Royal Horticultural Society - Research: Horticultural Themes

Introduction

Vine weevil ( Otiorhynchus sulcatus ) is often the number one insect pest in terms of enquiries sent by members to the RHS Advisory Service at Wisley Garden.

Vine weevil - photgraph copyright RHS The adult beetles have dull black, pear-shaped bodies 8-10mm long. They are active after dark between the spring and autumn, eating irregular notches in the leaf margins of many plants, especially rhododendron and evergreen euonymus. Although unsightly, this damage is rarely sufficient to affect plant growth and it is the larval stage which is the serious pest.

All vine weevils are female and each can lay up to 1,000 eggs during the summer. The larvae are stout, creamy white, legless grubs with brown heads; their bodies are slightly curved and up to 10mm long. They feed on the roots of many plants and the tubers of plants such as cyclamen and begonia. Plants growing in pots, window boxes or other containers are particularly at risk and may be killed during autumn-spring.

Most of the insecticides available to amateur gardeners give poor results as they have insufficient persistence to deal with the prolonged egg hatching and larval development period between early summer and the following spring. The adult beetles and older grubs are also tolerant of pesticides. A new insecticide, imidacloprid, can give prolonged protection to ornamental container-grown plants. It is available as a slow-release formulation premixed with a peat compost. This is sold as Levington Plant Protection Compost and can protect plants for up to 12 months. It is best used for potting up seedlings, plug plants, cuttings or tubers. Established plants would need the old compost washed from their roots before potting up in the insecticide-containing compost. An alternative form of the insecticide which can be watered into the compost of established container-grown plants is Bio Provado Vine Weevil Killer. This formulation should give protection for up to six months.

A non-chemical treatment for vine weevil grubs is the pathogenic nematode, Heterorhabditis megidis . This microscopic worm-like animal is watered into the potting compost. The nematodes enter vine weevil grubs and infect them with a fatal bacterial disease. The nematode requires a moist open compost, such as peat or coir, in the temperature range of 14-20°C. The best time for treatment is August-early September, when the compost will be at the right temperature and before the grubs have grown large enough to cause serious damage. The nematode can be obtained from the mail order firms listed below, most of which also supply additional biological controls for other pests, including glasshouse whitefly, red spider mite, slugs, caterpillars, glasshouse aphids and mealybugs.

Vine weevil

Introduction

Further reading