Vine weevil is an insect pest that infests a wide range of ornamental plants and fruits, especially those grown in containers. Adult vine weevils eat leaves, and the grubs eat roots.
What is vine weevil?
Vine weevil is a beetle that attacks a wide range of plants, both indoors and outdoors, but especially plants grown in containers.
It is one of the most common and devastating garden pests. The adult weevils eat plant leaves during spring and summer, but it is the grubs that cause the most damage over autumn and winter when they feed on plant roots, causing wilting, and often plant death.
Plants growing in pots or other containers, outdoors or under cover, can be severely damaged by vine weevil grubs. Plants growing in the open ground are less susceptible, although the grubs can 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.
You may see the following symptoms:
- Adult weevils cause irregular-shaped notches in leaf margins during the summer
- The plump 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 wilt and die during autumn to spring as a result of grubs devouring the roots
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 greenhouses, look under pots or on the underside of staging benches where the beetles hide during the day.
Trap adults with sticky barriers, such as Agralan Insect Barrier Glue, 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 beetles.
A biological controls 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 serious damage.
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.
Ornamental plants grown in 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.
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Adult vine weevils are responsible for the leaf damage, which can be unsightly but rarely affects the plant's 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 very persistent crawlers and climbers. Other related species that have recently become established in Britain, such as 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 to10mm (about 3/8in) long with pale brown heads. These eat the roots and also bore into tubers of cyclamen and begonia, and into stem bases of cacti and succulents , devastating many pot plants. They can also 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 many hundreds of eggs over a period of several months from April to September. The eggs are brown and less than 1mm (about 1/16in) in diameter, making them very difficult to see in the 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.
Vine weevil pupae like these develop from plump, white legless grubs, which cause the most damage, devastating many herbaceous plants.