Observing Garden Moths with a Simple Trap

Why the garden at night is worth watching

Most of us know our gardens by daylight — the robin on the spade handle, the bees working the lavender, the first brimstone of spring. After dark a different cast takes over, and much of it goes unrecorded. Britain has roughly 2,500 species of moth, and only a handful fly by day. The rest are nocturnal, and a good many are woodland species drifting in from hedgerows, scrub and the woodland edge.

Moths matter far more than their reputation suggests. Their caterpillars are the single most important food source for nestling birds, and the adults feed bats, hedgehogs and night-flying birds. They also respond quickly to changes in weather, hedgerows and light pollution, which makes them worth counting.

You do not need expensive kit to start. A simple trap, a notebook and a willingness to be up early will do.

How a moth trap actually works

Moths navigate partly by keeping a fixed bearing relative to light. A bright artificial light confuses that instinct, and many species fly towards it and circle. Your job is to give them somewhere safe to settle, rather than letting them batter themselves against a hot bulb until dawn.

The classic design does exactly that: a box with a funnel set into the lid, a light above the funnel, and something textured inside to rest on. Moths come to the light, drop through the funnel, and spend the remaining hours of darkness tucked away in the dark interior. Egg boxes are the traditional resting material because each cup gives a moth its own compartment.

Building one from things you already have

• A plastic storage box or bucket, at least 30 cm deep.
• A funnel — a cut-down plastic bottle or a plant pot. The spout must be wide enough for a large hawk-moth to pass through.
• Several egg boxes, or strips of corrugated cardboard, laid loosely inside.
• A low-heat light: an LED torch clipped above the funnel, a low-energy bulb, or a solar garden light switched on manually. Avoid high-wattage halogen bulbs, which exhaust moths.
• A clear plastic sheet angled over the light as a rain shield.
• A power source — a long extension lead, a battery pack or a rechargeable torch.

Stand the trap on a table or low bench rather than directly on grass, which simply fills your catch with slugs and ground beetles. If you would rather not build anything, hang a white sheet over a washing line, weight the bottom corners and clip a torch above it. Check the sheet every twenty minutes; many moths settle rather than enter a trap.

Choosing the right night

Timing makes the difference between a handful of moths and a hundred. Warm, still, humid, cloudy nights after a mild day are best. Wind above about 15 mph keeps everything down, and heavy rain spoils both the catch and the moths. A bright full moon can reduce numbers too, since there is plenty of natural light to navigate by.

In southern Britain, trapping from May to September works well, with July usually the peak; the north and west start later and finish sooner. Aim for nights when the temperature stays above 10°C in summer and above 6°C in spring and autumn. Switch on at dusk and check at first light, before the robins learn where the easy meal is. If you cannot get out early, move the trap into a cool, shaded shed or garage until you can.

Recording what you find

Keep a simple log: date, running times, temperature, wind, moonlight, and the species and numbers found. Expect to identify only a fraction at first. Work in groups rather than species — the large yellow underwing, heart and dart, common quaker, silver Y and angle shades are familiar garden visitors and good ones to learn first. Photograph anything awkward on a plain surface, using an egg box as a backdrop.

Your records have value beyond your own interest. County moth recorders rely on garden traps to fill gaps in national data, and a garden list kept over several years can show real change. Even if you never pass a record on, the seasonal rhythm — the first quaker in March, the last dark arches in October — is reward enough.

Handling and releasing moths unharmed

Handle as little as possible, coaxing individuals gently onto a finger, a soft paintbrush or into a small pot with a card over the top. Never pinch a moth by its wings: the scales rub off, and those scales provide its insulation and its ability to fly.

Release in the evening, in dense cover — a hedge base, a shrub, long grass — rather than on open lawn where a sparrow is waiting. If you trap in the same spot every night, move the trap a few metres now and then and vary your release points, so the same moths are not repeatedly caught.

Above all, garden for them. Let a patch of grass grow long, leave ivy to flower in autumn, and plant night-scented flowers such as honeysuckle, jasmine and evening primrose. Cut out unnecessary outdoor lighting, especially white LED floodlights that stay on all night. Do that, and your trap will have plenty to show you.