Mosquitoes

What bites here

The common house mosquito (Culex pipiens) and a group of marsh and woodland species between them do most of the biting. They come out at dusk and after dark, they hum, and their larvae grow in standing organic water: ditches, water butts, blocked gullies, cattle troughs, slow drains, the flooded edge of a marsh.

Coastal grazing marsh and estuary levels produce them in numbers that surprise people who think of mosquitoes as a foreign problem. So do some inland fens and wet woodland in a warm wet summer.

A wet fortnight and then warmth is the combination that matters, and not a wet day. It takes something like one to two weeks to go from egg to biting adult, so the mosquitoes out tonight were filled into puddles by the rain of a fortnight ago. Last night's downpour changes nothing about tonight.

Disease: the short and reassuring version

Mosquito-borne infection is essentially not a British problem. There is no established West Nile virus transmission here and no malaria transmission; cases seen in the UK are brought back from elsewhere. Saying so plainly seems better than leaving it out, because a gap where a disease section would be reads as though there is something we are not mentioning.

The Asian tiger mosquito (Aedes albopictus), which drives the whole story in Italy, is not established in these islands. It has been intercepted at ports and it is watched for; it is not a resident species, and our forecast does not count it for Britain.

What is left is the bite itself: itching, occasionally a large local reaction, and rarely a scratched bite that becomes infected. Broken skin that turns hot, red and painful over a day or two is worth showing to a doctor.

What our number means

How likely it is that you get bitten today, on a scale from 0 to 100, from temperature, season and the water available to larvae.

This one does change with the place. The inputs are the standing water around the point — wetland and marsh — and the rain of the past fortnight, and both of those we hold as real layers rather than as an assumption. Where the wetland layer has not been built for a region, the forecast page says the number does not vary by place, and means it.

One known weakness, stated here rather than buried: the flight threshold works off the mean temperature of the day rather than the maximum. On a cool day with a warm evening that understates things, and the evening is exactly when they bite.

What actually helps

Repellent works while you are out in it, which is most of what people want from it. The rest is about where the water is.

• cover the water butt with a fine mesh;
• empty and refill anything that holds standing water near the house once a week — buckets, watering cans, wheelbarrows, pet bowls;
• clear blocked gutters and gullies;
• expect the worst of it at dusk within a few hundred metres of marsh or slow water, and plan the evening around that rather than around the whole day.