Mouldy bait? Potassium sorbate

There may have been discussion of this previously, if so, happy for someone to point it out.

I am sure there are many of you out there that suffer from baits going mouldy. I have experienced it (particularly in wet warm conditions) with Nutella, peanut butter, apples and salted rabbit. Sometimes the bait may last many days, but at other times the bait can become mouldy in a couple of days. I check my traps every day so I can replace mouldy bait, but it an be rather time consuming. More importantly, if I go away then the traps may be left with mouldy bait for many days. This may well affect palatability of the baits.

I decided to try adding a preservative that is well known for its anti-microbial properties, namely, potassium sorbate. Potassium sorbate is extensively used in the food and cosmetic products. It is also used to stop (kill the yeast) fermentation in wine making.
From a safety perspective, potassium sorbate does not appear to be high risk to people, at worst it is a mild irritant. I avoid breathing it by wearing a mask and wash my hands, but that’s about it. It is odourless and tasteless. Many people are eating it and putting it on their face every day! It is also pretty cheap and available from brewing shops. I imagine it may be the magic ingredient in some fast food items that can be preserved for years :slight_smile: but (disclaimer) I really don’t know and am not pointing the finger at any particular restaurant.

So the questions I had were:

  • Does potassium sorbate extend the life of baits?
  • What concentrations are necessary?
  • Does the addition of potassium sorbate affect the palatability/attractiveness of the bait?

Whilst I acknowledge it would take many months of rigorous research to answer these questions, I have come to few qualitative conclusions:

Does potassium sorbate extend the life of baits?
Yes. It appears very effective in the case of apples and salted rabbit. It is also quite effective for Nutella, but I have not optimized the concentration. I think the concentration needs to be considerably higher than is typically specified in foods and cosmetics. I have not trialed it for peanut butter.

What concentrations are necessary?
In the case of apples and salted rabbit, I just sprinkle a little on the surface of the bait, no need to be too precise.

Nutella - I am currently using about 0.75% by volume. I lack an accurate balance to measure this as precisely as I would like. I am still experimenting to find the optimum level, ideally I would control for humidity, temperature, initial mould spore counts etc. This is rather beyond my expertise, but I may try to control for humidity and temperature.

Does the addition of potassium sorbate affect the palatability/attractiveness of the bait?
No. There seems to be no effect on catch-rates for rats (Nutella), apple/cinnamon (possums and hedgehogs). My catch-rate with salted-rabbit is extremely low, so I can’t draw any conclusions at this stage.

Just a couple of other unrelated points:

  • making peanut butter or nutella less viscous - add glycerol (another food additive)
  • making peanut butter or nutella more viscous - add silica powder (but be very careful not to breath this).

I use these method when I bait in my A24s depending on whether it is Summer or Winter. You may have other uses.

Happy to field any questions you might have, or hear of your experiences.

In a related matter, I will likely shortly post on using lures in Victor traps. Let me know if you are keen and this will speed me up.


Hi Dan. Great post thanks. First off I’ll say I hope you, and anybody else with a ‘try anything once’ mindset, when it comes to improving trap results, will come join our recently created Facebook group Trapping Outside the Box NZ. If that doesn’t work for you Dan could you pm me there somehow.

Had not considered potassium sorbet but do want to give vinegar a try. Both to kill mould in a trap, and as a diluted rinse for lures before they are used. Also shellac, which a study found preserved toxic baits for up to 9 months and stoats preferred it. I have also been trying honey, for it’s mould free properties, in screened SA2’s. It works great as I lure, but I have to work on convincing it not to run away. More in my discourse / rant in the pinned post at the group site.

Have a number of modified Victors that haven’t been working at all so any tips would be appreciated. Those were vertically on trees off the ground but have since removed them all.

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Thanks for your comments Ron. I will look for your Facebook group.

Do you paint shellac on the toxic baits? I can see how that would preserve them but I am not sure how the stoats smell them! Interesting.

As I mentioned in my post, you could try mixing the honey with silica powder. Not quite sure where you would get this (I already had some) but other inert oxide powders would probably work too.

