Here's the Waspy thing info I had back when I asked the question: (Information from Chicken Vet)
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The parasitic wasps always sound a bit scary but they are tiny, only approx 2mm in size when they hatch, they are totally harmless and you did not see an influx of another insect replacing the fly! ...............
......They are transported in bags with some grain as a filler, they are all unhatched pupae, as the temperature rises they hatch and then burrow into the fly pupae to lay their eggs thus destroying the emerging flies. You literally spread them by hand onto dung heaps or through the slats in a poultry house.
We store them in the fridge to stop them hatching and then when they are spread the hatching process begins. They are also completely organic but they only kill the fly pupae so to control adult flies you still ideally need to use a knock down product, however the beauty is if you use this once a fortnight for 6 treatments (12 weeks in total) during the worst of the fly season you dramatically reduce the amount of chemical used.