Hmmmm, I wonder...
The paper claims "1 - 5mm" for a mosquito, which seems plausible. Lets pick 5mm out of pure optimism. The new(ism) HQ Raspberry Pi camera modules are 12megapixel ~4000x3000 pixel sensors, if you assume you could recognise a mosquito based on it's flight/movement pattern rather than needing to have an image of the mosquito, maybe having a 2x2 pixel resolution of it might be enough, so you could use an appropriate lens and cover a 5mm x 4000pixel / 2 = 10,000mm or 10m wide by 7.5m tall region. That seems totally do-able. Even up to 4x4 pixel per mosquito at 5m x 3.75m would happily cover the wall of my bedroom with the windows. You _might_ even be able to do that with the cheaper 8 megapixel V2 camera module instead of the HQ one.
The background subtraction is fairly easy to do in OpenCV. Assuming there's not too much moving in the background (I might need to tie back my curtains?) I wonder if the path a mosquito traces out would be distinct enough to detect?
Research (read "the first google hit) suggests mosquitos fly at 1.6-2.4kmh - call it 2 - which is just over half a meter per second, so about 20mm per frame at 25fps, or 8mm per frame at 60fps. If we choose to use the 4px = ~5mm numbers from above, that means if you detect a thing that might be a mosquito, you don't need to search very far around it in the subsequent frame to track it - if it's a mosquito, it's likely to be still there and within 6 or 7 px of where it was in the previous frame. (I've never tried doing this on something so small, but that technique works really well tracking people across multiple frames in a video. See: https://data-flair.training/blogs/python-project-real-time-h... )
I don't think that at least x,y detection of a mosquito in a 12 or 8 megapixel camera is impossible. The 8MP raspi camera has a 63deg horizontal field of view, so that 5m wide assumed view from earlier would be at 5m distance give or take. If you could get your shoot-down laser close enough to your detection camera, you might not need to care about the z axis. The camera module is about 20mm square - at 5m range shooting the laser across the edge of the camera module pcb might be "close enough"? Might not work at short range, though once you're close you could scan the bean around a bit, it'll be super obvious in the camera view when you've got a hit (assuming the background doesn't;t reflect the laser brighter that a lit up mosquito? Great, now I have an excuse to paint my walls with VantaBlack...)
Another idea... Assume a similar setup - a camera aimed at a 5x3.75m cross section of space, with a line laser shining across it (one of those lasers shone through a cylinder that makes a line, like some power tools use to project guidelines). I'd guess it'd be possible (maybe even easy) to distinguish between dust mites and mosquitos lighting up as they cross the line laser? Perhaps have three co-planar line lasers 10mm or 20mm apart to make the timing of aiming your shoot-down laser easier, use the first flash as a detection signal and swivel your galvos to that region, and then use the 2nd/3rd flashes close by to quickly fine tune the aim without needing to swing too far?
(Now I just need to work out which if any of my half finished projects I should stop working on and half finish this one... ;-) )