When you refer to “modified” Victors, are your referring to the ones with the white plastic shrouds? I have to admit to removing all the shrouds because I failed to catch anything with the shrouds in place.

It was also recommended to me that I mount Victor traps on trees. I hardly caught anything like that and now all my Victor traps are in wooden tunnels. I do have a D-rat trap on a tree which seems to work fine, and the design protects the bait from the weather very well. I suspect there may be large variations in rat preferences depending on local geography, vegetation, availability of food types etc. but I don’t have any data to prove that.

I have caught around 500 rats (almost all ratus ratus) over the last two years. My next post will refer to the used of teracotta lures in standard Victor traps.

I don’t know how best to apply the shellac. I only know, from reading the abstract of the original study, that it works as I stated. I want to try painting (or dipping) Erayze with it. I refer to Erayze as being ‘the best of baits; the worst of baits’. My first experience with it, was in the Ruahines, clearing somebody’s months old previous set. From about seventy 200’s I cleared something like 22 stoats and 19 rats. Those were in plenty dry conditions - and followed a recent mast. In optimal mould conditions (August Taranaki) Erayze becomes an unrecognizable blob of mould in two weeks. Mould a problem with it everywhere but just not as fast. Want to try other preservative possibilities with it, like a rinse in dilute vinegar or would try a solution of your potassium sorbet. I would expect that painting it with shellac would mask it’s attractions as a lure, but won’t know til it’s trialed. If stoats prefer the shellac, might shellac flakes be used somehow as an attractant?

Yes they were modified Victors. We have some of the larger yellow trigger plates coming. I want to try them next, on the ground, in coraflute tunnels.

It will be interesting to hear what you find with the shellac. You are right, if shellac is so tasty to stoats why would you bother with the Erayze! I have also found mouldy blobs after two weeks during the Winter. I am in the Wellington area, but I imagine this would happen in much of the lower North Island bush (wet, cold and mouldy). The rate of mould growth is (unsurprisingly) dependent on whether the Erayze stays dry - being salted it can absorb water and weep, which doesn’t help. I am sure none of this is news to you :slight_smile:

I also replaced the shrouded Victor triggers with the yellow trigger plates. A great improvement.

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This is a really good chat ! Connovation’s D-Block Extreme has something that resemble shellac, it seems to work well and the animals find it easy enough.

Dan do you know if silica powder settles out over time?

Regarding the Victor Shrouds, we tried them also but had low interaction rates. We ended up making a hybrid that doesn’t need the rat to pull on the bait, just a foot on the yellow trigger plate. That seemed to be the key difference. We put them on the PF Franklin website if anyone is interested:

Thanks Andy, well it sounds like shellac might be a winner. I think the original shellac is a resin sourced from a certain beetle, so maybe this is also attractive to predators. I think one should probably avoid the modern synthetic alternatives, but I might be completely wrong :upside_down_face:. The problem for me is that I have caught so few stoats that I wouldn’t know if it wasn’t working! I am not sure whether this is because there are so few, or I am useless at catching them.

I will look at your link and the D-block.

Whether or not the silica sediments depends on the viscosity of the liquid and the particle size/density (and to be absolutely accurate the zeta potential of the surface!). But I wouldn’t expect the silica to sediment in a viscous liquid such as honey or nutella. At least I know it doesn’t in nutella from practical experience.

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Good info all. I’ve had a rethink on the shellac trials. Will give the very old, new technology a go. Terracotta lures look to be the way to beat mould and post kill bait theft. I ordered 50 of Conavations salmon along with a bottle of the salmon lure. Rough math says $1.40 each for the scented lures. I’m in Northland, full time volunteering for a trust. (A retired covid refugee from Canada. Much extended visa running out in two months. Will be two years here with ten months trapping in three regions.) DOC200s plus SA2s baited for possums but want to intersperse those with some cat lures.

Also have 50 modified Victors and will be changing 25 of them to the yellow treadle. Terracotta baited they will be, but will try a bit of chalk too. Have a variety of essences to try.

Thanks for the links.

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Thank you very much for volunteering your time and effort to kill pests!

I’m a Canadian that’s lived here since 2005, and I’ve really ramped up my pest control in recent years, and the number of insects, lizards, and birds has skyrocketed, and there is way less browsing and grazing, too.

