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Dave Edwards: possum proof baffles & bait bag rat excluders
29 April 2019

Section 1: Introduction


Bait interference: an expensive and frustrating problem

Dave Edwards is a well known figure within conservation circles. Over the past 16 years he has held various roles with community conservation trusts and is now enjoying life as a “foot soldier” for groups in the BOP -- maintaining bait lines and doing what is required to enhance the survival rates of kiwi and kokako.

Dave’s involvement with groups has provided him with valuable experience at the pest control “coal face”. He knows, only too well, the toll bait interference by non-target species exacts in terms of operational costs and the overall effectiveness of a pest control operation. It is an expensive and frustrating problem.

As a passionate conservationist he was determined to find a way to reduce bait interference. If he succeeded it would increase the survival rates of our most vulnerable wildlife species. It would also mean that financially strapped conservation groups would get more “bang for each pest control buck”.

Knowing that expensive or complex solutions were not going to work (not for cash strapped organisations staffed by volunteers)  he opted instead for a “keep it simple, keep it cheap” approach. He hopes that this approach will help ensure that any innovations he does develop, stand a good chance of being deployed in the field.

This article will look at two of Dave’s baffle designs that can be used in existing Philproof (large) bait stations and two new ways to deploy Feratox bait bags. It will also discuss the addition of a rope "ladder" to the base of Philproof bait stations to enhance rats ability to access bait.

All designs have been tested in the field.


Dave's inventions: a quick summary 

Dave is not a newcomer to the world of invention and innovation. Experience has taught him that a systematic approach to the testing and refining of ideas is critical. 

Digital footage from trail cameras in situ played a huge role in the design process for both the baffles and the Bio Bag/Bait Bag holders. Without Dave's painstaking analysis of footage he may well have missed the crucial details that led to designs being reworked or discarded altogether.


The possum-proof baffles ...

From the point of view of someone involved with Kokako recovery programmes, Dave believes that the use of baffles when using Philproof bait stations has the potential to make a significant difference to predator numbers and to outcomes for “struggling or translocated populations …”

This isn't the first time baffles have been tried but camera footage suggests that these ones work quite well.


Rope ladders for rats: enhancing rat access to toxins ...

Camera footage from the Philproof baffle trials revealed that rats often experienced difficulty getting into Philproof bait stations – which, during a control operation, could impede their ability to feed multiple times on the toxin and get a lethal dose. 

Rat access to bait in Philproofs has always been a bit of an issue – in fact it was a suggestion from Dave that led to Philproof adding “lips” to the outside of their bait stations -- the idea being that the lips give rats something to grip as they swing into the bait station from above. 

However the footage clearly showed rats were still struggling to access bait in Philproof bait stations. The "lips" were not enough.

Dave came up with the idea of attaching a rope to the floor of the bait stations. This proved to be a simple but effective solution and appeared to be popular with ground based punters ... (but more about that later)


Feratox Bio Bag delivery systems ...

His two “possums only please” bait bag options also look promising – particularly the suspended bags which suffered no rat interference at all.


While larger scale field trials may generate further refinements to his ideas, Dave is cautiously optimistic about the general direction of his research so far. The prospect of being able to run effective and economical “rat only” and “possum only” control operations would be a major step forward. He also considers that there may also be potential for significant savings in terms of operational costs --depending on the situation.




Section 2: The designs


Possum excluding baffles for existing Philproof bait stations

”Ship Rats are arguably the greatest threat to our native biodiversity. Given the rapid rate at which they breed the benefits of carrying out additional ship rat control as a supplement to normal possum and ship rat bait station operations has long been known, but conservation grps have lacked simple tools to carry out the work. In order to carry out efficient ship rat only control using Philproof bait stations, one needs to be able to exclude, or at least greatly restrict a possums ability to feed from a bait station, but at the same time not restrict rats from feeding at the bait station. Designs to restrict possums from feeding at bait stations during times of ship rat only control were developed. They had to be simple, user friendly and allow the use of all toxins that can currently be used in a Philproof bait station. Field trials were carried out at four sites within a Podocarp-broadleaf forest in the Bay of Plenty between October 2017 and mid-February 2018. The behaviour of possums and rats was recorded with Moultrie trail cameras

from "Field trial: Ship Rat control using modified Philproof bait stations


Knowing that hundreds of thousands of Philproof bait stations were already being used up and down New Zealand, Dave developed a couple of baffle options compatible with existing Philproof (large) bait stations, making them almost totally possum proof.

