Sometimes, when you are working with an insect or colonies of insects, it can be easier on them to put them to sleep first. Perhaps you want to move a ant colony from one place to another, but don't want them to injure their brood when they get worked up in a frenzy.
Perhaps you want your insect to be very still so you can paint codes onto it or so that you can make it magnetic. Maybe you want to reduce the injury to your cockroach during very delicate cyborg surgery.

Whatever the reason, here's a way to get a device that's

portable
inexpensive ($30)
Fast acting (some insects fall asleep in a second or two)
Safer than some methods (some other successful methods include dipping your insect in ice water, or putting in the freezer, but I have had less luck not accidentally killing them with those ways).

In short, it's just a small paintball C02 can modified with a on-off- valve.
For the purposes of demonstrating how docile you can make your subjects, I will also show in this instructable how to graffiti tag a bee.

Step 1: Go to Friendly Paintball Shop

The first step is acquiring a nice small, controllable, portable container of C02. Now if you want a huge tank of C02, you will probably want to go to something like: http://www.airgas.com/ . I wanted something I could toss in a backpack and use in the lab, at home, or in the field.

From the previous instructable https://www.instructables.com/id/Paint-Ants-for-Science/, jgencinc (https://www.instructables.com/member/jgencinc/ ) suggested finding a paintball shop.

In the Atlanta area there is a great one called Classic Paintball with super friendly people. Simply describe the problem to your paintball people, and if they are good, they will know that they need to remove the normal paintball valve, and install a hand-turnable one. This involves lots of twisting, sealant, and torches. They were nice and did it all for me! You will also need a little hose to attach to the tank. My total was 31 dollars for all of it!

Bring some sample insects to test out, (the paintball people might want to see their new device in action!) and make sure to dunk your new tank underwater to check for leaks.

Step 2: Gas and Manipulate the Subject

Slide the tube into the vessel where you are keeping the insect, and slowly turn the valve to release a trickle of c02 inside. If you go full blast, your insect will go flying around! Keep the subject in here for ~1minute.

Your insect will be passed out for a duration proportion to some combination of its mass, metabolism, and how long you keep it inside the C02 canister. Remember that C02 is heavy, and will sit in the bottom of the container, don't leave your insect in there too long (like an hour) or it will probably die!

If your insect starts waking up too early, just gas her again.

After this you can do whatever you want with your insect! Sharpie has these awesome new paint pens that work great for adding your tag to an insect. Now everyone in the neighborhood will know which bees have been visiting you!