Please apply for citizenship/dual-citizenship - we need all the trappers we can get!

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I’ve also had poor success with salted rabbit, but I haven’t used enough of it to know how much of that was due to luck. A large DOC study comparing fresh and salted rabbit concluded that salted rabbit is about 80% as attractive as fresh, so the authors recommended its use because it lasts longer. I’ve had success with Erayz, but it’s better in warm, dry weather. The only thing that I’ve ever used that doesn’t grow mold is hens’ eggs or fake eggs.

Corn starch should be a good way to thicken PB or Nutella, and it should make the bait even more appealing to rodents, given their love of grains.

From the reading I’ve done, the best mould-resistant rodent bait might be hard candy. It’s high sugar content and low water content apparently slows down the rate at which mould grows. In theory, hard candy inside a dry trap could last for some time. A piece of hard candy might work in a snap trap with a bait cup, and mouse-proofed candy in a DOC tunnel might work, but the odds of trapping a mustelid would plummet. They can be caught in unbaited traps, so a sweet smell has the potential to lure a curious individual. Insects will find hard candies more difficult to detect than soft sugar sources, and it would take them a lot longer to eat a lemon drop or popsicle than a soft bait.

You could try using a 50/50 of thick honey and molasses, and then stir in a thickener, like arrowroot or cornstarch. Straight molasses might work pretty well, too, because it is approx. 75% sugar. It would be way harder for insects to take because it’s so thick and sticky, and I bet some of them would get stuck in it, adding some fat and protein to the bait. I think birds would be more likely to investigate a honey bait than a molasses one, also, because it doesn’t smell as sweet as honey.
There are some baits on the market that combine PB and molasses, so that’s something to consider. Add a bit of salt to, and it should be a really effective, cheap bait. I’m definitely going to try this.

Vinegar is a rodent repellent, unfortunately. Apparently, apple cider vinegar is a good thing to spray in cages when you’re trapping rabbits or hares. I bet possums would investigate a trap that smells like ACV, too. A spray of ACV on an apple in a possum trap might keep it fresh a bit longer.

Update on use of Potassium Sorbate

I thought I would give you an update on the effectiveness of potassium sorbate (from you local home brew shop) on various baits. Note, potassium sorbate is odourless, tasteless, and I have seen no change in bait attractiveness to rats, mice or possums.

I’ll start with my main conclusions to-date, and then provide a few more details for those who are interested:

  • Fresh Apple (possums and hedgehogs) - having played with this a bit I am now satisfied that potassium sorbate is highly effective eliminating the formation of mould. You can expect your apple to last for several weeks - until it goes soggy or dehydrates. I have caught possums and hedgehogs in a Timms trap two weeks after putting out the bait. The coated apple seems to be unaffected by exposure to damp conditions (or even rain). IMHO this is a winner. :+1:

  • Nutella (rats and mice) - this is has been more of a challenge and I am still refining the composition. The problem is that potassium sorbate is not soluble in fats and therefore did not distribute well in the mix. I have solved this problem, but I am still hopeful I can improve the mix further. I will report back when I can be more definitive.

  • Salted Rabbit - I have rather given up on this! Potassium sorbate does NOT seem to be effective at reducing mould on salted rabbit. I am aware that others have had some success with acetic acid (vinegar), however I have my reservations about accelerating the corrosion of traps using this method, and it only appears to be partially effective. I will keep thinking. :-1:

Details of mix for fresh apple

  • 1 tablespoon of dried cinnamon powder (add ground cloves if desired)

  • 1/4 teaspoon of potassium sorbate

Mix together in a pestle and mortar (or whatever you have) to distribute sorbate in cinnamon.

Dip cut apples in powder mix - you’re done!

Picture of coated apples after several weeks in a Timms trap - look no mould :slightly_smiling_face: :slightly_smiling_face:!

Ron, do you have a link for that facebook group please? Thanks.

