Acaricide Resistance and the Move to Strategic Dipping

#Two numbers from the same study, and only one of them should worry you tonight

The 2006–2017 survey of acaricide resistance on South African commercial farms produced two figures for cypermethrin against the African blue tick: 74.3% and 66.2%. They get quoted interchangeably in the farming press. They are not the same thing and they do not lead to the same decision.

74.3% is the proportion of tick populations classified resistant. Of 369 Rhipicephalus decoloratus populations tested, 274 were resistant to cypermethrin. That is a prevalence: roughly three in four commercial farms sampled had a cypermethrin problem.

66.2% is the mean phenotypic resistance — the average degree of resistance measured within those populations. It is a severity figure, not a headcount.

Amitraz ran at 23.0% of populations resistant with a mean phenotypic resistance of 26.5%; chlorfenvinphos at 9.2% of populations with a mean of 13.2% (van Dalen & Jansen van Rensburg).

#Where the resistance actually is

The National Tick Resistance Survey tested 180 R. decoloratus populations from 77 local municipal districts across eight provinces by larval immersion between 1998 and 2001 (van Dalen & Jansen van Rensburg):

Acaricide group	Resistant	Emerging	Total
Amitraz (formamidine)	3.3%	3.3%	6.6%
Cypermethrin (synthetic pyrethroid)	29.4%	6.1%	35.5%
Chlorfenvinphos (organophosphate)	20.0%	16.1%	36.1%

Resistant to all three: 1.2%. Resistant to two: 25.8%. The worst multi-resistance was in KwaZulu-Natal at 54.5% and the Eastern Cape at 45.5%.

Put the two surveys side by side — carefully, because this is exactly where the trap in the opening section is set. The 6.6% in the table above is resistant plus emerging. The classified-resistant figure for amitraz in 1998–2001 was 3.3%. Compare like with like and it goes 3.3% of populations resistant to amitraz then, against 23.0% of commercial R. decoloratus populations resistant to amitraz by 2006–2017; and for cypermethrin, 29.4% classified resistant then against 74.3% later.

Two different sampling frames, two decades apart, so read that as a direction of travel rather than a measured rate of change. But the direction only goes one way, and the group that was almost intact in 2001 is the group that has moved furthest since.

One imported narrative deserves correcting here. Globally the story is that R. microplus is invading and displacing R. decoloratus because it is tougher and more resistant. On South African commercial farms over 2006–2017 the opposite held: the native decoloratus made up 94.9% of populations and was far more resistant, and R. microplus was not recovered from Gauteng, Limpopo or the Northern Cape at all. In communal systems the invasion story is real — R. microplus is expanding through the coastal Eastern Cape and elsewhere (Nyangiwe et al.), with amitraz and deltamethrin resistance documented on communal farms at King Sabata Dalindyebo (study). Both pictures are true. Which one applies to you depends on your fence line, not on the literature.

#Rotate by group, not by brand

There are five acaricide chemical groups registered in South Africa: organophosphates, synthetic pyrethroids, formamidines (amitraz), macrocyclic lactones, and chitin-synthesis inhibitors and growth regulators (Virbac SA).

Rotation means moving between groups. Two products with completely different names, different labels and different prices can carry the same active ingredient, and a farmer alternating between them is not rotating at all — he is applying continuous selection pressure to one group while congratulating himself on his programme.

Here is what that looks like in practice on a real farm.

A Highveld cattleman buys three dip products from two co-ops and alternates them across the season: one in October, a second in December, a third in February, back to the first in April. He believes he is running a four-way rotation. He reads the labels properly for the first time and finds product one and product three are both synthetic pyrethroids, and product two is a pyrethroid combined with an organophosphate. So across the whole season, every single tick generation met a pyrethroid — in a country where 74.3% of surveyed commercial blue tick populations were classified resistant to cypermethrin. His amitraz, the group with the lowest resistance prevalence in both surveys, sat unused in the store because it costs more per dip.

He has been paying for four dips a year to run one group. That is not a rotation. That is a resistance programme.

The fix costs nothing: read the active ingredient and the Act 36 of 1947 registration number off the label of every product in the store, write the chemical group next to it, and build the rotation from that list.

The legal position

Every acaricide and every drench you use is a registered product under one of two Acts. The Fertilizers, Farm Feeds, Agricultural Remedies and Stock Remedies Act 36 of 1947 registers stock remedies, which you can buy over the counter. The Medicines and Related Substances Act 101 of 1965 covers veterinary medicines, which are scheduled and generally need a prescription (SAVA).

The 2022 amendment to the Animal Diseases Regulations, GN 2318 in Government Gazette 47133 of 29 July 2022, defines an "effective remedy" as one registered under Act 36, or a medicine registered under Act 101, for the particular purpose (gazette text). Those four words matter. Using a registered product outside what it is registered for is not automatically compliant, however sensible it looks. The label is the legal instrument, and this course will never give you a dose, a dilution or an interval that belongs on it.

#The infrastructure problem behind the chemistry problem

#Making the move off intensive dipping

Lesson 1 ended with the Bushbuckridge result: halving the dipping frequency raised clinical disease in that setting (Rikhotso et al.). That is not a reason to stay on a 14-day interval forever. It is a reason to treat the change as a project with a plan and a reverse gear.

