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Vine weevil (Otiorhynchus sulcatus)

Plants growing in pots or other containers, in the open or under cover, can be severely affected by vine weevil grubs. In the open ground damage is less likely although the grubs sometimes cause damage, particularly to 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 during the summer. Plants wilt and die for no apparent reason during autumn to spring as a result of grubs devouring the roots. Adult weevils may be seen on the foliage at night or grubs found in the potting compost or soil.

Cause

Adult Black Vine Weevil are responsible for the leaf damage which can be unsightly but rarely affects the plant's growth. The adults are 8 to 12 mm long, dull black beetles with a pear-shaped body when viewed from above. Adult females emerge from the soil or compost in late spring as temperatures rise, feeding only at night and resting by day. They are slow moving insects that cannot fly but they are very persistent crawlers with modified legs which enable them to grip sheer surfaces such as glass and climb to feed. The adults are all female, reproducing parthenogenically.

The vine weevil starts to lay between 500 and 1600 eggs over a two to three month period one to two months after emergence. The eggs are very small, less than 1 mm in diameter, and are typically laid on the compost or soil surface, very close to the stem of a choice plant.

Far more serious is the damage caused by the soil-dwelling larvae which are plump, creamy white, legless grubs up to 10mm long with pale brown heads and typically adopt a 'C' shape. These feed on roots and also bore into tubers and succulent stem bases, devastating herbaceous pot plants. They also kill woody plants by gnawing away the outer tissues of the larger roots and stem bases. The final two stages are immobile.

Black vine weevil over winter as larvae, pupating for three to four weeks as temperatures rise in the spring. They pupate about 20 cm below the surface of the soil in an earthen cell formed by the larvae.

Non-chemical control

On mild spring or summer evenings inspect plants and walls by torchlight and pick off the weevils. Shake shrubs over an upturned umbrella or newspaper to dislodge and collect more. In glasshouses, look under pots or on the underside of staging where the beetles hide during the day. Squash and dispose of adults regularly. Place a damp piece of sacking on a path to provide a daytime resting place. Lift the sacking during the day and dispose of adults. With containers, don't keep the compost from year to year. Complete replacement every year can significantly reduce the level of vine weevil problems in the following season.

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.

Biological control of the larvae is available as a microscopic pathogenic nematode (Heterorhabditis megidis) available by mail order (see suppliers of biological control).

Apply in August or early September when the soil is warm enough for the nematode to be effective (14-20oC/57-68oF) and before the vine weevil grubs have grown large enough to cause damage. The nematode can also be applied on garden soil but gives poor results in dry or heavy soils. It works best in open potting composts such as peat or coir.

Chemical control

Easy Control Using insecticides for ornemental plants - not for use on edible crops Until the launch last year of Levington Plant Protection Composts, there was no insecticide available to amateurs that would protect plants from root damage by the larvae of Black Vine Weevil. This unique compost is the new 'anti vine weevil' compost which has been treated at manufacture with the insecticide Intercept (imidacloprid). This insecticide works by systemic action to give control of the soil dwelling insect pests, vine weevil and sciarid fly for up to one year and is taken up in the plant to control sap-sucking foliar pests greenfly and blackfly for up to 3 months. It has also been shown to give useful control of whitefly.

Pot on seedlings and baby plants in Levington Plant Protection compost. As the plants get bigger repot into more Plant Protection Compost. Make sure only Levington Plant Protection Compost is used in the container because the Intercept insecticide cannot 'flow' from treated compost into untreated root ball. Any larvae in, for example, the root ball of a plant being potted on will not be controlled and may still kill the plant by eating all the roots. With plants already infested, the best advice is to wash the roots clean of old compost before repotting into Levington Plant Protection Compost.

Both products are licensed for use on ornamental plants in containers only. Armillatox is approved for use against the egg stage in the life cycle. It needs to be sprayed over the soil surface at 14-day interval between March and October; few eggs are likely to be laid at other times of the year.

There are three varieties of Plant Protection Compost: - Outdoor Containers, Houseplant Containers and Ericaceous Containers.

Always use fresh Levington Plant Protection Compost. Use the compost as soon as possible and don't buy more than you require at any given time. This is because the efficacy of the compost will gradually decline if stored unused for several months. Best results are achieved with fresh compost. It is for this reason that manufacturers Scotts have introduced a 'use by' date on the bags, and undertake not to stockpile the compost at their factory. Once you have opened the bag, use within four weeks.

Do not dilute the compost. It has been pre-dosed with exactly the right amount of insecticide to control the pests. Adding bulky materials to it will reduce the concentration of Intercept in the growing medium, jeopardising its ability to work.

A better option for established pot plants is to apply imidacloprid as a liquid drench (Bio Provado Vine Weevil Killer). This formulation gives protection against the grubs for up to six months but treatment in mid to late summer will control the young larvae and prevent damage occurring later in the autumn to spring period.

Both products are licensed for use on ornamental plants in containers only. Armillatox is approved for use against the egg stage in the life cycle. It needs to be sprayed over the soil surface at 14-day interval between March and October; few eggs are likely to be laid at other times of the year.

Warning:

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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