Estimating rodents killed from bait take reports

I note in the front, landing page of TRAP.NZ website, that it shows
" 208,313 Rats" have been controlled.
Is this count from traps only, or does it include an estimate from bait station take?
If from bait stations, how is that estimate derived? Do you assume that each fatal dose of bait taken roughly “equals” one dead rat? Or is there some standard/accepted ratio that says for each x kg of bait of this type, we expect y rats or ice are killed?
It would be really good for reporting if we could say from our baiting efforts that eg “given a bait take of 880 bait blocks over the year, an estimated <n?> rodents have been removed from our project area”.

It’s not a simple calculation.

In our group we use Ditrac, which is a first generation poison. It means rats need to eat quite a lot to actually die. Which in turn means the bait stations need to be kept full.

I emailed the manufacturers (Bell Labs in the states) and this was their reply:

Thanks for your inquiry. Using a known lethal dose 50 (LD50) estimate of 3 mg/kg for diphacinone determined in lab studies, a 250 g rat that consumed ½ of a block of Ditrac (14 grams) all at once would have a 50:50 chance of dying. Other lab studies using multi-dose administration of diphacinone estimate an LD50 of 0.3 mg/kg, daily for 5 days. From this, we can estimate that a 250 g rat consuming 1/20th of a block of Ditrac daily for 5 days (total of ¼ of a block, or 7 grams) would also have a 50:50 chance of dying.

Depending on the frequency of consumption, ¼-1/2 of a block of Ditrac approaches the lethal range for 50% of rats. To get closer to 100% lethality, the dose would be higher. Because of the delayed action of anticoagulants, the rat may feed for several nights before dying, and is likely to consume more than what is required to kill it.

It is difficult to predict how much bait will be eaten, this varies considerably with food availability, population size, and daily food requirements. However, if all stations are rapidly emptied, placements should be at the maximum allowed by the label and checked daily until consumption starts to decline to avoid under-baiting.

From all of that, we basically work on the basis that a rat needs to eat about 1 28g block of bait to die.

It’s a rough guide, but better than nothing.

Thanks David

The bait we use is Contrac (active ingredient bromadiolone). One meal of one 28g block is enough to kill them, & they don’t need to come back for more to be killed. Death is likely some days away however, so they may come back on subsequent days and eat more. Or they may be mice/small rats and need less to kill them.

Estimating based on
one bait taken = one rat
sounds sensible to me - at least until/unless someone works out that’s not correct. I was told someone was doing a study … but sadly they didn’t reply to my request.
Many thanks.

Contrac is a bit stronger. So it should kill more rats for less bait. But it’s still not a bad estimate.

My understanding is that a whole 28g block of Contrac will kill most rats. You used the LD50 measure, I’ll have to think about that some more, & how much it might impact on the estimate.
But I’m not asking in order to inform the baiting protocol, but to estimate number of rats killed based on bait eaten (or at least bait taken, which could be different).
So when you estimate the number of kills for NZ, do you use the same weight, 28gms, regardless of the bait on offer?