultra Vine Weevil Killer) or thiacloprid (Provado Vine Weevil Killer 2) as a liquid drench applied to the compost. These insecticides give protection against the grubs for up to two and four months respectively; treatment in mid- to late summer will control the young larvae and prevent damage occurring later in the autumn to spring period. Neither product can be used to treat edible plants or ornamental plants growing in open soil.
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.