The stuff in apricot kernels is amygdalin, which is converted to cyanide by the intestinal flora in the gut when ingested. It's just as toxic for rats as it is for humans, but maybe they're not getting a big enough dose. Next time your apricots are in season, save the pips after eating the fruit and bust them open to get the kernels, then try using them as rat poison. If the rats don't fill up on the apricot fruit, they may be tempted to eat more of the kernels.
Commercial rat poison is brodifacoum (a super anticoagulant - makes them leak blood internally). Cyanide works differently, so maybe a combination of the two will be more effective than rat bait alone.
Actually, if you use rat bait and have small children or pets, it's worth knowing that vitamin K is the antidote to rat poison. It might be advisable to keep some tablets on hand in case of emergency. A bottle of pills is only a few bucks and they last a long time in the fridge.
A couple of days after we moved into the house I now live in, my dog got quite ill. She became very lethargic and her gums were quite pale. We had no idea what was wrong at the time so we took her to the vet. He declared that she'd been poisoned, probably by rat bait. We later found rat bait in the wood shed (left there by the previous owners of the house). The dog evidently found it before we did and ate some. Vet gave her a vitamin K injection followed up with daily vitamin K tablets for a couple of weeks. She recovered OK.