Why have my leaves been stripped to the veins overnight?

This kind of damage is alarming because of the speed. A gooseberry bush or a Solomon's seal can go from full leaf to bare twigs in a few days, because the larvae hatch together and feed together. The plant usually survives — the timing of the loss matters more than the amount — but only if you find them, and they are never on the top of the leaf.

Step by step

1. Turn a damaged leaf over immediately.Every one of these feeds from underneath. A row of small caterpillar-like larvae along the vein of a leaf, often lifting their tail ends in unison when disturbed, is sawfly. Nothing else does that.
2. Identify by the plant, which is very nearly diagnostic.Gooseberries and redcurrants stripped in late spring: gooseberry sawfly. Solomon's seal reduced to stems: Solomon's seal sawfly. Roses with leaves grazed to a transparent skeleton: rose slug sawfly. Aquilegia leaves vanishing: aquilegia sawfly. Lilies and fritillaries: lily beetle grubs. Viburnum tinus and guelder rose with lacy leaves in spring: viburnum beetle.
3. Look for the ones that hide under their own mess.Lily beetle grubs cover themselves in a wet coating of their own excrement and look like a blob of black slime on the underside of a leaf. That is the animal, not dirt.
4. Pick them off, and do it thoroughly.This is the single most effective control on a garden scale. Go over the whole plant including every leaf underside, and drop them into a container. On gooseberries, start checking in late spring at the centre of the bush low down, which is where the first generation always appears.
5. Check again in a week, and again after that.Sawflies have two or three generations a season, so a bush cleared in May is stripped again in July if it is not rechecked. Lily beetle adults keep arriving through spring and summer and keep laying.
6. Look for the adults as an early warning.Adult lily beetles are bright scarlet, about 8mm, sitting openly on lily leaves — they drop and land on their black undersides to hide, so cup a hand beneath before grabbing. Adult viburnum beetles are dull brown and appear in late summer.
7. Accept the damage on an established plant.A defoliated gooseberry in June will still make a crop and will releaf. A mature viburnum shrugs it off. The cases that genuinely matter are young plants, repeated defoliation over several years, and lilies, where lily beetle can destroy the plant.
8. Encourage the birds.Blue tits and other small birds take large numbers of sawfly larvae. Nest boxes and a garden with cover are a real control, and the reason the same bush is not stripped every year.

Tips

• Gooseberry sawfly starts in the middle of the bush, low down and in shade, where nobody looks. Check there first and you catch it before the leaves go.
• Shaking a branch over a light-coloured sheet dislodges most larvae in one go and shows you how many there are.
• Viburnum beetle overwinters as eggs in small pits along the previous year's shoot tips. Pruning those tips out in winter removes the next generation.
• Lily beetle can be reduced dramatically by checking lilies every few days from March, when the first adults appear, rather than in June when the grubs have already done the damage.

Common mistakes

• Looking only at the top of the leaves — None of these feed there. A plant can be crawling with sawfly larvae and look completely clean from above.
• Clearing them once and considering it done — There are several generations in a season. The second one arrives on a bush the gardener has stopped checking.
• Assuming the black slime on a lily leaf is disease — It is the grub, wearing its own excrement. Wiping it off with a gloved finger is the treatment.

Questions people ask