How do I deal with insects indoors and out in summer?

Excluding rather than killing, the specific answer for each of the common summer insects, and when it becomes a job for someone else.

Difficulty: beginner
Time: 45 min
Read: 4 min
Safety: warning

Short answer

Almost all summer insect problems are solved by removing what attracts them and blocking the way in. Flies want food waste, wasps want sugar late in the season, mosquitoes want standing water, and ants are following a scent trail. Killing the ones you can see never ends any of them.

Insects arrive indoors for a reason, and the reason is usually findable. Sprays deal with what is in the room; finding the attractant or the entry point is what actually ends the problem.

Safety

Never disturb a wasp or bee nest yourself — call a pest controller for wasps, and a local beekeeper for a honeybee swarm, which is usually collected free. Anyone with a known sting allergy should carry their prescribed treatment. Keep insecticides away from food surfaces and out of reach of children and pets, and never spray near a naked flame.

Step by step

Fit fly screens on the windows you open.Magnetic or fitted mesh on the kitchen and bedroom windows. It is the single most effective measure for flies and mosquitoes and it lets you keep windows open in heat, which matters more in summer than at any other time.
Deal with what flies are breeding in.Food waste bin, outdoor bin, drains, a forgotten bag of rubbish, or something dead in a loft or under decking. Adults you can swat are a symptom; the breeding site is the problem.
Wash the bins, not just the bags.Residue under the rim and in the lid mechanism is where kitchen flies breed. Both the kitchen bin and the outdoor one, with hot water and detergent.
Remove standing water for mosquitoes.Water butts uncovered, saucers under pots, a full watering can, blocked gutters, a bird bath. They breed in still water and a few days of it is enough. This is the whole mosquito answer.
Follow the ant trail rather than spraying it.Ants lay a scent trail to a food source. Find where it enters and what it leads to, remove the food, and wipe the trail with soapy water or vinegar to break the scent. Then seal the entry point.
Use bait rather than spray for ants.A bait station is carried back to the nest and deals with the colony. A spray kills the ants on the worktop and the trail rebuilds within a day.
Understand wasps' seasonal behaviour.Early in summer they hunt insects and largely ignore you. From late August the colony stops producing young, the workers lose their food source and become interested in sugar — which is when they become a nuisance at a table.
Do not swat a wasp.A crushed wasp releases an alarm pheromone that attracts others. Move calmly away, and cover sweet food and drinks — a can left open outdoors is a specific hazard because a wasp can get inside it.
Leave nests to someone else.A wasp nest is a pest controller's job and is usually inexpensive. A honeybee swarm should go to a local beekeeper, who will normally collect it free. Never attempt either yourself.
Manage light at night.Insects are drawn to light through open windows. Close curtains before turning lights on, or fit screens. Warm-toned bulbs attract considerably fewer than cool white ones.
Deal with cluster flies and their entry points.Large numbers appearing in a loft or a spare room in late summer are usually getting in through gaps at the eaves or around window frames. Sealing those is the durable fix.
Use a fan against midges and mosquitoes.They are weak fliers and moving air disrupts them effectively. A fan at a seating area works better outdoors than most repellents.
Check food storage and pet bowls.Fruit left out, an open cereal packet, a pet bowl of food standing all day. These sustain a population you then treat as an infestation.

Tips

Fly screens cost very little and remove the trade-off between ventilation and insects, which is the actual summer problem in most houses.
Cover the water butt. It is the most common mosquito breeding site in a British garden and a lid solves it entirely.
Wasps become a problem in late August rather than in July. Knowing that stops people treating an early-summer wasp as an emergency.

Common mistakes

Spraying ants on the worktop — It kills the visible ones and leaves both the scent trail and the nest. Bait carried back to the colony is what ends it.
Swatting a wasp near others — A crushed wasp releases an alarm pheromone that draws more. Move away calmly and remove the sugar that attracted it.

If it doesn't work

Flies in the kitchen every day

Cause: A breeding site — usually the bin, a drain, or forgotten food — Fix: Wash both bins including the lid, check the drains, and look for anything overlooked in a cupboard or under a unit.

Ant trail returns after cleaning

Cause: Scent trail intact and the nest untouched — Fix: Wipe the trail with soapy water or vinegar, seal the entry point, and use bait rather than spray.

Mosquitoes indoors at night

Cause: Standing water outside and open unscreened windows — Fix: Cover the water butt, empty saucers and cans, and fit screens to the windows you open.

Hundreds of flies in a loft in late summer

Cause: Cluster flies overwintering, entering at the eaves — Fix: Seal gaps at the eaves and around frames. Vacuum the existing ones rather than spraying.

Questions people ask

How do I stop flies getting in during summer?

Fit fly screens to the windows you open, and find what they are breeding in — usually the kitchen or outdoor bin, including under the rim and in the lid mechanism. Killing the adults never ends it.

Why do wasps become a nuisance in late summer?

From late August the colony stops producing young, so the workers lose the sugary secretion they fed on and start looking for sweet food elsewhere. Earlier in summer they are hunting insects and largely ignore people.