There is a stretch of the Long Island summer when the mosquitoes stop being an annoyance and start being a number. It usually starts about now.

The Suffolk County Department of Health Services announced this week that 11 mosquito samples collected on July 15 and 16 tested positive for West Nile virus. The positives came from Bay Shore, Port Jefferson Station, Setauket, Copiague, West Babylon, Sayville and, most of all, Lindenhurst, where five of the 11 were trapped.

That is not a headline about an outbreak. It is a headline about a surveillance system doing exactly what it was built to do, in the weeks it was built for.

What the samples actually say

Ten of the 11 were Culex pipiens-restuans. That is the northern house mosquito, and if you want to understand why the virus shows up in ordinary residential neighborhoods rather than out on the marsh, that species is the whole answer. Culex pipiens does not need a wetland. It needs a bucket. A clogged gutter, a tarp with a dip in it, a catch basin, a birdbath nobody has emptied since June, a kiddie pool that has gone the color of weak tea. Stagnant, nutrient-rich, small.

The eleventh sample, collected in Sayville, was Culex salinarius, a salt marsh species. It is the one that fits the map you would expect. It is also the one that was outnumbered ten to one.

To date this season Suffolk has recorded 12 West Nile-positive mosquito samples and three positive for Jamestown Canyon virus, a less familiar mosquito-borne virus that the county also screens for. Two crows found dead in Ronkonkoma on June 24 tested positive as well. Dead birds are part of the early warning system here, which is why the county asks residents to report them rather than bag them and forget it.

No human case has been reported in Suffolk this year.

Trucks on Fire Island this week

The other half of the county response is scheduled and specific, and it is happening as you read this.

The Suffolk County Department of Public Works has ground spraying planned for three Fire Island communities, weather permitting. Atlantique and the Summer Club are set for Wednesday, July 29, from 5:30 p.m. to 9 p.m. Davis Park and Water Island follow on Thursday, July 30, in the same window. In each case the treatment covers all streets and boardwalks in the community, applied as an ultra low volume aerosol from the ground, using a product called Anvil, whose active ingredient is sumithrin.

The county publishes precautions with every notice, and they are worth reading once rather than never. If you can, stay inside during spraying and for roughly 30 minutes afterward. Close windows and doors, and shut off or close the fresh-air intake vents on window air conditioners, then reopen them half an hour later. Bring in pet bowls, laundry and small toys beforehand, cover ornamental fish ponds, and pick anything from the garden you were planning to eat soon before the trucks come through. Rinse produce anyway. If spray reaches your skin, wash it; if it reaches your eyes, rinse them with water immediately.

Children and pregnant women are advised to avoid exposure where it is practical to do so.

Who should actually worry

Most people who get West Nile never know it. The infection is mild or silent in the large majority of cases. The reason public health departments take it seriously anyway is the tail: headache, neck stiffness, vision loss, high fever, confusion, muscle weakness, paralysis, and in the worst outcomes, death.

The county singles out two groups for extra caution: people 50 and older, and anyone with a compromised immune system. If that is you, or the person you are packing the car for, the calculus on a screened porch and a long-sleeved shirt at dusk changes a little.

The practical defense has not changed in twenty years and still works. Empty standing water on your property, every week, including the containers you do not think of as containers. Repair screens. Use an EPA-registered repellent. Shift the barbecue an hour earlier, because Culex pipiens is most active from dusk into the night.

Numbers worth having

To report a dead bird on your property: Suffolk County Department of Health Services, Bureau of Public Health Protection, 631-852-5999, 9 a.m. to 4 p.m. Monday through Friday. The county encourages you to submit a photograph.

To report a mosquito problem: Department of Public Works, Division of Vector Control, 631-852-4270.

Spraying information hotline: 631-852-4939. Notices are also posted to the county news page, and residents can sign up for SuffolkAlert to get them by text, email or phone.

Ticks have taken most of Long Island's attention this summer, and for good reason given what the lone star tick has been doing out east. But August belongs to the mosquito, and the county's own trapping data is the closest thing there is to a weather report for it. Eleven samples in one collection is not a reason to stay indoors. It is a reason to go turn over the wheelbarrow.