What is vine weevil?

Vine weevil is a beetle that attacks a wide range of plants, both indoors and outdoors, but is especially damaging to plants grown in containers.

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.

The adult beetles feed on the foliage of many herbaceous plants and shrubs, especially Rhododendron, evergreen Euonymus, Hydrangea, Epimedium, Bergenia, Primula and strawberry. Adults rarely cause enough damage to affect the vigour of plants.

There are several other species of weevil closely related to vine weevil, the adults of which cause similar damage but are usually less problematic as larvae.

Symptoms

You may see the following symptoms:

• Adult weevils are approximately 9mm (about 5/16in) long and dull black with dirty yellow mark on the wing cases. They cause irregular-shaped notches of leaf margins during the summer
• The plump c-shaped white legless grubs have light brown heads and are up to 10mm (about 3/8in) long. They are likely to be found among the roots. Plants can wilt and die during autumn to spring as a result of grubs devouring the roots

Control

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.

Non-Pesticide 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, 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

Biological control

• 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/41-68ºF) 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/ 54-68ºF)
• 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

Pesticide control

• 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
• Inclusion of a pesticide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener

Downloads

Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)

Biological control suppliers (Adobe Acrobat pdf document)

Biology

Adult vine weevils cause notch like leaf damage, which can be unsightly but rarely affects plant growth. The adults are 9mm (about 5/16in) 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 excellent crawlers and climbers. There are several other related species found in Britain which can cause similar damage to foliage as adults. Some that have recently become established include Otiorhynchus armadillo, O. crataegi 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 (about 3/8in) long with pale brown heads. These eat the roots of a wide range of plants and can bore into tubers of cyclamen and begonia, and into stem bases of cacti and succulents. They can kill woody plants by gnawing away the outer tissues of the larger roots and stem bases.  Most plant losses occur during September to March, when the grubs are becoming fully grown.

All Otiorhynchus sulcatus adults are female and each can lay several hundred eggs during spring and summer. The eggs are brown and less than 1mm (about 1/16in) in diameter, making them very difficult to see in soil. Larger yellowish-brown spherical objects seen in potting composts are likely to be controlled-release fertiliser pellets added by the nursery that raised the plants.