I thought Sally made some really insightful observations about tunnel traps and A24s which you might want to listen to, if you haven’t already!
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Thanks for the link.
I was listening to Sally’s interview and have had pretty much exactly the same experience over the years using A24’s in our project area. There was only one time a few years ago (mast year) that we ever found rats under the traps a week after rebaiting. I pulled in all the A24’s from our backcountry lines and installed them next to DOC 200’s in hopes they might keep them free for mustelids. I’ve recently been trialing different lures (terracotta etc) in the bait pods with cameras on them for at least 3 months and there has been ZERO activity picked up on any of the A24 traps. 
I keep wanting them to work, but still not convinced.
Yes I also find tunnel traps far more effective. However I wonder if an A24 could be combined with a tunnel to produce a far more effective unit. The issue would be clearing the rat carcasses, but I think it would be possible.
I also think an automatic lure feeder is required.
On the positive side, the units are well engineered, robust and the kill mechanism is very effective (provided the gas hasn’t been used up or leaked out).
I did kill five rats in one night with an A24 but that was in the initial knockdown phase. Since then progressively fewer.
I’ve also wondered if housing A24’s in some sort of tunnel could make them more effective, but I can’t imagine how the problem of carcass-disposal could be solved.
A corpse catapult? An acid bath?
I’ve only found a few rats under my A24’s, but quite a few mice/small rats, and no mustelids, so far. They’re definitely a good phase one weapon, when starting predator control in a new area, or if there’s a mast year that overwhelms single-kill traps, but they aren’t a good trap for medium and long-term control on the mainland, generally-speaking, according to studies and my personal experience.
I do think that A24’s have the potential to work well when paired with DOC tunnels at sites with high kill-rates, though, because predators that are wary about entering a tunnel might interact with an A24, instead.
- Some predators might prefer the lure in the A24 to whatever’s inside the tunnel(s), especially if it’s past its gone stale, moldy, rotted, or if mice/insects/invertebrates have eaten most of it.
- A DOC tunnel lured with meat, paired with an A24 using the nut or chocolate lure, would appeal to different tastes, which should result in a higher overall interaction-rate. I’ve had way higher kill-rates since I started to use combinations of lures. This applies to snap-traps, too. Try using PB and Nutella at once, or PB and Goodnature’s Blood Lure, etc.
- Having a trap at a site that uses a long-lasting lure, and a trap that uses a short-lasting lure, is better than having one with a lure that loses it appeal within a few days. When trapping stoats, for example, it’s better to have fresh rabbit in half of your tunnels, and salted rabbit or Erayz in the other half, to prevent all of your tunnels becoming unattractive within a few days.
- If a predator is regularly visiting an A24 to scavenge, and there’s nothing under the trap, it might explore the DOC tunnel, instead. Likewise, if there’s a rotten body under the A24, the DOC tunnel might be explored as Plan B.
- A predator that’s an expert at stealing lures out of DOC tunnels, might be killed by an A24. Having 2 traps that work so differently, side-by-side, should make a difference.