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Tasmania worms, flies and lice update - July 2017

Tasmania WormBoss Worm Control Programs

Tasmania WormBoss Drench Decision Guides





Perth: Paul Nilon, Nilon Farm Health (

The last month has delivered essentially no rain and since late June the frosts have piled up to reduce soil temperature and almost stop pasture growth. It’s shaping up as a tough winter.

Pre-lamb shearing is well underway. As the fleece comes off, most of the cross bred (XB) ewes are looking quite good (compared with the obscenely fat state last year), but many Merino ewes are coming up short; i.e. Condition Score (CS) less than 3. There are insufficient pasture resources to improve the sheep and still have something for lambing. As most Merino flocks are 6 weeks off lambing there is still time to supplement.

It’s not surprising that worm egg counts (WECs) in Merino ewes have started to move up. There is no need to panic, but many WECs in mixed age ewes are higher than 400 eggs per gram (epg). You only need to drench now (additional to a pre-lambing drench) if your lambing is still a long way off.

Below is a summary of a particularly ugly Drench Test (at least by our standards). This test has a few anomalies (due to a low a control count).
(Editor’s note: Overall efficacy is sometimes referred to as percent reduction.)


White (Wh)

Clear (Cl)

Abamectin (Aba)




Overall (%) Efficacy







Brown stomach







Black scour







1Calculated result.
2 This negative value indicates the efficacy was zero and WEC in treated group were higher than for the untreated control group.

Notwithstanding the test’s limitations, it is highly likely that there is substantial mectin resistance. Less certain is whether the mectin resistance in black scour worm is real or a test artefact. Now, this place and all the others where we have found substantial mectin resistance have one thing in common: unrelenting use of long-acting mectin drugs pre-lambing. My concern about mectin resistance is simply the fact that long acting mectins are our tool of choice for the really disastrous years, and we should do all we can to keep them working.

We are confident from the modelling work (Dobson and Barnes [formerly CSIRO Armidale] in Australia, and Leathwick in New Zealand) that long-actings (capsules and injectables) are potent resistance promoters. Our Kiwi brethren get particularly toxic about long-actings given as a pre-lambing drench on two grounds.

Firstly, drug is transferred to lambs via the milk and results in lambs receiving a sub-lethal dose (for the worms) of mectin (this was first identified in Australia by Michelle Dever et al. 2015).

Secondly, depending on how worm-free the lambing paddocks are, you may be preferentially selecting a resistant population by tail selection.
(Editor’s note: survival of incoming- or ingested-larvae in lambs due to a lower concentration of drench.)  

You know the chorus: limit the use of long-acting drugs (whether capsules or injectables) to high-risk situations. If readers have different information I would love to hear it. Nothing would make me happier than being able to confidently tell clients that using a long-acting preparation pre-lambing is without consequence for resistance. But the published information and my observations suggest that I am right.

Therefore, use a long-acting drug if you are in a high-risk situation.  High-risk does not mean that Richmond may make the finals, and you have to go watch them play.  The best way to predict this is to map your WECs against the paddocks.  If there has been sustained contamination since the autumn you can bet that the paddock is high risk.  Nearly all prime lamb producers and many Merino people will get away with a short-acting drug.  Without resistance data use Zolvix®, Startect® or an abamectin triple.  If you pick the risk incorrectly the fall-back position is a lamb marking drench, but lost production at a key time for lamb growth is likely to have occurred”.

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