The terrace in summer: treat the edges, not the air

An outdoor space has two insect problems that get treated as one. There is the mosquito in the air at nine in the evening, and there is the route everything else uses to get from the garden into the house through the terrace door. They need different answers.

The air is a losing game

Anything you put into the air outdoors is gone with the first breeze, which is why the candle on the table only works when there is no wind and you are sitting inside its little circle. It is fine as far as it goes. It is just not a plan.

A surface treatment works differently.

Where insects actually sit

Insects rest on surfaces far more than they fly. On a terrace they sit on the underside of the parasol, on the shaded face of a wall, on railings, on the frame of the door and in the shadow under the furniture. Those are the surfaces worth treating, and the ones nobody thinks to treat.

• The frame and threshold of the terrace or balcony door, inside and out.
• Window frames and sills on the same side of the house.
• The underside of the table, and the frames of chairs and benches.
• Railings, and the shaded face of the wall behind the seating.
• The underside of the parasol and the pole.
• The edge of the decking or paving where it meets the wall.

Doing it properly

Clean the surfaces first and let them dry, because outdoor surfaces carry dust and pollen and the coating will bond to that instead of to the wood or the metal. Then a thin, even layer, overlapping slightly along each edge.

Test a hidden corner of cushions, parasol fabric or outdoor upholstery first for colour and finish, and let textiles dry fully before anyone sits on them.

Rain and reapplication

Outdoors, that is the ceiling rather than the expectation: sustained rain on an exposed surface will shorten it, and a covered balcony will hold it longer than an open terrace.

The practical version is to treat at the start of the season, again in high summer, and to touch up the door frame and the furniture after a stretch of bad weather. Treat in the morning of a dry day so the surfaces have the whole day to dry.

Do the boring part too

Empty standing water: saucers under plant pots, the bottom of the watering can, a blocked gutter, the cover on the barbecue. Mosquitoes breed in a surprisingly small amount of water, and no treatment on a railing competes with a nursery two metres away.