Warmer days and warmer nights are making it harder for sprays to work on some flies and people may just have to grin and bear the critters, says a Rotorua pest controller.
"If the house is very hot and sticky and you are sleeping with just a sheet the flies won't be as affected [by fly spray] as if it was cooler. You'd expect to find them on the floor dead when you get up but instead they are making breakfast for you," says Chris Brunel, manager of Rotorua's Bay Pest Services.
"Those tiny house flies that don't really head for the window but sit on the end of your nose and on tea towels, they are very difficult to get rid of."
Mr Brunel said people getting their homes sprayed needed to be realistic and realise it won't completely get rid of the flies.
With temperatures not going down too low at night, some of the pesticides had limited value.