Caterpillars

What you are looking at

Holes in the middle of leaves, chewed edges, and margins that are ragged rather than cut cleanly. Below the damage there are dark pellets of frass on the leaves and on the soil, and that is the sign that separates a chewing insect from a disease or a disorder.

Where the animal itself is hiding depends on the species. Cabbage whites feed in the open, in groups, on the upper leaf surface. Winter moth caterpillars pull two leaves together with silk at a shoot tip and eat inside. Tortrix moths roll a single leaf into a tube. Cutworms come up out of the soil after dark and are gone by morning.

Count the legs before you do anything

A great deal of caterpillar advice is applied to animals that are not caterpillars, and the distinction changes what works.

Three pairs of jointed true legs at the front is common to both. What differs is the fleshy prolegs on the abdomen behind them. A caterpillar, meaning the larva of a butterfly or a moth, has at most five pairs, and each one carries a ring of tiny hooks. A sawfly larva, which is a relative of the wasps, has six pairs or more, and no hooks.

Loopers are caterpillars that have gone the other way. Geometrid moths, including winter moth, have only two or three pairs of prolegs, all at the back, so the animal has to arch its body into a hoop to move at all. The gait identifies the family from a distance.

Sawfly larvae are the ones stripping a gooseberry bush, a Solomon’s seal or a rose in early summer. Nothing sold or recommended for caterpillars affects them, which is why so many gooseberry treatments fail.

Most of them are not a problem

Britain has about 2,500 species of moth. Perhaps a dozen do damage in a garden worth acting on, and the rest are the foundation of the food web the garden sits in. A single brood of blue tits is fed several thousand caterpillars, and the timing of the whole breeding season is built around the flush of them on oak and fruit trees in May.

So the useful question is not what is eating the leaves. It is whether the plant being eaten is one you needed intact.

An ornamental shrub with some chewed leaves in August recovers completely and costs you nothing. A cabbage reduced to veins is a crop lost. A box hedge with box tree caterpillar in it will be dead in two seasons. Spend the effort on the second and third, and let the first alone.

The few worth acting on

Large and small white on brassicas and nasturtiums, from May to September, in two or three generations. Large white lays clusters of yellow eggs on leaf undersides and the caterpillars feed in groups; small white lays singly and its caterpillars are green and burrow into the heart of a cabbage.

Winter moth on apple, pear, plum, cherry and roses. Flightless females walk up the trunk between October and March, and the caterpillars hatch with the buds and eat the developing leaves and blossom.

Box tree caterpillar, which has gone from unknown to the most destructive garden pest in the south east of England in a decade, and which is a different order of problem from everything else here.

Cutworms in a vegetable bed, which cut seedlings off at soil level overnight and get blamed on slugs every spring.

What to do about them

For brassicas, net. Fine mesh over hoops from the day the plants go out, with a gap between the mesh and the leaves, because a butterfly will lay through netting resting on foliage. That one measure removes the problem for the season, and it does it without touching anything else living in the garden.

Where a crop is already infested, look for the eggs rather than hunting the caterpillars. A batch of large white eggs rubbed off with a thumb is thirty or forty animals dealt with in two seconds.

Then hand pick, twice a week, under the leaves and along the midribs. On a garden scale that is enough on its own.

For fruit trees, put grease bands on in October. The winter moth female cannot fly, so she has to climb, and a barrier around the trunk and the stake stops the whole thing before it starts.

For box, act quickly and use something targeted. Bacillus thuringiensis or Steinernema carpocapsae applied to the hedge and nothing else, at the point the caterpillars are small.

Leave the parasitised ones. A dying large white caterpillar with a cluster of small yellow cocoons beside it is producing a generation of Cotesia wasps that will find the next batch for you.

Why no spray is listed here

A broad-spectrum insecticide applied to a garden kills the caterpillars of every butterfly and moth it reaches, along with the parasitoid wasps and predatory insects that were controlling them, and it removes food that nesting birds are depending on that fortnight.

Against the one problem where a treatment is warranted, box tree caterpillar, a targeted biological gives a better result anyway, because it reaches the animals inside the webbing that a contact spray runs off.

The whole of the rest of this page is netting, a thumb and a grease band, and between them they cover almost every case.

What it gets mistaken for

Sawfly larvae look like caterpillars and are not, and the proleg count settles it. They strip gooseberries, Solomon’s seal, roses and aquilegia, often working from the middle of the bush outwards, and they leave no webbing.

Slugs and snails eat holes with smooth rounded edges and leave a slime trail and no frass.

Vine weevil adults cut neat semicircular notches in from the leaf margin and never make a hole in the middle.

Leaf-cutter bees remove smooth semicircles from rose and lilac leaves with edges as clean as scissor cuts. They are pollinators and should be left entirely alone.

Earwigs chew ragged holes in dahlia and clematis flowers at night, and on a fruit tree they eat more aphids than petals. The damage is mostly to blooms rather than foliage.