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  Flies remain a headache
for UK farmers. Apart from the hassle factor, flies
have been associated
with reduced weight
gains and greater
incidence of disease.
Fly Parasites
In nature flies suffer with parasites just as mammals do. The UK fly parasites are pteromalid wasps. They live 10 to 30 days and generally produce 30 to 60 eggs. Cooler temperatures extend their life while it can get too hot for several species in hotter countries.

In the UK we have permission from DEFRA and English Nature to release two indigenous species, Muscidifurax raptor and Spalangia cameroni. At optimum temperatures (around 25oC) Muscidifurax has a lifecycle of around 18 to 24 days while Spalangia has a lifecycle of 21 to 28 days.

Using the Parasites
The bags we use in the UK contain approximately 50,000 parasitised fly pupae. In deciding how many parasitised pupae to release it is important to bear in mind that the size of fly breeding area is more aligned with fly population than are stock numbers.



  
These species are not too fussy about which species of fly they will parasite. However, Muscidifurax raptor tends to prefer to parasitise house flies and Spalangia tends to prefer to parasitise stable fly (Stomoxys). Spalangia is a little more active over a greater range of temperatures than Muscidifurax.

Depending on how active the parasites are when released, peak parasitism usually occurs two weeks after release. In hot weather parasites are only active for about 1 week.

Parasite Trials
One published trial involving fly parasites was carried out by Miguel Zapater of Buenos Aires in Argentina. In the summer of 1996 he recruited four trial farms (farms 1 to 4) and monitored two farms that used no parasitic fly treatments. (farms 5 & 6 - ‘control farms’) Each trial and control farm had two sheds typical to the farm and comparable with each other. Three cards measuring 12 cm by 7 cm were hung in each shed for 7 days. Fly ‘spots’ on each card were counted and recorded. The results were as follows:

  
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