I have some excess feeder crickets that I no longer have a use for.
What is the most humane way to get rid of them?
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First of all, don't release them into the environment without checking with your local authorities whether it's okay to do so.
Chances are that the captive crickets you have bought are not local to your area. The introduction of non-native animals to an environment unused to them can disrupt the local wildlife by introducing new predators, spreading disease and introducing competition for food sources.
The most humane and efficient way of disposing of them, especially if you have large numbers, is to tip them into a bag, seal it closed and place the bag in the freezer for 24 hours. (You can double-bag it if you're worried about contaminating your food.) After 24 hours all the insects will be dead* and you can dispose of them with your normal household waste.
Insects are ectothermic, which means that in cold temperatures their metabolism just slows down until they die. The insects go into a cold-induced coma from which they do not recover. In fact freezing - albeit a more slow reduction in temperature - is the preferred humane method of culling for some insect farms:
when they're culled we lower the temperature so that there's no violent death or change in state (because insects are exothermic their metabolism slows until they go into a coma-like sleep without any pain). I can't think of a more humane way to raise our meat.
Our crickets are harvested by freezing. As with many insects, crickets will slow down in low temperatures, and reach a state called diapause. Once they've gone dormant, we freeze them in a deeper freeze. Not only does this humanely kill the crickets, it also ensures that the bugs remain as fresh as possible until they're ready to be prepared as food or dehydrated and processed.
You could attempt to emulate the more gentle freezing method, perhaps by use of a water bath or by repeatedly taking them in and out of the freezer. Or you could accept that freezing, even the quick kind, is probably as humane a death as the death they would have got from being eaten ;)
I have also heard the suggestion that you can crush their heads with feeding tongs or tweezers as a humane way of killing them. However, it's not very practical if you have hundreds of them. There is also some debate as to how humane this method is, because the insects remain alive for a while after having their heads crushed. I found a method for killing cockroaches on a forum which suggests a more humane adaptation of the "crushing method":
If you need to swiftly kill them [...] crush the head and make two quick incisions on their ventral side (between the last two pairs of legs and in the middle off the abdomen) you will humanely euthanize the roach. The roach's nervous system runs ventrally and making those two incisions after crushing the head will [e]nsure that the body does not go through excessive pain, since it will still be (by many definitions) alive after the head has been dealt with.
This suggestion was for a single insect at a time and is even less practical than simple head-crushing if you have large quantities. However, if you need to kill only a few insects it sounds like a humane option.
* Note that you may see reports of frozen crickets reviving, but these crickets have typically been frozen under carefully-controlled conditions. For example, the ones in this study (the one referenced by the report I linked to) slowly cooled the crickets from 25°C to 0°C over 1 hr 40 minutes, then held then at 0°C. In contrast, a typical home freezer is -18°C and the crickets will reach that temperature within minutes due to their low body mass:surface area ratio, subjecting them to cold shocks, unrecoverable tissue damage and death.
For more information see @WYSIWYG's answer here.
Inert gas asphyxiation may be effectively painless, and certainly more humane than death by extreme temperature.
Here is an article describing a study done on the effects of nitrogen gas on kittens and puppies.
Nitrogen can be purchased in canisters, and any airtight container with a relief valve will work; simply cycle the air out of the container until the crickets go completely dormant, and leave it sealed for a day or so. This is best done outdoors, or with proper ventilation otherwise.