There are many species of fly in New Zealand. Most pose a threat to hygiene and are a nuisance to humans and animals. Flies are vectors of a large number of diseases including gastroenteritis, dysentery, typhoid, polio, salmonella and tuberculosis.
Some, such as sandflies, are a direct irritant, biting humans and sometimes causing painful reactions.
The true flies are insects of the order Diptera (meaning: two winged) and include common pests like houseflies Musca domestica, lesser house flies Fannia canicularis, blowflies Calliphora spp, cluster flies Pollenia rudis and fruit flies Drosophila spp.
Other flying insects such as clothes moths Tineola bisselliea and T. pellionella, midges and crane flies Tipula paludosa enter our homes and workplaces and are also considered pests.
Numerous bristles on house flies' legs pick up and distribute germ laden particles wherever they go from rotten animal waste to your sandwich.
In warmer climates, breeding continues throughout the year. Eggs are often laid in moist decaying vegetable matter and animal waste.
The flying insects encountered in rural and farm premises can be of many types, but it is generally accepted that those which are regarded as pests are those which spread disease through contamination, or cause physical damage and general nuisance.
The insects most associated with the spread of disease in lifestyle blocks, commercial premises and on farms are the true flies. There are many thousands of species of true fly, however, relatively few interact with humans. Those that do are among the most destructive of pest species, spreading disease to man and domesticated animals, as well as contaminating food.
Adult flies fly. It is this characteristic that makes their status as pests so important, allowing them mobility to visit many diverse habitats. By nature, many flies breed and feed in areas of unsanitary conditions, with larvae feeding on decaying organic matter. The adult female uses complex sensory systems to choose suitable areas of rotting vegetation and decaying animal matter in which to lay her eggs and for the larval stages to develop into pupae. The adults emerge from the pupae in these unhygienic sites and, as they do, they become contaminated with disease causing organisms. They may fly to sensitive food preparation, processing and consumption areas, seeking feeding sites for themselves as adults.
The likelihood of contamination of human food with pathogens has been demonstrated over the years by a number of experiments. In these, disease-causing agents have been found to survive on outside body surfaces of flies. They also exist in the fly gut and blood system.
Flies are fluid feeders and, although they feed on solids, they need to liquidise the food before they can suck it up. They do this by producing large quantities of 'saliva' from glands. This is then poured onto the food via the salivary canal of the mouth parts. The flies also frequently vomit some fore gut contents onto the food while feeding. In addition, during the feeding process, flies frequently defecate. This in turn can spread pathogens from the hind gut of the fly onto food and food preparation areas.
If the food on which the flies have been feeding and defecating is prepared for human or animal consumption, disease and suffering is frequently the result. Food poisoning outbreaks can occur from a minute dose of pathogens.
Two groups of flies are grouped under the term house fly (or housefly) these are of the Musca and Fannia genus. Fannia are lesser house flies and are a little smaller that Musca house flies. Lesser house flies are often the type that are seen flying in angular patterns around the centre of a room. They can keep flying all day and will only land at night. This makes them a little more difficult to control using surface treatment alone.
Blue bottle and green bottle flies have shinny metallic looking bodies and are larger than houseflies. They are associated with rotten meat where their eggs are laid and the larvae (maggots) feed before pupating and emerging as adult flies. An infestation of blow flies suggests a dead animal carcass in the vicinity e.g. a dead rat in the roof space. Check for possible sources and carry out treatment as suggested below.
The fruit flies are also known as vinegar flies. Drosophila melanogaster is the common fruit fly and has been used extensively in the development of the science of genetics over the last fifty years.
Fruit flies are small usually about 3mm long. They breed on decaying vegetable matter or sweet liquids. An infestation of fruit flies suggests rotting fruit, stale beer or wine, blocked drains etc. somewhere close by. Check all areas and then treat as suggested below.
Because of the particular habits of cluster flies they will be dealt with in a separate article.