Thanks for the post Dan, interesting … I have one query - you mention"when I bait in my A24s". What do you mean by this? Are you using something other than the supplied ALP? Thanks, Tony

Hi Tony

I use a small HDPE bottle (which I screw into the bait chamber) filled with a nutella/potassium sorbate mix. The bait seems to last a bit longer than the manufacturers product and is a lot cheaper (plus no additional packaging etc). You could just use unadulterated nutella if you prefer, but it will form surface mould over time - I am still working on the best way to reduce mould formation. I have made progress but it is more challenging that I had envisaged. I don’t want to send people off in the wrong direction, so I am holding off recommending anything just yet.

Bottle for A24 - made from 60 ml nalgene bottle (cut in half)

It is much easier to stop mould formation on apples, as I have described in my post. The very high sugar content of nutella, combined with exposure to moisture and mould spores, is the challenge.

Sometimes I muse that if the govt/Predator Free spent some money employing a Food Scientist to formulate a range of edible, moisture-stable baits, we could make better progress. I would prefer these formulations to be open-source rather than made into unnecessarily pricey (no disrespect to the manufacturers :-)) commercial products.

IMO we have rat traps (I mainly use Victors, DOC and AT220) that work pretty well, often the problem is rapid bait spoilage or loss, rendering the trap completely ineffective. We need to encourage people to trap and make it cheap and less labour intensive. Just my two cents…

Hi Dan thanks for reply … the mayo lure used in the AT220 seems to last a long time now, though of course it is kept away from the air inside the bottle and regularly replaced automatically by the pump. I wonder what they added to keep it lasting better than the original lure that would turn to jelly in quite a short time … I am very interested in finding a way to make use of my A24 and A12, both of which are currently in the garage as it is not worth the money to reinstall them. I think the actual kill mechanism is very good, but I haven’t found a way to get possums or rats to stick their heads in the trap! I think I caught 3 mice and had 3 other supposed kills (no bodies) in the A24 in 2 years which is a very poor return on investment. Maybe what you are doing would work - are you catching rats in the A24? Doc200-style traps in tunnels plus Victors in tunnels have been very successful as have Timms traps, Possum Masters and the AT220 before regular trapping followed by an Ospri project put a halt to regular possum visits. I am disappointed I haven’t caught a rat in the AT, just 2 possums, but location may be the problem for rats which I generally catch on different parts of the property. Cheers, Tony

I can only speculate with the A12 as I don’t own one - as others have commented- perhaps it isn’t a very natural movement for a possum to put its head in a blind hole. However, to contradict that, many possums put their heads in Timms traps, so maybe that isn’t the whole (excusing the pun) story. I have video evidence that some possums a very reluctant to put their head in Timms traps, or take toxic bait, and leg hold traps are the only option.

I have two A24s - initially, when I had a large populations of rats, I found the A24s quite effective, sometimes killing several rats in a night. However in recent times the A24s have not killed any rats. This is most likely because I have killed most of the rats who will put their head in a blind hole. I have found Victor-tunnel traps by far the most effective. Having put an A24 side-by-side with a Victor-tunnel, the Victor continued to be very effective whilst the A24 recorded no kills. I think the lateral constraints of the tunnel, and its open mesh end, are critical to the Victor’s effectiveness. I have wondered if it would be possible to build the A24 into a ramped tunnel, but I haven’t taken the time to try this, and I don’t know if it would do the trick. I agree the A24 is an expensive trap if it does not work better than a Victor!

AT220 - I am quite impressed with the design and operation of this trap. I have caught several possums with the mayo lure but no rats (unlike others). When I previously tried mayo in my Victors I had no success there either. My rats love nutella, so I think (a lot of speculation in this post!) the preferences of rats may depend on location. My rats don’t much like peanut butter either.

Yes I have caught quite a few rats in the AT220 since I swapped the mayo for my own liquid mix of nutella, so the problem is not the AT220. In fact the AT220 has outperformed an adjacent Victor-tunnnel trap two-to-one over the last six weeks.

Photo of rat attracted to nutella mix in AT220:

(Getting carcasses stuck in the trap is the subject of another thread.)

One thing I want to try is diluted Connovation chocolate (or others for that matter) lure. I have found the lure very effective when used in conjunction with terracotta bite blocks (see my post on terracotta lures) in Victors, so I can’t see why it wouldn’t work.