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Guide to Vine Weevil (Otiorhynchus sulcataus) and how to get rid of them

Recently, there seems to be a lot of people reacting to the apparent upsurge in the numbers of Vine Weevil grubs being found in all sorts of areas, particularly in pots and containers of their Hostas.

We will try and set out a platform for people to understand and use, to help reduce or eradicate the instance of Vine Weevil in their garden. Firstly we need to understand what we are dealing with, and when & how to treat.

Vine weevil is an insect that can feed on a wide range of ornamental plants, including Hostas, as well as some fruits, especially those grown in containers. In simple terms, adult vine weevils eat leaves, and the grubs eat roots.


Graphic of Life Cycle of Vine Weevil
Life Cycle of Vine Weevil

Biology


Adult vine weevils cause notch like damage to the foliage. However this rarely affects plant growth, but is not pretty to look at. The adults are 9mm (about 5/16in) long, dull black beetles with a pear-shaped body when viewed from above. Adult weevils may be seen on the foliage at night; during the day they hide in dark places. They are slow-moving insects, but they are excellent crawlers and climbers. They can’t fly.

A much more serious problem is that of the larvae, or grub. This is soil dwelling, plump, legless, white with a brown head. Quite small, only ½” long. They will eat the delicate roots of your Hostas, reducing their performance and sometimes killing the whole plant.

It’s not just your Hostas, but they can kill woody plants by gnawing away the outer tissues of the larger roots and stem bases. Most plant losses occur during September to March, when the grubs are becoming fully grown.

All these particular types of vine weevil adults are female, and each can lay several hundred eggs during spring and summer, reproducing asexually. The eggs are brown and less than about 1/16in in diameter, making them virtually impossible to see in soil or compost.

Vine weevil can feed on a wide range of plants, both indoors and outdoors, but can be especially damaging to plants grown in containers. It is a very widespread and common insect. The adult weevils eat leaves during spring and summer, but it is the grubs that can cause the most damage over autumn and winter when they feed on plant roots. This damage can result in wilting and plant death.

Plants growing in pots and containers, outdoors or under cover, are most likely to be severely damaged by vine weevil grubs. Plants growing in the open ground are less likely to be damaged, although heavy infestations of grubs can occur on strawberriesPrimula, polyanthus, Sedum, Heuchera and young yew plants.

 

Identify


Vine Weevil Leaf damage

Adult Vine Weevil

Adult weevils are approximately 9mm (about 5/16in) long and dull black with dirty yellow mark on the wing cases. They cause irregular-shaped notches of leaf margins during the summer.


Vine Weevil Larva

You are much more likely to come across the grubs. Usually when changing compost, or re-pottting. The plump c-shaped white legless grubs have light brown heads and are up to ½” long. They are likely to be found among the roots, particularly of plants in containers. Plants can wilt and die during autumn to spring as a result of grubs devouring the roots.


Management


Vine weevil is a very widespread insect in Britain. Gardeners with vine weevil should keep up their guard because stopping control measures after the apparent disappearance of the weevil can allow numbers to build up again.

Check susceptible plants frequently so action can be taken before a damaging population has developed. Look for notched leaves particularly on thick leaved evergreen plants such as Rhododendron & Laurel, & even on your Hostas, as a sign adult are present. 

When choosing control options, you can minimise harm to non-target animals by using the methods in the non-pesticide section below. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.


Non-Pesticide


A bit like looking for our old friends slugs & snails, we need to go out, on a mild evening preferably, and pick off the slow moving adults. (Oh joy!)

In greenhouses, look under pots or on the underside of staging benches where the beetles hide during the day.

Trap adults with sticky barriers placed around pots or on greenhouse staging. Many commercial barriers are available on the internet

Encourage wildlife in the garden. Vine weevils and their grubs are eaten by a variety of predators such as birdsfrogs, toads, shrews, hedgehogs and predatory ground and drove beetles

Remove as many larvae as possible from compost. Use a very fine sieve to separate the compost from the grub.

 


Vine Weevil Nematodes

 

Biological Control


Control of vine weevil biologically can be by applying the correct live microscopic nematodes at the correct time, at the correct amounts.

They are available from suppliers of biological controls and as sachet formulations available from some retail outlets. They are suitable for use in containers and in the open ground. Nematodes have the potential to infect other animals. They should therefore be used with care and only when there is a specific problem to treat. For best results apply in August or early September when the soil temperature or potting media is warm enough for the nematodes to be 12-20ºC before the vine weevil grubs have grown large enough to cause serious damage. For a spring application, the soil temperature should be at least a minimum of 5C.

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 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. Remove them in the evening and replace.


Pesticides 


Rewela Cottage does not recommend that you use pesticides. Most pesticides (including organic types) reduce biodiversity, including natural enemies, impact soil health and have wider adverse environmental effects. Where vine weevil is a real problem, start with the natural control before moving on.

Pesticide treatments are likely to kill natural enemies and so reduce the likelihood of natural control.  If you do decide to use pesticides, the shorter persistence products (that are usually certified for organic growing) are likely to be less damaging to non-target wildlife than those with longer persistence and/or systemic action. Be aware that products such as Neem oil are not registered for use in the UK and we cannot advise on their use.

Plants in flower must not be sprayed due to the danger to bees and other pollinating insects. 

A liquid drench can be applied to the compost of most ornamental plants grown in containers. Treated with the systemic neonicotinoid insecticide acetamiprid (e.g. Bug Clear Ultra Vine Weevil Killer. This insecticide gives protection against the grubs for up 4 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 cannot be used to treat edible plants or ornamental plants growing in the ground. 

Systemic pesticides are widely considered to be the most environmentally damaging, remain active for a long time and will kill beneficial invertebrates


A note to the wise…


Think carefully before buying Nematodes.

Nematodes are live and only have a short shelf life before applying. 2 weeks in a fridge. About 6-8 weeks once applied.

Do I need to order several batches so I can apply again? Probably ill-advised.

Ensure conditions are right for application. The compost/soil needs to be warm (5C). It needs to be moist and kept moist. If the compost dries out, the nematodes will die before they have done their job. Equally, don’t let the compost get too wet, or they will simply rot away.


Hose End Feeders

Application


Simply mix the harmless powder with water & apply to the soil around the plant using a watering can fitted with a coarse rose or use an applicator such as a hose-end feeder for larger areas.

Apply to the root ball of each plant or soak pots to make sure the nematodes come into contact with any grubs present.

One of the most popular vine weevil treatments is Nemasys. There are others also on the market. Whichever one you use, follow the instructions carefully. Remember, this is a perishable product.

You may not eradicate them first time round, but keep trying. As soon as you think they've gone, they are simply multiplying ready for a new invasion.



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John Plant

Rewela Hostas

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