About a month ago our lawn was replaced, as were 2 other houses next to us. We have been watering it every day as per our landlords instructions. We now have light streaking over some of the lawn and tons of swarming gnats/flies/something, flying up from the grass and hovering over the grass. They are tiny and everywhere, in sunlit areas and shade. We cannot use the garden because they are so annoying. When I walk on the grass I get dead black things on the sole of my feet. All 3 houses have the same problem. What are they and what can we do? Thank you.
I am fairly certain that you have been overwatering your new lawn. New sod should be getting 1 inch of water every 2-3 days in the form or rain or with a sprinkler. You can measure this with a can with the top removed. If you place it on the lawn where you are watering, you can easily tell how much water is reaching the lawn. Watering should be done in the early morning so that the grass blades have a chance to dry during the day. Occasionally you can lift a corner of the sod to see if the soil underneath is moist but not soggy. If the sod becomes too wet, the roots will rot and the entire plant may die. The gnats that you are seeing are likely fungus gnats that have developed because of the wet conditions. They should disappear once you reduce the amount of water. After the sod has rooted into the soil you should only water so that the lawn receives 1 inch of water every week if you have clay soils and twice a week for sandy soils. This will encourage deep, strong roots which will make it more resistant to pests and diseases.
The City of Toronto has an excellent information page on lawn care which you might find useful.