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Not if they are white-ish. This thing will not make a mark on white copy paper even after 10-20 sec exposure. Dark colored wall paper might be a problem but then you won't be able to see the critter anyway with it blending in. In practice when you hit a critter it will scamper away or fall to the floor stunned (like moths). It all depends on the accuracy and steadiness of your aim. I've blasted small wasps nests larva on the porch ceilings with good success. In general the darker the color of your target the more immediate the effect.
Sure, anything larger than what I've used should work assuming you can get to the internals. You'll also need to customize the trigger with a high current switch and also drill out for the charging port. There's always trade offs to consider but it's all about having fun!
You can put the laser inside the body and use it like a pistol but you'll be very challenged to hold it steady enough to burn a mark in a target. Also the batteries may not fit and you'll have heat build up to monitor and deal with. Putting the laser at the end gives you the needed aiming stability. One hand holds the grip the other hand steadies the tube. These are just my opinions having made five of these things so far. I'm still searching for the optimal configuration...
Laser Based Bug-blasterView Instructable »