Ectofly, Crovect and Dysect are all synthetic pyrethroids which basically have an ability to kill a certain number of flies per dose. How long they last is very dependent on the season and the current fly challenge. It works like blotting paper and when a fly lands and then subsequently flies off and dies, well it takes an amount of active ingredient with it and this is why some years (like this year) when the challenge is very high any of these products struggle to achieve the stated cover period. I explain it as "You hang a fly paper up in your kitchen in December and it needs throwing away in April because it's full up, it can't physically kill any more flies - You hang the other fly paper up out of the pack in May and guess what, it needs throwing away because it's full up in just a few weeks!" Same product, it's only the challenge that has changed. Change products and brand names all you like but it's all down to the level of challenge in the environment with these types of fly control products. Sorry it's such a long post...!
Great post well put.