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South Australia worms, flies and lice update - March 2015

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides


(This report was too late for the March 2015 ParaBoss News email)

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

The extremes of weather in the past 2 months have not been conducive to worm survival, but WECs (worm egg counts) have continued to indicate one in three mobs (on average) requiring a “late summer” or pre-lambing drench. Sixty plus days of negligible moisture following the 50+ mm rain in early January will have stifled the reproductive aspirations of most worms. However, as the cooler evenings and morning dews start to increase in frequency and intensity any worm eggs are more likely to survive desiccation leading to infective larval stages on soil/pasture for unsuspecting grazing ruminants. There is little pasture or ground cover across the State and so infective larvae have little protection in those situations. However, any moisture in the next month will encourage regrowth in perennial plant species assisting the buildup of infective larvae awaiting consumption by hungry stock grazing close to the ground. As a result, significant worm burdens can develop over the next 4–6 weeks.

That begs the question….is a pre-lambing drench warranted? I have been pleasantly surprised how good the condition score of ewes have been recently while monitoring ewes and pastures during LifeTime Ewe workshops. However, the scarcity of feed going into autumn generally indicates decisive action on worm control prior to lambing is paramount. Worm egg counts will soon indicate whether a drench is needed, but this decision should include ewe condition (needs to be > 2.5), feed on offer (preferably > 1,000 kg DM/Ha or a dense pasture averaging 3+ cm height) and future use of the paddock being grazed i.e. are lambing ewes to remain in this paddock or will it be spelled for several months. 

It is difficult to predict weather patterns at this time of year, but if the rain forecast is bleak and worm counts are approaching 100 epg or more, then targeted drenching is indicated. These decisions need to be made on an individual mob basis using local knowledge. However, it is important to remember if you are drenching at this time of year to make sure you use a different effective drench group to the chemical used in the last six months i.e. do not use the same chemical on consecutive occasions over summer, to minimise the risk of resistance developing.

Another consideration is the increased risk of worm pickup associated with high intensity stocking/grazing e.g. trail feeding or confinement feeding if there is a risk of faecal contamination of the grain or hay provided. Once again, WEC monitoring will soon indicate whether a pre-exisitng worm burden needs to be removed before subjecting ewes to these management strategies in the lead up to the opening break.

For March 2015 state outlooks, please follow the links below:
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