Vine weevil is not usually a pest of short-term potted herbs, but can be a serious pest on those kept for longer periods e.g. stock plants. Herbs are more at risk if grown on nurseries where alpines, hardy nursery stock or other susceptible ornamental crops such as cyclamen, fuchsia and primula are also grown.
The adult is a large (about 9 mm long), wingless black weevil with a rough body surface and yellow speckling on its back (Fig. 1). Vine weevil adults are all female and produce fertile eggs without the need for mating. The white, spherical eggs, about 0.7 mm diameter, are laid in the growing medium, turn brown after a few days and hatch into larvae after 2-3 weeks. The plump, legless larvae are white or cream, up to 10 mm long and have a tan-coloured head capsule (Fig. 2). They tend to lie in a C-shape and can be found in the compost or soil around the finer roots or chewing their way into the larger roots or stem base of the host plant. Pupae are white, up to 10 mm long, with the developing legs and antennae clearly visible externally.
Adult weevils chew characteristic notches around the leaf edges (Fig. 1). The larvae feed on roots and stem bases, leading to stunted plant growth, leaf yellowing, wilting and even death.
Sources of infestation and favourable conditions
Infestations of vine weevils may develop from a few adults that have survived from previous infested crops. Perhaps the commonest original source is infested potted material brought onto the nursery from elsewhere. The adults have no wings so they cannot fly and are unable to walk long distances to a nursery. The weevils are nocturnal and shelter during the day in plant debris or under pots and trays, often remaining undetected by growers. At night, however, they are very active, walking over floors and climbing up benches. Adult weevils usually emerge from pupae in May and early June, and lay eggs between June and September. The larvae usually feed between July/August and the following April, but in protected structures or in warm springs and autumns, development can be quicker, leading to earlier adult emergence and extended egg-laying periods. This can result in all stages of the pest being found at one time.
Integrated Pest Management (protected herbs)
- Potted material that might be brought onto the nursery should be carefully examined for vine weevil larvae, and rejected if any are found.
- Strict nursery hygiene will restrict the sources of vine weevil, particularly on nurseries also growing susceptible ornamentals. Infested plants should be removed and disposed of promptly and carefully, and bench and floor coverings should be kept free from plant debris.
- If typical leaf notching is noted a nocturnal examination using a hand torch may confirm the presence of vine weevil adults. Some may then be picked off and destroyed.
Various species of insect-pathogenic nematodes are available for vine weevil control. These include Steinernema kraussei (Nemasys L, Exhibitline sk), Heterorhabditis megidis, and Heterorhabditis bacteriophora (e.g. Larvanem, Nemasys H & Nematop). S. kraussei is effective at a greater range of soil or compost temperatures (5-30°C) than other nematode species (10-25°C), depending on the product. Thus it can be applied at cooler temperatures, i.e. later in the autumn, or earlier in the spring, than other nematode species. Nematodes are best applied in August or September, against young larvae, but S. kraussei can be applied as late as November. They can also be applied in March/April to early May, before the larvae pupate and emerge as adults. The nematode suspension is applied as a compost or soil drench. The nematodes search the compost for a suitable host vine weevil larva and enter the body, where they release symbiotic bacteria that kill it. The dead vine weevil larva turns reddish-brown or pale brown before decomposing (Fig. 2). The nematodes reproduce in the decaying larva and then move off to infect other vine weevil larvae. Nematodes need to be applied very carefully, following all supplier recommendations, e.g. the suspension of nematodes should be kept agitated during application, and the compost kept moist so that the nematodes can survive and move within the compost.
Monitoring within IPM
- Check susceptible plants regularly for signs of adult leaf feeding or larval damage to roots.
- Container-grown strawberry or primula plants can be used as indicator plants for adult feeding activity, as they are favourite host plants.
- If using nematodes for vine weevil control, infected larvae around roots should be showing the characteristic discolouration 1-2 weeks after nematode application.
Chemical control (protected herbs)
There are no pesticides with approval or Extension of Authorisation for Minor Use (EAMU) for the control of vine weevil on protected herbs. Pesticides approved for compost incorporation for vine weevil control on ornamentals may not be used on herbs, even if sold to garden centres as ‘ornamental’ herbs. Some growers of ornamentals and soft fruit use night-time sprays of a pyrethroid pesticide to kill adult vine weevils, but these sprays are incompatible with IPM, as pyrethroids are harmful to biological control agents for up to three months after application.
- Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants. Also see Section A – Principles of IPM.
- Pesticide approval information in this guideline is current at 29 April 2013.
- Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/. General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
HDC Factsheet 02/03. Vine weevil control in hardy nursery stock.
HDC Factsheet 01/03. Vine weevil control in soft fruit crops.