How to Keep Flies Under Control
- Flies inside - It is best practice to keep flies from entering with the use of mesh screens on windows and doors or keeping doors and windows closed. If flies gain entry; aerosol fly sprays can knock down flies in a room but only work for a short time i.e. they are non-residual. Be sure to remove or cover fish tanks and follow all precautions stated on the can. Stay out after spraying an aerosol as any people or pets in the room would be inhaling insecticide. Automatic dispensers that regularly puff insecticide (natural pyrethrum plus synergistic enhancer) into a room can be useful for deterring and controlling flies but they should always be sited away from food and food preparation areas.
- Surface treatment using residual type insecticides containing insecticides such as deltamethrin or permethrin will give long term control of flies. Flies are killed after they land on the treated surfaces and insecticide will not be inhaled by people or pets using the rooms. Spray the surfaces that you see flies landing on; these are often the edges of doorways and window frames, light fittings, ceilings, tops of walls and wall corners.
- Note: Pyrethroids are broken down by UV light and have a shorter life in bright sunlight. They are also slower to work in higher temperatures so on warm days the flies may take several hours after contact to die.
- Some flies don't 'like' variable airflows. An oscillating fan can help to make a room unattractive to flies.
- Flies outside - It is more difficult to deal with flies outside as it may not be possible to treat areas not within your control and there can be no way to proof against flies. However, there are ways to minimise the nuisance. Search for possible breeding sites of flies. Remove or treat with insecticide any decaying animal or vegetable matter that might provide food for maggots (fly larvae). Check and clear drains.
- Spray exterior walls, window frames, door frames, pergolas and other fly alighting surfaces using residual surface spray.
- Baits are being developed for the control of flies. These may be useful in areas such as chicken coops, animal houses etc. where spraying can be less effective because of dust and where there is a great deal of decaying matter to attract the flies.
- Citronella candles and other repellents can reduce fly numbers in limited areas such as decks and around barbecues. However, windy days will dissipate the repellents rapidly.
- Personal insect repellent can be useful in preventing flies as well as mosquitoes and sandflies coming close to us. There are synthetic and organic insect repellents available.
Keep flies to minimum by following these simple principles.
- Remove or limit what is attracting the flies.
- Remove or treat breeding sites.
- Stop them entering by physical barriers.
- Use residual surface treatments where entry cannot be prevented.
- Use automatic aerosol dispensers in areas of high fly nuisance, but never where food is handled.
- Use aerosol insecticide as a quick knockdown but stay out of the sprayed area for as long as possible after use.
Generalised Life Cycle of Flies
- The complete metamorphosis cycle can be as short as 7 - 9 days in ideal conditions
- Eggs to Maggots 8 - 24 hours
- Maggots to pupa 4 - 5 days
- Pupa to Adult fly 2 - 7 days
- Adult flies normally live 1 - 3 months.
More about flies
- Flies are great travelers, up to 32km recorded.
- Flies can over-winter in larval, pupal and adult stages.
- Flies are active by day, resting up by night.
- Maggots are cannibalistic and destroy large numbers of each other.
- Eggs and maggots, if swallowed, can cause intestinal injuries.
- Flies are a very important part of the food chain.
- Predators include wasps, spiders, birds, bats, frogs, fish and lizards.
- Prior to feeding, flies often vomit on to their food matter and trample it to dissolve tissue to a liquid before sucking up the resultant juices.
- Flies do not have lungs but breathe through spiracles, small holes on either side of the body.
- Flies are very quickly attracted to perspiring human skin.
- Musca domestica is not native to New Zealand.
- Under perfect conditions eggs become maggots within 8 hours.
‘The Pest Advice’ – by David Brittain
David has worked in pest control since starting work with Rentokil in the UK in 1982. He worked for the Department of Agriculture of Northern Ireland for 8 years studying DNA typing and the epidemiology of bovine tuberculosis before starting his own pest management business. David moved to New Zealand in 2004 and joined Target Pest to set up an urban pest management division and manage marketing for the company. He moved to Kiwicare in 2007 and has been involved in promoting proper use of pesticides and developing new products. David is a member of the executive of the New Zealand Biosecurity Institute and webmaster for the NZBI, Kiwicare, KCL Commercial, Aldwin Paige and writes the Pest Advice Blog.