Now considered to be the most serious pest of ornamental plant production, black vine weevil (Otiorhynchus sulcatus) and other closely related weevil adults feed on susceptible plant foliage leaving notched edges while the larvae feed extensively on roots. Damage causes in excess of £30m in annual losses for this sector and that total will only increase with the restriction in useful control options.
Some of the key insecticides available against vine weevil are granule formulations, mixed into the compost before potting to prevent infestation by larvae. They can be used, bearing in mind the restrictions now in place, at any stage up to final potting (non-edible crops only) and there is no guaranteed total control if untreated liners are potted on into treated media because larvae can survive around the roots.
Parasitic nematodes can be drenched into affected soil/growing media, with some working effectively down to 5°C soil/growing media temperatures.
Otiorhynchus armadillo and Otiorhynchus salicicola were discovered in the UK in 2002 and are believed to have arrived on nursery stock from southern Europe. They are likely to be controlled by measures used against black vine weevil.
How to recognise it
Otiorhynchus sulcatus adults are approximately 1cm long with roughly textured black bodies and slightly speckled yellow, owing to tufts of yellow hairs. They have no wings and are nocturnal, hiding in crevices and leaf litter during the day and generally feeding at night.
Otiorhynchus armadillo and Otiorhynchus salicicola look similar but can be distinguished by a tooth-like protrusion at the top of their legs. Otiorhynchus salicicola is also larger, at 1.2cm long.
Larvae are white and legless with glossy orange/brown heads. Fully grown, they are 1-1.5cm long, plump, often curled into a C-shape and sometimes discoloured.
In northern Europe, adult black vine weevils occur only as females, so the species reproduces by unfertilised eggs. In unprotected crops, most adults emerge from pupae in May/June, depending on the season. About four weeks later, they start to lay eggs on the surface of the soil or growing media, close to the host plant, until September/October. Some estimates suggest they can lay as many as 1,500 eggs at a time, but not all are viable.
Larvae hatch after about two weeks and, when young, feed on peripheral roots of plants. In time, they can digest woodier tissue and will graze on the bark of woody stems, burrow into root systems and mine into fleshier parts, such as cyclamen corms, when they become more difficult to control.
Larvae overwinter in a dormant state, in cells in the soil or growing media, resuming feeding when temperatures rise in spring. They pupate in May and June, the adults emerging three or four weeks later. Some adults can also overwinter in glasshouse structures or debris and may start laying eggs earlier than newly emerged adults.
There is usually only one generation but in the warm conditions of interior landscapes and heated glasshouses, when the life cycle is speeded up, there may be several generations and all stages
may be present at the same time.
- Adult feeding leaves distinctive half-moon notching around leaf margins.
- Larval damage can show as stunted growth. Infestations can go unnoticed until leaves or whole plants wilt suddenly.
- Adult weevils may feed and lay eggs using different plant species so some plants displaying leaf notching will not necessarily have larvae too.
Treatment: biological control
Several products based on nematodes are available, applied as a suspension through conventional spray equipment, irrigation systems (maximum pressure 3 bar) or even as a drench with a watering can or through a Dosatron. The nematodes attack the larvae, releasing bacteria that kill the pest and break down its tissue for the nematodes to digest. A new generation of nematodes is produced inside the larva’s body that leave in search of more larvae.
Apply from August to November when larvae are young and from March to May before they pupate.
Nematodes can persist for four-to-six weeks in consistently moist soils and composts.
Larvae attacked by Heterorhabditis spp. nematodes change colour to red-brown. They require soil/compost temperatures above 12°C. Larvae attacked by Steinernema kraussei do not turn a different colour. S. kraussei requires soil/compost temperatures above 5°C.
Metarhizium anisopliae, an insect-killing fungus against vine weevil larvae and adults, is also available and can be compost-incorporated or applied around established plants.
Treatment: cultural control
- General hygiene, such as disposal of old stock, weed control and cleaning up plant debris.
- Check new arrivals of plants for signs of infestation or damage.
- Strategic placement of plants particularly attractive to adult weevils can help to monitor for their presence.
Treatment: chemical control
Active ingredient Metarhizium anisopliae strain F52
Formulation Met52 granular bioinsecticide* (Fargro)
Action(s) Effective on vine weevil pre- and post-incorporation into soils and growing media. Compatible with biological controls.
Active ingredient Thiacloprid
IRAC code 4A
Formulation Calypso* (Bayer), Exemptor* (Bayer)
Action(s) Granular formulation (Exemptor) for non-edible crops as well as a drench option (Calypso) available. Compatible with some biological controls.
Active ingredient Imidacloprid
IRAC code 4A
Formulations Couraze (Solufeed), Imidasect 5GR (Fargro)
Action(s) Granular formulation (growing media incorporation) for non-edible crops with additional label restrictions. Compatible with some biological controls.
Fully updated by Dove Associates
Use plant protection products safely. Always read the label and product information before use.
* Extension of Authorisation for Minor
Use (EAMU) required for use in ornamental plant production outdoors and/or under protection.
Dove Associates shall in no event be liable for the loss of or damage to any crops or biological control agents caused by the use of products mentioned.