Garden pest control - Vine Weevil (Description and control instructions)

Scientific name: Otiorhynchus sulcatus

Vine weevils have been increasing in significance to gardeners over the past few decades, due to the increased use of ornamental containers and container grown plants from nurseries. Recently a couple of species previously unrecorded in the UK have been spotted in the London area; they have probably arrived in plant pots and can overwinter in the milder environment of the city.

They are difficult to control once established because of their nocturnal behaviour, the subterranean habits of the larvae, and the lack of natural predators or parasites. Commercial growers used to use Aldrin to control the pest, but it was withdrawn in 1989.

Their presence is shown by semi-circular notches eaten into the edges of leaves, most notably on evergreen shrubs like Rhododendron, Euonymus, Azalea and Camellia. This damage by the adults is not fatal, just unsightly (although is a good indicator that there will be eggs, and larvae hatching in the autumn). The real damage is done by the larvae which feed on the roots, often killing the plant, especially potted and young plants. The first sign of the presence of the larvae is usually yellowing leaves, poor growth and a wilting plant which does not respond to watering. Unfortunately it is often too late to save the plant. Rescue is possible if the damage is not too extensive, wash off all the compost to remove the grubs and remaining eggs, then replant in fresh growing medium. Some plants are more at risk from attack than others, eg. Sedums, Primulas, Fuchsias and Impatiens, but most plants in pots are at risk.

The adults are all female, reproducing by parthogenesis (ie. they don't need a mate), and are flightless, but are very good walkers able to climb sheer surfaces. If one is spotted on a plant, arrange a tray or sheet directly below it as their favourite trick is to fall to the ground at the slightest disturbance and if this is not anticipated they are difficult to find.

They emerge from the pupa stage in late spring and after feeding on plant material for 21 to 45 days they are ready to lay eggs; between 500 to 1600 eggs over a one to two month period. The round eggs are about 0.8 to 1 mm across, laid in the soil close to a plant; white at first they become brown later (slow release fertilizer pellets are much larger and usually the yellow outer coating crushes easily, the eggs are relatively hard). They hatch 10 to 12 days later and the larvae burrow down to the fine roots. A prepupal stage develops in December and remains like this until late spring when it pupates fully for a few weeks before the adult emerges.

Conditions for eggs and larvae are optimal when soil moisture is moderate to high in July and August. Heavy mulches help to maintain moisture levels, so removal of excessive mulch layers and minimal watering of plants during this period is detrimental to their survival. (Unfortunately surface rooted shrubs such as Camellias need to be well watered at this time to set the flower buds, so this must not be missed.) Excessively damp soils in the autumn also force larvae to move up the base of the plant where girdling can occur, so good drainage around the plants will ease the problem.

Indoors, the warmer conditions mean that they can reproduce all year round so all stages of the life cycle can be present at all times. If the plants are in pots and on a stand place the legs in water as the adults do not swim so cannot climb up - only useful for rust resistant legs and not wood either.

A natural predator is the Centipede which eats both eggs and larvae.

Treatments

• Provado - the systemic insecticide, Imidacloprid applied to the growing medium as a drench, lasts for a few months and kills the larvae which do the damage. It cannot be used on edible or cropping plants such as strawberries.
  
  
• Levington Plant Protection Compost - contains Imidacloprid a systemic insecticide, and used as the growing medium gives protection for about six months. Follow the instructions carefully and buy only what is needed as it loses its potency if stored.
  
  
• Nematode - Steinernema kraussei - a microscopic eel worm which kills the larvae, is a recently discovered species that works down to 5°C so it can be applied over a longer period outdoors. It is best used between August and November when the weevil eggs are hatching and again from March to May when the soil is warm enough and the larvae are active. The protection lasts for about four weeks - trade name Nemasys Vine Weevil Killer or Grubsure.
  
  
• Nematode - Heterorhabditis megidis another species of eel worm which enters the grub and carries bacteria that kill it. The nematode reproduces using the dead grub as food. It is also temperature sensitive and works best at around 12°C. They die out if no grubs are present.
  
  
• Traps - corrigated cardboard has been used, made into a roll and left for the adults to hide in during the day. Moist sacking laid on a path provides a dark daytime hiding place for the adults, which can be collected during the day when they are relatively inactive. Night-time sorties with a torch might show results, but the adults are quite active and difficult to catch.
  
  
• Foliar insecticide - plants which are attacked by adults can be sprayed with insecticide starting in the late spring and repeated during the summer to kill them before the egg-laying period in late summer
  
  
• Sacrificial plants - primulas, polyanthus and cyclamen are some of the favoured plants for adult weevils to lay their eggs beside. Some of these potted in Levington Plant Protection Compost and left beside your treasured plants will attract the adults, which will lay their eggs in the compost and the larvae will be killed when they hatch and begin feeding. You could use Provado to treat the sacrificial plants instead.