Fly control - Its a Numbers Game - By Dr Lesley Lunn

Understanding fly control

Despite us having had access to extremely effective insecticides over many years, flies remain a pest. The problems associated with flies are numerous. House flies and Stable flies are the most common pests. House flies in particular are associated with:

  • Fly worry
  • Wound infections
  • Spread of disease

Stable flies are not as worrisome for humans, but cause intense irritation in animals due to their biting method of feeding when they suck blood.

Control.... A Fly In The Ointment?

The world of fly control is bewildering and people are often confused regarding which remedy to use when. Fly problems are simply a result of the adult fly that is allowed to reproduce excessively! To control flies we need to interfere or stop fly reproduction.

People are often under the impression that the application of an insecticide on the victim animal will offer total fly control. Such a one-dimensional approach simply does not achieve success. The misuse of pesticides has led to the development of fly resistance to many of the currently available insecticides. We therefore need a multi-faceted or integrated approach. The most effective control remains the interruption of the fly’s life cycle.

The life cycle can briefly be described as follows: EGG – LARVAE – PUPAE – ADULT FLY

The complete life cycle of both house flies and stable flies can be completed in as short a period as 2 – 3 weeks (It is normally 3 weeks but in some cases, can be completed in only 8 days).

An integrated fly control programme incorporates control methods that can be applied- where possible – to interrupt the fly life cycle and thus target specific elements of the life cycle. In general it is relatively difficult and not cost effective to try and kill eggs or the pupae. Thus, most efforts are directed at control of larvae or adult flies.

How is this achieved?

  1. Sanitation (Breeding site Control): Wet manure is an ideal breeding ground for flies. Water management is therefore critical. Manure should be spread out to allow drying, as fly larvae are very susceptible to desiccation. If spreading is not feasible, then the manure should be piled in heaps to allow the build-up of heat. This will drive fly larvae to the surface of the manure heap where they will fall prey to fly predators (like birds) or die. Stable flies breed most efficiently in lawn clippings that have, for instance, been put into compost heaps.

The larvae are initially small worm-like organisms that grow in size until they are ready to move to next stage – production of pupae. In order to pupate, the larvae have to move out of their breeding medium, (manure/grass compost) and burrow down into sand where they pupate below the level of the soil. If manure heaps or compost pits are placed on an impervious surface like a cement base, the insects have to move out of the breeding material and are then exposed to normal predators like birds. If a perimeter around the impervious surface includes a depression like a gutter in which water can be maintained, the fly larvae will drown in the water without any insecticides.

 

2. Chemical: Pesticides should be focussed on areas where the ADULT flies rest which are areas that receive morning sun early in the day and shady places during the heat of the day (rafters, fence posts, walls). Insecticides should not be applied where flies breed as it will harm insects that parasitise flies. Spot-on or spray treatments may be applied to control face flies on horses or biting flies on dogs’ ears. Other methods of control which can reduce house fly numbers in particular are flytraps, fly baits, insecticidal paint-on’s and specific chemicals which prevent larvae growing. The latter may be applied to breeding sites. Mechanical sticky tapes can also reduce house flies. If manure management is applied by creating compost pits, these pits can prove successful by attracting the flies to lay their eggs in these readily available breeding sites, but preventing the larvae from pupating.

 

  1. Biological: certain wasp species (e.g. Mucidifurax zaraptor, Spalangia cameroni) are natural enemies of the fly. They are harmless and do not sting people or animals. They can be purchased in the form of parasitised fly pupae. Parasitised fly pupae are strategically spread around areas where flies breed. Regular release of these pupae in the environment is recommended – particularly at the beginning of summer. The wasp lays its eggs inside the fly pupa. The legless immature wasp larva then consumes the immature fly inside the fly’s own pupa! Instead of an adult fly emerging from the pupa – which can lay up to 800 eggs – we now have between 1 and 50 harmless wasp emerging which will seek out other fly pupae to parasitise. Each wasp initially kills about 50 flies and after about 3 weeks another generation of beneficial wasps emerges to repeat the process. The wasp population needs to be continuously replenished because flies lay up to 800 eggs, but the wasp can only attack up to about 50 fly pupae. The life cycle of the fly is also much shorter than that of the wasp.

Preventing a build-up in the fly population is the best way to deal with flies – which is much easier than to try and get rid of them. It should however be recognised that where fly control measures are applied in a restricted area and the neighbours do not follow a similar programme, flies will move from wherever they are in large numbers (and competing with each other) to areas where there are low numbers of flies present.

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