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South Australia worms, flies and lice update - January 2017

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





Adelaide: Colin Trengove, Sheep Health Lecturer (UA Roseworthy campus) (

Following on from one of the wetter years on record for much of South Australia rainfall episodes have continued in the first half of January giving sheep parasites a distinct advantage. Much of the state has received 20–80 mm and this is sufficient to encourage a green pick and worm larvae to emerge and survive for a week or two.

The majority of worm egg counts during December/early January indicated sufficient worm burdens to warrant a drench and so these counts would be expected to rise unless they received an effective drench at the time of the Worm Egg Count (WEC). It will be important to check egg counts again late January/early February to evaluate the need for a late summer drench. If a drench is warranted it is important not to expose the resident worm population to the same chemical as used for the last drench—especially if that chemical was used in the last 3 months i.e. the average life expectancy of most worm types we are concerned about.

Editor's note: After the first summer drench, the pastures in some areas may carry few larvae, so the drenched sheep may carry only or mostly drench-resistant worms that survived the first summer drench. In this circumstance, rotating to a new drench group that is effective against the drench-resistant worms the next time can be useful. >> Read more.

It might be argued that, with the abundance of dry feed available following on from amazing spring growth, sheep won’t be grazing close to the ground. Admittedly, most sheep are in great condition and so able to minimise worm populations in their gut. However, significant summer rainfall will encourage all stock to chase the green pick ensuring newly emerged infective worm larvae are consumed in the process.

The dilemma arises that summer drenching is generally discouraged as it promotes drench resistance i.e. any worms that survive a summer drench will be directly responsible for the majority of the (drench resistant) worm population in the following winter. However, delayed drenching will encourage a larger proportion of worms surviving through to winter either in the gut or on pasture and so causing a higher worm risk in 2017.

The long-held principles of worm management still apply:

  • Monitor WECs frequently to decide if drenching is necessary;
  • Summer drench only if monitoring indicates necessity; and
  • Adopt refugia management i.e. either delay moving a mob to a clean paddock for a week post-drenching or only drench 80% of the mob. This is to ensure “drench susceptible” worms are transferred to the clean paddock to dilute the impact of any “drench resistant” worms that survive drenching.

Editor's note: The recent ParaBoss Technical Workshop also highlighted that drench resistance can be slowed substantially by using the most effective drenches available on a property and by using combination drenches (providing they are also shown to be effective). More information can be found here.


For January 2017 state outlooks, please follow the links below:
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