Gail Tait, of Kaitangata, asks :-

Where do flies live when it is cold?

Anthony Harris, an entomologist at Otago Museum, responded.

Houseflies (Musca domestica) and blowflies of that family Calliphoridae (e.g., Calliphora quadrimaculata, the native bluebottle) whose larvae eat dead organic matter including meat, live in much the same places during both summer and winter and will breed throughout the year at a constant rate in an incubator set at a constant temperature between 20 and 34 degrees centigrade. At 20 degrees C., from blowfly eggs laid on meat, larvae (maggots) hatch, feed, and change into pupae seven days later. Adult blowflies emerge after a further 8 days.

Blowflies develop very rapidly in summer and there are many adults about (the life cycle takes 2-3 weeks). In winter, their cycles slow down because of the lower temperatures and in cool places they can take two months and three weeks to complete their life histories. Consequently in winter, even if there is a lot of food outside, blowflies can only breed slowly, so few of them are seen.

Some flies hibernate, and can suddenly appear on a warm day when it is hot enough for them to fly. Others appear in spring when warmer weather causes them to emerge from pupation.

An exception to the foregoing is the cluster fly, Pollenia pseudorudis, which arrived from North America in the last 25 years and became a nuisance in New Zealand in the last 8 years. Although it belongs to the blowfly family Calliphoridae, the cluster fly is exclusively an earthworm parasite and does not carry germs or eat dead meat. The adult cluster fly, 7 mm long, is a little larger than a housefly. It comes indoors in large numbers in March, to hibernate over winter, entering wall cavities and the space between the ceiling and roof on the sunny sides of buildings. Big fat bodies in the abdomen give the cluster fly a store of energy for the winter months. On warm winter days, cluster flies emerge and fly around rooms. They vacate buildings in spring, laying eggs on grass. The first stage larvae burrow through the soil to attack earthworms. So cluster flies can suddenly appear in hundreds and fill a room in the middle of winter.

Consequently, house flies and blowflies are generally in low numbers in winter, but flies that hibernate, such as cluster flies, can suddenly appear in hundreds in winter. Some species of small native flies are seasonal, appearing in early spring or late autumn.