Vine weevil

Two problems wearing one name

The adult and the grub do such different things that most people never connect them, and they search for them separately.

Above ground, something is cutting neat semicircular notches out of the edges of rhododendron, camellia, euonymus, bergenia and primula leaves from late spring onwards. It looks deliberate, almost stamped, and it is disfiguring rather than harmful. The plant carries on.

Below ground, a container plant that was fine in October is dead in March. It lifts out of the pot with no resistance because it has no roots left. Heuchera, primula, cyclamen, begonia, fuchsia and strawberry are the usual casualties.

Same insect. The notches are the adult feeding, and they are the warning that eggs are going into the compost.

What to look for

The adult is about nine millimetres long, dull black with a scatter of dull gold flecks, pear-shaped, with a short broad snout and antennae elbowed halfway along. Its wing cases are fused, so it cannot fly. Everywhere it goes, it walks.

It feeds at night and hides by day under pot rims, in leaf litter, behind labels and in the crown of a plant. Almost nobody sees one until they go out with a torch, at which point a garden that appeared to have none turns out to have plenty.

The grub is plump, creamy white, legless and curled into a C, up to about ten millimetres, with a hard orange-brown head. It is unmistakable. Tip out a pot of a plant that has collapsed and they are against the inside of the pot and around the root ball.

Almost every vine weevil in Britain is female and lays without mating, which means one insect arriving in a pot of compost is enough to start a population. Each lays several hundred eggs across a summer.

The calendar that decides what you do

Adults emerge from around May and feed for two or three weeks before they start laying. Egg-laying then runs through summer into early autumn.

Eggs hatch within a few weeks, and the young grubs eat fine roots. Through autumn they grow, and by winter they are large enough to strip the bark off the stem base as well, which is the point at which the plant is finished. They pupate in spring and emerge as the next generation of adults.

Two windows matter. Torchlight collecting between May and August removes females before they lay. Nematodes in late August and September kill the grubs while they are small and the compost is still warm enough for the treatment to work. A nematode drench applied in January to cold compost, when the grubs are large and the damage is done, is the commonest way this control gets a poor reputation.

What to do about them

Buy carefully. Slide the root ball out of any container plant before it comes home and look at the compost. That single habit keeps this out of a garden better than anything you can do once it is in.

Grow the vulnerable plants in the ground where you can. The same heuchera that dies in a pot survives in a border, because the root system is bigger than the grubs can get through and there are predators in the soil.

Between May and August, take a torch out after dark. They climb pot sides and stems to feed and they drop and play dead when you touch them, so hold a sheet of paper underneath and knock the plant. Every female you collect is several hundred eggs that will not be laid.

In late August or September, water nematodes into every susceptible container. Use Heterorhabditis while the compost is still above 12 degrees C, and switch to Steinernema kraussei once it cools, since that one works down to about 5 degrees. Keep the compost moist for a fortnight afterwards so they can move through it, and treat again in spring.

When a plant does collapse, tip it out, pick out every grub, wash the roots, and repot into fresh compost. The plant will often recover if any root remains, and the old compost should go in the bin rather than onto a border.

What it gets mistaken for

Caterpillars and slugs both make holes in the middle of a leaf. Vine weevil only ever works in from the margin, taking neat scallops out of the edge, and it never leaves a slime trail or a pellet of frass.

Root rot produces the same sudden collapse of a container plant. Tip the pot out and the difference settles it. Rot leaves black, soft, foul-smelling roots and no animals. Vine weevil leaves clean compost, chewed-off roots and grubs.

Chafer grubs and leatherjackets in a lawn get called vine weevil regularly. A chafer grub is much larger, has three pairs of obvious legs and a fat grey rear end. A vine weevil grub has no legs at all, which settles it in a second.

Leaf-cutter bees take smooth semicircles out of rose and lilac leaves, and the edges are cut cleanly as though with scissors rather than chewed. They are harmless, they are pollinators, and they should be left entirely alone.