The park began looking into the chips after 17 saguaros were stolen in January 2007, the second such theft in recent years… The devices are about the size of a grain of rice and are injected with a hypodermic needle…Cactus theft can be highly lucrative. Presumably sophisticated criminals will get their own chip readers, but the program may deter amateur thieves.
The program could include spot checks at nurseries, which sometimes buy from thieves who forge removal permits for the protected plants…Chips would be put only in small cactuses near roads - the ones thieves target. "They're looking at saguaros that are generally 4 to 7 feet, something a couple guys could manhandle into a pickup truck.”
Because the chips are passive - they have no batteries or moving parts - they could last more than 100 years…
The chips cost about $4.50 each, and chip readers cost about $600 for handheld models and $2,500 for larger, more accurate ones.
Link found at J-Walk.