Tony Dickerson answers your questions
Q
My containers have vine weevil in them. How do I get rid of them?
Kim Nolan, U oxeter, Staffordshire A
Vine weevil is one of the most widespread, common and devastating garden pests. The adult weevils eat plant leaves during spring and summer, but it’s the grubs that cause the most damage over autumn and winter when they feed on plant roots. Plants growing in pots and containers, outdoors or under cover, are most likely to be severely damaged by vine weevil grubs. The plump, C-shaped, white, legless grubs have light brown heads and are up to 10mm (about ⅜in) long. They’re likely to be found among the roots. Plants wilt and die during autumn to spring as a result of grubs devouring the roots. There’s a biological control for the grubs in the form of a microscopic pathogenic nematode ( Steinernema kraussei) which is widely available from suppliers of biological controls, such as Simply Seed (www.simplyseed.co.uk; tel: 0115 727 0606). It’s suitable for use in containers and in the open ground. Other nematode species are also effective, but Steinernema kraussei is the most widely available. Apply in August or early September when the soil temperature of potting media is warm enough for the nematode to be effective (5-20C/41-68F) and before the vine weevil grubs have grown big enough to cause serious damage.
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.
Alternatively, ornamental plants grown in containers can be treated with acetamiprid (such as Bug Clear Ultra Vine Weevil Killer) as a liquid drench applied to the compost.
This insecticide gives protection against the grubs for up to 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 can’t be used to treat edible or ornamental plants growing in the ground.