Naturally occurring parasites and predators of flies are found in manure and other habitats fly larvae grow in.
These parasites and predators reduce fly numbers.
Biological control agents play an important role in controlling fly populations. Conserving these naturally occurring bio-control agents is the most important and easiest biological control system available to producers.
The primary management strategy is to preserve existing populations of biological control agents. Unlike some of the chemical fly treatments, biological agents are a soft and slower acting tool for fly control and their use has to be carefully planned and implemented well ahead of the occurrence of major fly waves.
Types of bio-control agents
Mites
There are various mites inhabiting manure and other larval substrates. These mites eat fly eggs and larvae, reducing the number of adult flies produced. Mites travel between areas by ‘hitchhiking’ a ride on adult flies (Figure 1).
Figure 1. A red mite attached to a house fly. Image courtesy of the Peter Green, Queensland Department of Agriculture and Fisheries.
Fungi
Fungi infect and kill flies by sticking to the flies’ exoskeleton when they are feeding or resting. This fungus grows and kills the fly, and then produces fungal spores which other flies pick up (Figure 2).
Figure 2. House fly infected with a fungus. Image courtesy of the Peter Green, Queensland Department of Agriculture and Fisheries.
Parasitic wasps
Parasitic wasps are harmless to people and livestock and at 2-3 mm long, are so small, they are seldom noticed.
The wasps attack fly pupae and either feed on the contents or lay a single egg that hatches inside the fly pupa. When the egg hatches, the wasp larva feeds on the fly pupa, killing it, and 3 weeks later a new wasp emerges from the pupal case (Figure 3).
Up to 35% of developing flies in Australian feedlots are killed by parasitic wasps.
Figure 3 A parasitic wasp emerging from a house fly pupa. Image courtesy of the Peter Green, Queensland Department of Agriculture and Fisheries.
Using your bio-control agents
It is important to look after the bio-control agents that occur naturally.
Avoid using insecticides where possible or use them judiciously when treatment is necessary. Most insecticides also kill parasitic wasps and mites. Fly populations recover more quickly than parasitic wasps due to their shorter life cycle, resulting in reduced biological control during this lag period.
Plan ahead. Bio-control agents are slower acting so their use has to be carefully planned and implemented well ahead of major fly waves.
Keep fly breeding substrates (manure, spilt feed, rotting vegetation) dry, to impede fly breeding and promote parasitic wasp and mite breeding.
Parasitic wasps are most effective when present in large numbers. You can increase parasitic wasp numbers, by augmenting the natural population with commercially available Spalangia endius. Research shows that this will increase the level of parasitism of fly pupae.
Releasing parasitic wasps
To maximise the impact of naturally occurring parasitic wasps, you can augment the natural population by releasing commercially available Spalangia endius. These wasps are reared using house flies and then sent by express mail while still inside the fly pupae.
The parasitised pupae must be placed in the feedlot immediately as the wasps will arrive almost ready to emerge and attack new fly pupae. Only wasps (no flies) will emerge from these pupae.
Augmenting the natural wasp populations with Spalangia endius won’t immediately reduce fly numbers but will reduce the growth of fly populations.
When?
Start releasing wasps at the start of the fly season (generally October in southern Queensland) and continue until fly numbers decline due to seasonal conditions. Monitor fly numbers in your area to identify when to start and terminate wasp releases.
How often?
Release wasps regularly over a season. Wasps emerge over several days after placement in the feedlot and adult Spalangia endius live for seven to fourteen days.
If wasps are released fortnightly, some will still be alive at the time of the next release. Weekly releases will provide a better overlap, but are more costly.
Wasps released early in the season have the opportunity to breed and increase in numbers.
How many?
Release 50 – 200 wasps per animal per week depending on the size of the animal facility.
Use 100 wasps per animal per fortnight in large intensive animal facilities and
200 wasps per animal per fortnight on smaller or less intensive holdings.
Where?
Place parasitised pupae in release stations (Figure 4) or in protected areas on, or near, the ground. Distribute across the whole feedlot and target major fly breeding areas. Use of release stations protect the pupae from heat, direct sunlight and predators such as birds and ants.
In Figure 4 the release station is made from a piece of PVC pipe (250 to 400 mm long, 100 mm diameter). It has 10 – 20 round openings which are covered with fly mesh to prevent pupae from falling out but allow wasps to leave the container.
Use wire or ties to attach the release station to fence posts or rails – approximately 200-500 mm above ground, close to fly breeding areas but out of reach of inquisitive animals.
Mix the parasitised pupae 2:1 with vermiculite (hydrated mineral; Grade 3) and place in the release station. This provides ventilation and space for wasps to move after emergence.
When refilling the release stations, you can drop the old fly pupae and vermiculite on the ground as they are both non-toxic.
Figure 4. Release container for parasitic wasps. Image courtesy of the Peter Green, Queensland Department of Agriculture and Fisheries.
Related links
For more information about obtaining Spalangia endius visit the ‘Bugs for Bugs‘ website.
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