Do this on the farm

The six-step transition from intensive to strategic dipping.

Confirm which diseases are actually present — identification of the ticks on your animals plus laboratory confirmation of the pathogens circulating, through your vet or animal health technician.
Vaccinate the herd where the diagnosis warrants it, with your veterinarian choosing the product.
Hold intensive tick control right through the immunity window. The ARC's own instruction is to vaccinate first and maintain intensive control until immunity has developed. For planning purposes it gives at least six weeks for babesiosis and heartwater and at least eight weeks for gallsickness — build your calendar on the longest figure, because the FAQ itself is not fully consistent on the shorter ones.
Only then lengthen the interval, and lengthen it by one step, not by half.
Monitor. Standardised tick counts on a fixed set of ten index animals, on a defined body region, plus body temperature checks. Count ticks, do not count days.
Expect some clinical cases in year one and be equipped to treat them. Have your veterinarian on call and the treatment decision made in advance, not at 6pm on a Sunday.

Write your abort criteria down before you start. The published research does not give you thresholds, so set yours with your vet and put them on paper where you cannot renegotiate them in the moment. Sensible ones look like: tick counts on the index animals still climbing after two consecutive dips at the new interval; a second clinical case in the same month; any case in an animal whose vaccination status you are unsure of; or a break in the monitoring itself, because a transition you have stopped measuring is not a transition, it is a hope. Hitting an abort criterion means going back to the previous interval and re-diagnosing — not pushing on.

Two hard limits: do not attempt this without a veterinarian, and do not attempt it on pregnant animals.

Careful — this one bites

The gate is where resistant ticks, resistant worms and controlled diseases arrive. Every incoming animal, without exception:

Isolate on arrival, away from the main herd and off the pasture you care about.
Treat for internal parasites with a chemical class you do not rely on — a quarantine drench — and hold the animal long enough to void.
Sheep get two treatments with an effective remedy before they may be mixed with the resident flock. The interval between those two treatments is set by the mite's life cycle and by the product label. Read the label and confirm it with your state veterinarian. It is not in this course.
Check for lice, mange and ticks, and identify what you find.
Confirm the status documentation — brucellosis, movement permits, identification.
Vaccinate to your farm's programme, allowing the full immunity lead time before the animal joins the herd.

Contested — here is both sides

Does cutting chemical input actually pay in South African animal health?

The strong case for: it is the only strategy that preserves the acaricides you still have, and South Africa's resistance data make that preservation existential rather than optional. Amitraz went from 3.3% of populations classified resistant in 1998–2001 to 23.0% of commercial populations by 2006–2017 — two different surveys, but only one direction. There is no new chemical group arriving to replace the ones being burnt.

The strong case against overselling it: no South African trial demonstrates that a reduced-input animal health system beats a well-targeted conventional one on production or profit in the same season. Bushbuckridge found strategic dipping raised disease incidence initially.

The honest claim is narrower than the movement's rhetoric. Reduced input buys you durability — chemistry that still works, and a herd whose immunity does not depend on an unbroken dip schedule. It costs you some current production and some transition risk. That is a good trade over a decade. It is not a free lunch in year one, and a course that sold it as one would be setting you up to blame yourself for a predictable dip.

Where it is simply wrong: in a naive herd, in an endemically unstable situation, or with a controlled disease on the property. There, low input is not regenerative — it is negligence.

#What to do with this on Monday

Walk into your store with a notebook. Write down every acaricide you own, its active ingredient, its chemical group and its Act 36 registration number. If two of them share a group, you have been rotating brands, not chemistry, and you now know why the ticks are still there in April.

Then look at the equipment those chemicals go through. A resistance problem you can fix with a nozzle set is the cheapest one you will ever have.

#Check yourself

4 questions — answers explained as you go

1A farming magazine reports "66.2% cypermethrin resistance in South African blue ticks". What does that figure actually measure?
Why: The same study reported both numbers, and they answer different questions. 74.3% of populations were classified resistant; 66.2% was the average severity within populations. The prevalence figure tells you how likely you are to have the problem; the phenotypic figure tells you how bad it is once you do. The two are frequently quoted interchangeably, and confusing them is the difference between booking a resistance test and writing off a chemical group.
2A farmer alternates three differently branded dip products through the season. What must he check before calling it a rotation?
Why: There are five acaricide chemical groups in South Africa and rotation is between groups. Two products with different names can carry the same active, so a three-brand programme can be a one-group programme. The label carries the active ingredient and the Act 36 of 1947 registration number, and that is where a rotation plan starts.
3A communal survey found 94.6% of dip tanks non- or partially functional. Why does that matter for resistance, not just for tick numbers?
Why: A tick that meets a full lethal dose dies and passes nothing on. A tick that meets a weak dose and survives is exactly the animal whose genes get selected. Under-strength dip wash, blocked nozzles and unreplenished tanks train the population you are trying to kill, which is why fixing the equipment often beats buying a new chemical.
4What is the first step in moving a herd from 14-day dipping to a strategic programme?
Why: The transition starts with a diagnosis, not with a calendar change. Vaccination follows the diagnosis, intensive control is held right through the immunity window, and only then does the interval lengthen. Doing it in the other order is what the Bushbuckridge trial was measuring when clinical disease went up.