No prisoners! A new toxin for NZ pest mammal control

On my recent re-read of The Lord of the Rings I was struck by events that have concerned me since I read LotR as a kid, the genocidal nature of Tolkien. At the end of the major battles; Helm’s Deep, the Pelennor Fields  and at the Black Gate (and even the skirmish in Ithilien and the battles in The Hobbit and The Silmarillion), pretty much all of the forces of Sauron and Saraman that are present are completely destroyed.

“Hard fighting and long labour they had still, for the Southrons were bold men and grim, and fierce in despair; and the Easterlings were strong and war-hardened and asked for no quarter. And so in this place and that, by burned homestead or barn, upon hillock or mound, under wall or on field, still they gathered and rallied and fought until the day wore away. Then the sun went at last behind Mindolluin and filled all of the sky with a great burning, so that the hills and the mountains were dyed with a great burning ….. And in that hour the great Battle of the field of Gondor was over; and not one living foe was left within the circuit of the Rammas. All were slain save those that fled to die, or to drown in the red foam of the River.”

No one survived? Out of the tens of thousands of orcs and men, not one group was able to surrender? Or a particular organized company wasn’t able to battle there way north a few leagues to where a fresh orc host was encamped on the road to Rohan? Or a bunch of scouts, well used to sneaking about, didn’t slip away in the tumult? As a kid this seemed fairly unlikely. For the sake of the story I could accept speaking giant eagles, smoldering fire demons and a wizard who always seems to be missing when the really important stuff happens because that is part of Tolkien’s world. When Tolkien strays into areas that shouldn’t be different from our world but are then that really seems to jar. Casualty rates in battles over history, while very variable, seem to sit about 20% +/- 10%. So 100% seems reasonably unrealistic.  Later as an ecologist, especially one who works occasionally on mammal pest management, I found out why such high casualty rates are even more unlikely. It’s really hard to kill them all!


“by burned homestead or barn, …. still they gathered and rallied and fought until the day wore away”

New Zealand has a problem with vertebrate pests. Possums, rats, mice, stoats, rabbits, hedgehogs, pigs and so many more all combine to impact on a native biodiversity as well as causing problems for agriculture and health. One advantage that New Zealand has over most other parts of the Earth is that we have no native terrestrial mammals, other than two bat species (who feed on insects and nectar for the most part). What that means is that we can put toxins into the native environment in the form of baits which are generally only eaten by introduced mammals, which are effectively all pests. If we are particularly targeting possums, but rats also eat the toxic bait and die, it doesn’t really matter as they are pests as well. So we have gotten very good at spreading toxic baits through aerial drops over our wild areas to remove resident pest populations. This allows our forests to recover and our native species a chance to compete and survive. But can we kill them all? That is particularly tricky. Typically a good control operation may reduce target species to 90-95% of the original population. That takes a lot of work but leaving a few of the pests is OK as they have minimal impacts until their populations grow again. In areas where we are trying for utter destruction or, to take a line from a favourite movie, “Nooooo prisoners!” (Lawrence of Arabia), it is killing the last 5% that takes most of the effort in time and resources.

We have improved our control (which is shorthand for killing!) methods by working out the best density of spreading bait, when to do operations, figuring out how different mammals interact with baits, stopping bait shyness from developing and so on. We have looked at all steps from how to deliver the toxins, how the target species locates the baits, how they then interact with the baits and what happens after the baits/toxins are ingested. One area in which we are continually looking for options is in new toxins to use. New Zealand is very reliant on the toxin 1080. This is a very effective broad spectrum toxin. However, there is a reasonable amount of public resistance to the use of this toxin (mostly unfounded!) but it would certainly be useful to have options that were targeted at particular pest species. One potential toxin is zinc phosphide. Zinc phosphide is used in some parts of the world for rodent pest control. Although it has been registered in New Zealand as a toxin for possums there has been little use as no one has been able to demonstrate that it works effectively in the field. Until now.

In a study by Lincoln University researchers, including PhD student Lee Shapiro and Dr James Ross, zinc phosphide in a paste form was trialed at six sites in the North Island. Accredited pest control contractors were asked to use their preferred ground control technique but using zinc phosphide paste instead of their usual toxin. In order to identify the success of their operations, lines of leg-hold traps were placed out and trapping rates were estimated before and after the trial. In the New Zealand Journal of Ecology they report that the paste was successful in controlling the local possum populations with a mean reduction of 82% (with a maximum of 92% and a minimum of 70%). While this sounds like a high kill rate (and it is!), existing toxins like 1080 and feratox, in the hands of the same experienced operators, expect kill rates of over 90% for a possum operation. So while there is still a bit of tweaking to do with the methodology to be assured that zinc phosphide is another reliable tool for the toxin toolbox, it is looking very promising. With more field trials and research we might be able to get the zinc phosphide kill rate up and over 90%. Now that’s a rate that even Tolkien and the men of Gondor might have found acceptable.

2 thoughts on “No prisoners! A new toxin for NZ pest mammal control”

  1. Great article, very interesting. I am now wanting to watch those movies.

  2. Fred Harvey says:

    Another way pesticides can spread and cause potential harm is by volatilization. Volatilization occurs when a pesticide turns into a gas or vapor after it has been sprayed, allowing it to travel through the air and spread to different pieces of land. (Vapor Drift) This can be harmful for wildlife, such as frogs.

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