Scarlet lily beetle

What you are looking at

A beetle the colour of a pillar box, around 8 mm long, with a black head, black legs and black clubbed antennae. It sits in plain view on the upper surface of a lily leaf and makes no attempt to hide, which is unusual enough to be diagnostic on its own.

Reach for it and it drops. On the way down it turns over, so it lands on its back with its black underside upwards, and against dark soil it disappears. Hold one and it squeaks, a thin creak made by rubbing two body segments together.

Turn the leaves over. Short rows of orange eggs along the veins are the next generation. Wet brown or black blobs stuck to the underside are larvae, each one an orange grub carrying its own droppings on its back.

The grubs do most of the harm

Adults chew holes through leaves, buds and open flowers, and a few beetles over a fortnight leave a plant looking pecked rather than ruined.

The larvae are the ones that take a lily down. They feed in groups, work from the top of the stem downwards, and can strip a plant to a bare green stick in under a week. Because they are on the undersides and dressed in excrement, a gardener looking at the plant from above sees leaves vanishing with nothing apparently eating them.

The excrement coat is a defence and it works. Birds that would take a naked orange grub off a leaf leave these alone, which is why no amount of feeding the garden birds reduces the population.

Two waves, March to September

Adults come out of the soil from late March and feed for a week or two before mating. A female then lays in batches on the leaf undersides through spring, several hundred eggs over her life. The larvae feed for two to three weeks, drop into the soil, and pupate there.

New adults emerge from midsummer, feed through late summer and autumn on whatever lily foliage is left, and then go back into the soil, leaf litter or the crown of a nearby plant for the winter. They can spend the winter well away from the lilies, so clearing the bed does not clear the problem.

The beetle established itself in Surrey around 1940 and stayed local for decades. It moved through most of England and Wales from the 1980s onwards and has been recorded in Scotland since. Gardeners who never saw it before 2000 now meet it every April.

What to do about it

Check the plants every few days from the moment shoots appear. Fifteen minutes a week from late March, spent turning leaves over, holds a bed of lilies without anything else.

Cup a hand under the beetle before you touch it, or hold a jar of soapy water beneath and knock it in. Wipe the egg rows off the leaf undersides with a thumb. Take the larvae off with a gloved hand or a damp cloth.

Keep going after the flowers are over. The midsummer generation is the one that overwinters, and every adult removed in August is a plant not attacked next April.

Grow them in pots if the beds have defeated you. A pot turns for inspection and stands on a surface the larvae cannot pupate in.

Leave the sprays alone. Lily flowers are full of bees, the beetle drops off the plant before the spray lands, and the wasps that have started to parasitise the larvae in southern England are killed by the same product.

What it gets mistaken for

Cardinal beetles are also red, longer and flatter, with a red head rather than a black one, and they hunt other insects on flowers and dead wood rather than eating leaves. Leave them alone.

Ladybirds are domed and spotted, and lily beetle is oblong with no spots at all.

Slug damage on lilies starts at ground level and moves up, with rasped edges and slime trails on the soil and stems. Lily beetle works down from the top.

Vine weevil notches the leaf margins of many plants in small clean semicircles and does its real damage underground to the roots. The adults are dull matt black and slow, with an obvious snout.

Damage to day lilies, hostas or lily of the valley is something else. Those are not lilies in the sense this beetle recognises, and it does not breed on them.