Q What are vine weevils?
A Vine weevils (Otiorhyncus sulcatus) are brownish-black insects, around 1cm long, with jointed antennae and pale-brown flecks on their backs. Like aphids, the females do not need males to reproduce, so it only takes one weevil to start an infestation. The larvae are plump, creamy-white and maggot-like, up to 9mm long. They are usually seen curled in a C-shape, with brown heads and no legs. There are other similar weevils that do much the same sort of damage. Clay-coloured weevils (O. singularis) eat leaves, and their maggots gnaw the roots and bark of woody plants. They are hard to distinguish from vine weevils, but this does not matter much as they are dealt with in the same way.
Caption: Vine-weevil grubs eat the roots of plants
Q Why have vine weevils become such a problem lately?
A Vine weevils have always been a serious pest of greenhouse plants, including grape vines, but commercial growers were able to keep them under control using Aldrin, a soil insecticide. However, this pesticide was banned in 1989, leaving growers with no alternative means of controlling vine weevils effectively. As a result, the pest got out of control in nurseries; and container-grown plants supplied to garden centres spread the problem to gardens nationwide. A different insecticide, SuSCon Green, is now available to commercial growers and should ensure that any new plants you buy are free from the pest.
Q What damage do vine weevils do?
A Adult weevils feed on leaves, chewing irregular notches in the margins. Their mouthparts are incapable of making holes in the middle of the leaves. (This sort of damage is likely to be caused by caterpillars or slugs.) Vine weevils are especially fond of camellias, euonymus and rhododendrons, though they will also feed on other shrubs and tough herbaceous plants like bergenias. The grubs feed on the roots of a wide range of plants, both indoors and out. However, their particular favourites include the following: begonias, bergenias, cyclamen, fuchsias, heuchera, polyanthus, primulas, sedums, succulents and saxifrages. They are also partial to strawberries.
Q Are the vine-weevil grubs more damaging than the adults?
A Yes. The first you’ll probably know of an attack is when a plant wilts for no apparent reason and does not respond to watering. Closer inspection will show that the plant has become completely severed from its roots. Plants are easily pulled up and come away without their roots attached. Plants in containers are especially prone to attack, though gardens can also become infested. The damage caused by the adults is unsightly but not serious.
Q Can you explain the life cycle of vine weevils in more detail?
A The adults are active between the end of June and October. In daylight you might spot the odd one walking across the patio or on the wall of the house. If you go out at night with a torch there’s a good chance you will see them feeding on the foliage.
The female weevils lay their eggs in the compost or soil among host plants. As a rule, this happens in August and September, though a lot depends on the weather during the previous winter and spring. In a mild year, egg-laying could start in early summer. Each female is capable of laying 1000 or more eggs. These eggs are less than 1mm across, white at first, later turning brown. They are difficult to see and much smaller than the granules of slow-release fertiliser found in container-grown plants, which are often mistaken for insect eggs.
Vine-weevil grubs usually hatch after two weeks and immediately start to feed on the roots of their host. Fully-grown grubs burrow deeper into the soil to pupate, usually in late spring.
Most adults do not emerge until the following June or July, when the whole cycle starts over again.
Q Is the vine-weevil life cycle different indoors?
A In a heated greenhouse, vine weevils can feed and breed throughout the year, so all stages of the life cycle can be found.
Q What can I do to protect my plants from vine weevils?
A There are a range of options. The most direct is to keep an eye open for the adults and destroy any that you find. As the adult weevils tend to feed during the night, it’s worth going out with a torch. If only a few pots are affected, try sifting the grubs out by hand. Squash any you find before repotting the plants in fresh compost and putting the rest in the bin.
Q What about barriers for vine weevil?
A Vine weevils cannot fly so barriers are useful. Fit parcel tape smeared with non-drying insect barrier glue around tubs or pots during the egg-laying season (August and September outdoors, or all year in a greenhouse) to trap the adult females before they lay their eggs. For the glue to be effective you must keep it free of debris. Anything stuck to the glue, such as a leaf or dirt, will act as a bridge. Insect barrier glue is available from some garden centres or by mail order.
Q Are there chemical controls for vine weevil?
A Yes, when Which? Gardening trialled controls we had most success with Scotts Bug Clear Ultra Vine Weevil Killer. It can only be used on pot-grown plants and can't be used on edible plants.
Q What about biological controls for vine weevil?
A There is an effective biological control that is made up of millions of nematodes (microscopic worms). They kill the vine weevil grubs by penetrating their bodies and releasing lethal bacteria. They are supplied in a concentrated form and have to be mixed with water to be activated. The soil or compost also needs to be kept moist for a fortnight after applying the nematodes.
Q Are there any restrictions on using the nematodes for vine weevil?
A Nematodes are best used straight away, but they can be stored in the fridge and used within 12 days. The main downside with using nematodes to control vine-weevil larvae outdoors, or in an unheated greenhouse, is that they are very sensitive to temperature.
This control works best if the soil remains above 12°C for at least 14 days after use. This makes August and September the most effective application period, though you can apply in April or May before the adult weevils emerge from their pupae if it is warm enough. By October it’s generally too cold to deal with them outdoors.
Suppliers of biological controls for vine weevil
Biowise 01798 867574 biowise-biocontrol.co.uk
Defenders 01233 813121 defenders.co.uk
Green Gardener 01603 715096 greengardener.co.uk
Harrod Horticultural 0845 402 5300 harrodhorticultural.com
Just Green 01621 785088 just-green.com
The Organic Gardening Catalogue 0845 130 1304 organiccatalogue.com
Scarletts 01206 240466 scarletts.co.uk