Professional Pest Controller Magazine Issue 102

26 February 2021

Opinion: Cholecalciferol - the new kid in the block


It’s rare for the UK pest management sector to get a new active ingredient, but when it does happen, it’s worth reflecting on.

We caught up with regular PPC contributor, Dave Archer and asked him for his thoughts on cholecalciferol, actives from times gone by and the future of rodenticides.


Old-school actives

Since the late 1970s, as an industry, we’ve lost certain rodent control products from the market place for a multitude of factors; mainly regulatory and products reaching the end of their registration lifetime.

These products had different modes of action to the current anticoagulants and were potentially hazardous to the environment, operators and non-target species.

As an example, in the early 1980s as a local authority pest control services manager, I mixed and used zinc phosphide to control increasingly anticoagulant resistant strains of rats that emerged in West Berkshire.

Zinc phosphide, was a product which, when pre-baited with whole soaked wheat for a period of a week or so, literally killed rats within minutes.

Obviously, this was an extremely hazardous product which had the capacity to produce disastrous secondary effects if misused.

Every rat that consumed bait had to be located and removed from the site. Some rats died close to the bait base (soaked whole wheat) and it was not unusual to kill many hundreds of rats overnight with a single treatment.

Sodium cyanide (marketed as Cymag) was sold as an off-white powder which, when it came into contact with moisture, produced hydrogen cyanide gas – deadly to any animal that inhaled it.

The treatment was fraught with hazards as one can imagine but, for controlling large infestations of rats in farm embankments where a fast kill was required, it was a very effective control measure.

As it produced a gas that killed rats in their burrows there was no chance of secondary poisoning.

Since the 1970s, anticoagulant-based rodenticides have, by natural or enforced selection, produced populations of rats that are increasingly resistant to the active substance and, more worryingly, are capable of passing this genetic resistance onto their offspring.

The new old active

There have been few ‘new’ rodenticides produced in the past two decades and, in the main, these have been produced as more potent anticoagulants, capable of dealing with strains of rats tolerant to less toxic compounds.

Rodenticides with a different mode of action to anticoagulants have been produced but these may not have been suitable for certain environments.

Now the industry has a new product available with a different mode of action to anticoagulants, not seen since the 1980s.

This new product, Selontra, marketed by BASF, is a cholecalciferol-based product and is presented as a ready-to-use soft block formulation, with the addition of vegetable oils and Bitrex.

I understand additional formulations are being considered and other companies are developing their own cholecalciferol formulations.

Field trials detailed by the manufacturer show excellent palatability in rodents to this product, with the added benefits of greatly reduced secondary poisoning problems, and no resistance problems as can occur with anticoagulants.

Additionally it is claimed infestations of rats can be controlled in as little as seven days.

Cholecalciferol occurs naturally in the body and, when a lethal amount is ingested, it eventually causes calcium deposits in the major organs, eg kidneys.

Rodents will stop feeding after 24 hours once a lethal dose is consumed (less than one block for an average size and weight of brown rat, ie half of its daily food intake).

The product is certainly an attractive proposition given the small amount of compound needed in tandem with the kill time, and the fact that secondary poisoning does not occur.

There is no single antidote to Selontra, but there is an antidote treatment regime for accidental poisoning.

The product must (as with all poisons) be used in exact accordance with labelling requirements, which includes its use in approved bait boxes or covered and protected bait points.

What’s around the block?

What of the future? Will we see the eventual demise of anticoagulants for the control of rodents in favour of baits with different modes of action, or the use of modern electronic trapping campaigns, as are already coming to the fore?

Perhaps – but to my mind, the more choice available to the informed and competent pest controller to control ever-changing populations of rodents can only be deemed beneficial.

Your comments

What do you think the future of rodent control looks like? Will it be all internet-reliant smart devices, new rodenticide modes of action or laser shooting drones?

Send us your thoughts and we might print them here.

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