Pests & diseases
Vine weevil
A beetle whose adults notch leaf edges and whose grubs eat roots, especially of plants grown in pots like strawberries.
Vine weevil is one of the most damaging pests of container-grown plants in the UK, and a common cause of a potted plant suddenly collapsing for no obvious reason. It is a small, dull-black beetle about 9mm long, with a short snout and a slightly pear-shaped, knobbly back. The adults do cosmetic damage, but it is the grubs hidden in the compost that do the real harm.
The two stages of damage
Adult vine weevils are active from spring through to autumn, mostly at night. They chew distinctive notches out of the edges of leaves — neat little semicircular bites that look as if someone has been at them with a hole punch. This nibbling is unsightly but rarely fatal; it is mainly a warning sign that adults are about and laying eggs.
The serious damage comes from the grubs (larvae): plump, creamy-white, legless, C-shaped maggots with a pale brown head, up to about 1cm long. They live in the compost and eat roots, working their way up to gnaw the base of the stem. A plant can look perfectly healthy one week and then wilt and keel over the next, because by the time it shows above ground the root system has already been eaten away.
Why pots are the worst
Vine weevil can attack plants in open ground, but it is far more destructive in containers. A pot is a small, enclosed world: a few grubs can devour most of the roots in it, whereas a plant in a border has more root to spare and more natural predators to keep numbers down. Compost is also a tidy, undisturbed nursery for the eggs. Container favourites such as strawberries, primulas, heucheras, fuchsias and cyclamen are regular victims — anyone growing strawberries in pots or grow bags should keep a close eye out.
How to control it
There is no need for harsh chemicals on edible crops. The most reliable home remedy is a biological one: nematodes. These are microscopic worms (Steinernema kraussei is the type sold for this job) that you mix with water and drench into the compost, where they seek out the grubs and kill them. They are watered on rather than sprayed, are harmless to people, pets, bees and earthworms, and are sold by garden centres and online suppliers ready to use.
Timing matters: apply nematodes when the soil is warm and moist — late summer to early autumn is the prime window in the UK, catching the young grubs before they have eaten much, with a second dose possible in spring. Beyond that, pick off and squash any adults you find on warm evenings, tip out and check the compost of any plant that wilts unexpectedly, and stand vulnerable pots on hard surfaces rather than soil. Vine weevil grubs are very different from sap-sucking pests like aphids, so the cure is different too — the answer is in the compost, not on the leaves.
In a UK garden
Vine weevil is widespread across UK gardens and is at its most damaging in containers, where the root-eating grubs do their worst over autumn and winter and finish off potted plants by spring.
Example
Neat semicircular notches bitten out of the edges of strawberry or primula leaves over summer are the adult's calling card; tip the pot out in autumn and you may find creamy C-shaped grubs among the bare roots.