However it took time to work out what worked and what didn’t …

During the first two months ten baffle designs and variations of a design were trialled. Most of the baffles fell by the wayside fairly quickly as possums had a greater reach and dexterity than I had expected. Designs that showed promise were further developed and retrialled over the next two months.”

from "Field trial: Ship Rat control using modified Philproof bait stations"



  • Philproof bait stations used in this trial were positioned 270-300mm off the ground
  • Only large Philproof bait stations were used in this trial. Camera footage of rats accessing bait showed rats gripped the rounded ledge or lip that protrudes from the base of these bait stations to help them enter the Philproofs and access the bait. Dave is in the process of designing a similar floor with baffles attached for Philproof mini bait stations.
  • “To condition animals to feeding from the bait stations they were kept full of prefeed pellets for 7 days before the introduction of any baffles”.  This gave the animals an incentive to overcome the restrictions presented by the baffles
  • “Bait stations were checked every 3-4 days and refilled to the 650 gram level. Camera SD cards were swapped at the same time so that the data could be analysed before the next check” 
  • No toxins were used during the trials
  • Backriders were the greatest variable


What about bait interference by other predators during the trial?

“During the trial I recorded both feral cats and stoats taking an interest in the bait stations. I guess they were attracted by the rat activity as they had a good look at and around the bait stations then wandered off”

from "Field trial: Ship Rat control using modified Philproof bait stations"


 Lets have a look at the two baffle options …



Baffle #1


Consists of a reversable bait station floor. On one side there is a perforated, “raised plastic baffle screwed to the underside of a standard floor”. The perforations allow the scent of bait to disperse and were also used as footholds by the rats as they climbed the baffle -- enhancing their access to the bait.

A key feature of this simple baffle design is that it is always there when you want it. The baffles are already in place so users do not have to carry tools or spend valuable time attaching the baffle to the floor.



For ship rat only control simply turn the floor over so the baffle is up. To return the bait station to its normal state the bait station floor is turned over and slid back into place.

This design did not appear to hinder rat access to baits but drastically reduced possum bait take to just 6% (adults 1.25%  backriders 5%)


NOTE: Dave made the point that during a normal August bait station operation backriders are not normally independent enough to be feeding from the bait station




Baffle #2

“To try and reduce the amount of prefeed backriders could remove from the bait station a second baffle design was developed”

from "Field trial: Ship Rat control using modified Philproof bait stations"



This design uses a restricted opening combined with the same raised baffle idea shown in Baffle #1. This baffle is removable and simply clips into place when required. The baffle is fixed in place by two dowels that are permantly fixed to the front of the baffle. These are pushed through two holes drilled  through the front of the bait station. The baffle is held in position by the floor. An alternative fixing option is to use a piece of #8 wire secured by a hole on each side of the base, at front of the bait station.

The addition of the restricted front opening prevented bait access to “backriders” which were small enough to reach beyond Baffle #1. This design also “had the advantage of the lip and floor of the front baffle for rats to grip and aid … entry into the bait station”.

Baffle #2 remains in place when filling the bait station

This design did not appear to hinder rat access to baits but reduced possum take even further to less than 1%


A word about Philproof Minis … 

Design differences between Large and Mini versions of Philproof bait stations make it more difficult for rats to access baits from the Philproof Minis.

The lack of a front plate on the floor of Minis seemed to make it harder for rats to swing in from the top. After observing camera footage it became obvious that rats needed something to grip as they try to swing into the bait station from above. Even the more recent Philproof Minis, which have lips on the outside, didn’t work as well as the large Philproofs in this respect. Many rats fell off or struggled to get inside the bait station. The addition of a rope transformed the situation …

“One of the mini's I used is the new design with lips on the outside which certainly helps but the rat is still stuck on the outside with not much to grip as he tries to swing into the bait station. Interestingly enough when I put ropes on the floors the problem with rats falling off went away, I had a look at a couple of videos frame by frame and the rats are using the top 70mm of the rope to hold onto as they swing into the bait station.

… on the Mini 87% of rats gave up on swinging in from the top and just used the rope from the ground.” 

email from Dave Edwards, 14th February 2019


Using rope to enhance rat access to bait stations

To try and improve the ease with which a ship rat could access bait in Philproof bait stations Dave carried out trials at four sites. A piece of 12mm rope measuring 320 mm long was attached to the bait station floor. Wire was wound around the rope and threaded through a 2mm hole drilled near the outside of the floor.

Bait stations were set at 280 -300 mm from the ground to the floor of the bait station.




