Plum moth

What you are looking at

A plum that has coloured early. While the rest of the tree is still green and hard, a scatter of fruit turns purple, softens and falls, two to four weeks ahead of schedule. Picked up and cut open, it has a pinkish-white caterpillar about 12 mm long with a brown head, curled in the hollow around the stone, sitting in a mass of dry brown crumbs.

From the outside there may be nothing at all, or a pinhole in the skin with a bead of clear gum beside it. Most people find this pest at the kitchen counter rather than on the tree.

Damsons suffer worst, then plums and gages. The wild blackthorn in a nearby hedge carries the same moth and is where a new infestation often comes from.

The year the moth has

Adults are small and grey-brown, about 15 mm across the wings, and they fly in the evening from late May or early June through to August. They are easy to miss and almost nobody sees them.

A female lays single eggs on developing fruit and on the leaves beside it. The caterpillar hatches, bores in near the stalk, and feeds in the cavity around the stone for three or four weeks. The fruit responds by ripening early and abscising, which suits the moth: it puts the caterpillar on the ground with a food supply, near the trunk it wants to pupate on.

The caterpillar then leaves the fallen fruit, finds a crevice in the bark or the litter, and spins a tough silk cocoon. In a warm year a second, partial generation follows in August and September. The rest overwinter in those cocoons and emerge the following May.

Every part of the control follows from that cycle. The fruit on the ground holds next year’s moths with a fortnight’s head start, and the loose bark on the trunk is where they will spend the winter.

Plum moth or plum sawfly

Both put a grub in a plum and the treatments differ, so the distinction is worth five minutes.

Plum sawfly attacks early. The grub goes into fruitlets in May and June, tunnels to the stone while the fruit is still small and green, and then leaves through a hole several millimetres across. Around that hole is wet black frass with a strong unpleasant smell, and the fruitlet drops in June. One sawfly grub will often ruin three or four fruitlets in succession.

Plum moth attacks late. Its caterpillar goes into a fruit that is already sizing up, stays inside until the fruit falls, and leaves dry brown crumbly frass with no particular smell. One caterpillar stays in one fruit.

June drop with black smelly holes is sawfly, and August fruit with dry frass round the stone is moth. Pheromone traps for plum moth catch no sawflies at all, so a trap that stays empty while the fruitlets are dropping tells you which one you have.

Do the traps work

On a garden tree, yes, and better than they do commercially. A pheromone trap catches male moths, and where there is one tree and one trap, enough of the local males end up on the sticky sheet that many females go unmated. Growers with a block of trees get less benefit from the same trap, because there are always more males arriving, which is why the commercial literature describes traps as monitoring tools and the garden experience is better than that.

Two things stop them working. Hanging the trap in June, after the flight has begun, misses the first and largest wave. And leaving the same sticky base in place all summer means the trap is full and inert by early July.

Nothing sprayed is available or worth pursuing here. The caterpillar spends its whole feeding life inside the fruit, where no product a gardener can buy will reach it, and the window before it bores in is a few days wide and impossible to judge without the trap you would have needed anyway.

What to do about it

Hang a pheromone trap on the tree by the middle of May, and replace the sticky base and the lure through the season as the instructions say.

Gather the fallen fruit twice a week from late June onwards and destroy it.

Thin the crop in June so that fruits do not touch, and pick the whole tree, including the part you need a ladder for.

Tie a band of corrugated cardboard round the trunk in August and burn it in autumn.

Feed the birds near the fruit trees over winter and leave the bark alone.

Cut and use the sound halves of maggoty fruit rather than throwing the lot.

What it gets mistaken for

Plum sawfly damage comes earlier, smells, and leaves a wide exit hole with wet black frass.

Codling moth does the same job on apples and pears and never touches plums.

Pear midge attacks pears alone, and its fruitlets turn black, swell oddly at the base and drop in May, with many small white maggots inside one fruit rather than a single caterpillar.

Nut weevil puts a single grub in a hazelnut or a filbert kernel and leaves a neat round exit hole in the shell.

Brown rot works from the outside in, spreading a soft brown patch over the surface with concentric rings of buff pustules, often on fruit a bird or a wasp opened first.

Wasps and earwigs found inside a plum in August are enlarging damage that something else began.