its been a long day in NYC but if I remember your specs correctly you are keeping them (the flies) in the drains. try to keep them out of the drains in the first place. Difficult task I know most of the time they are brought into the location from an outside vendor.
If it works out let me know I come from a pest control family, the old man still owns a company on Boonton & I am currently working for a "major" pest control company. the industry is always looking for a better way to resolve problems. You make 'em cheap. we'll buy them & jack the price up %150

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Also if you come up with a prototype let me know I can pass it through a couple of entimologists I know.