When analysing footage from the baffle field trials Dave noticed that with the large Philproof bait stations approximately 85% of ship rats entered the bait station from the top and 15% jumped up into them from the ground. For reasons unknown, 85% of the rats that entered from the top did so from the left side (as viewed from the front with the bait station mounted on the tree). For this reason, Dave placed the rope on the left side of the entrance.

The following video shows rats using the rope to gain access to the bait station 


During the rope trials 270 rat entries into the bait stations were recorded. Of those 77% used the rope to go from the ground to inside the bait station, 17% entered from the top -- most of which still used the top section of the rope to help them swing into the entrance of the Philproof, and 6% were unknown as the rat was already too far inside to tell when the camera activated.

Except for having the odd chew on the rope, (like they do with everything) possums did not take much interest in the rope.

Given that several doses of anti-coagulant toxins are required for them to be effective, the rope would appear to enhance the chances of rats consuming the required dose – particularly if Philproof Minis are being used to deliver the toxin.


Now for something different ... 


Ship Rat excluders for Feratox Bio Bags

Dave was asked to develop a “cheap and easy method to reduce Ship Rat interference with Feratox bait bags”. Field trials were carried out using [Moultrie] trail cameras “in a native block that does not get animal pest control so has good numbers of possums and Ship Rats. No toxins were used during the trial. I used Feratox prefeed bags”.

He developed and trialled two different types of delivery systems that prevented or seriously reduced rat interference with Feratox Bio Bags/bait bags

In his discussion at the end of his report on these trials Dave pointed out that

“Prefeeding Feratox Bio Bags is recommended however what I found was that the first possum on the scene ate all three prefeed bags, so unless the possums that wandered past later scored prefeed elsewhere it is likely that when Feratox is put out possums could be exposed to it without ever eating any prefeed.”

from "Ship Rat excluder for Feratox Bio Bags"



Bait station #1 (0% rat bait take)

“The first method was simply to suspend the prefeed bags from a light angle iron support screwed to a tree. The prefeed bags were suspended about 600mm out from the tree on a light .6mm diameter (22 gauge) wire about 600mm long. After trialling several heights [it became clear that] a distance of 500mm from the ground to the bottom of the bait bag was a good height for the possums to feed at …”

from "Ship Rat excluder for Feratox Bio Bags"



This video shows the suspended bait bag delivery system in action



After seeking feedback from conservation colleagues on the design Dave modified it so that it could collapse back against the tree when not in use. The revised design pivots on a 4mm bolt and locks into place under it’s own weight when in the down position.



These modifications would reduce the chances of the suspended arm:

  • injuring conservation volunteers
  • being broken by falling branches or similar

Yellow plastic covers could also be used to cover the end of the metal arm to increase visibility and prevent injury. This practise is common within the construction industry and seems to work well.



Bait Station #2 (5% rat bait take)



This free-standing Bio Bag/bait bag holder is capable of carrying as many bags as can be stapled onto the holder. However Dave feels two bags at a time is probably a good number.

The flip-floppy nature of the metal baffle on this bait station proved to have multiple benefits

  • It effectively blocked rat interference from below
  • Its lack of stability and the smooth metal surface meant that Ship Rats jumping onto the baffle simply slid off, onto the ground
  • Possums were also seen to use “the stiffer No. 8 wire [stem of the bait station]… to steady themselves” while they fed on the bait bags


Dave’s take on this option?

“The method using the wire in the ground and a metal disc had ship rats get 5% of the prefeed bags. While I would consider this an acceptable result the rats were making repeated attempts to either get up the wire or jump onto the metal disc, so the results could have easily shown rats taking more baits. I would tend to favour the method using  suspended bags as it gave better results, would be cheaper to make, and because it is fixed in place would be less prone to outside interference.”

from "Ship Rat excluder for Feratox Bio Bags"


Components that were trialled but discarded

  • Plastic baffles: Rats were able to “get a grip onto the plastic and haul themselves up onto the top of the disc and get the prefeed bag.”
  • High tensile fencing wire: It’s very flexible but “a few times when a possum pulled it over to get the bait bag it slipped from his grip and whipped around, startling the possum.” 



Connovation is currently exploring the commercial possibilities of these innovations.


If you would like more information on any of the designs discussed in this article feel free to contact Dave at





Edwards, Dave   “Field trial: Ship Rat control using modified Philproof bait stations” (2018)

Edwards, Dave   “Ship Rat excluder for Feratox Bio Bags” (2018)

Interview notes, February 2019

Email conversations, February – April 2019