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South Australia worms, flies and lice update - November 2017

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





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

By mid-November, summer is starting to have an impact with harvest in full swing in all areas under 500mm annual rainfall. The exception being parts of the Adelaide Hills, Fleurieu and the mid and lower south east where green is still the norm. Worm larvae would still be surviving on pasture, but as daily temperatures reach 30°C plus on consecutive days they will be dehydrating rapidly…humans away from air conditioners and shade alike.

Worm egg count (WormTest) monitoring indicates a general reduction in worm burdens across South Australia, but individual mob counts over 2,000 eggs per gram (epg) are still evident in weaners and hoggets while others are zero – the latter presumably post-drenching (DrenchCheck) checks. Some counts from the south east have risen markedly post-drenching with a triple active product - most likely (hopefully) indicative of heavy pasture contamination rather than multiple drench resistance. This highlights the opportunity to check drench efficacy as high counts provide the ideal situation to see if particular drench groups are as effective as often they are assumed to be. Ultimately the preferred option is a drench trial (DrenchTest) using lambs or weaners to simultaneously check the effectiveness of multiple drench groups when there are plenty of worms about.

Most of the recent Worm Egg Counts (WECs) have been done in the 500+ mm rainfall areas presumably for two main reasons: 1) harvest is the consuming activity in lower rainfall areas; and 2) lower worm burdens are assumed in lower rainfall areas due to the earlier haying off of pastures. While larval survival on dry pastures is much reduced, the latter can be a false assumption as often worms in adult sheep cease to lay eggs when conditions are not conducive to larval survival. It pays to check WECs in adult sheep every 2-3 months for changes in egg output from scour worm populations plus 3-4 weeks after 20+ mm rain in areas prone to Haemonchus (barber’s pole) infections. For example, this would be a consideration in areas south of Bordertown that received 30-40 mm rain over two days last week. Additional consideration of the very real flystrike risk goes without saying.

Monitoring WECs during November in all age groups is advised to gauge worm burdens across the flock and risk of pasture contamination in summer. Summer drenching is only recommended where counts exceed 100 eggs per gram of faeces. Counts below this are unlikely to significantly contribute to pasture contamination in summer given the expected dry hot weather conditions as well as sheep generally being in good condition at this time of year.

A major consideration is minimising the development of drench resistance as it is most likely to develop or accelerate during summer. This involves encouraging refugia and there are three options: 1) don’t drench during summer; or 2) retain the drenched mob on the paddock of origin for a week after drenching before moving to a new paddock (stubble included); or 3) leave 20% of the mob (those in best body condition) undrenched. The principle is to ensure that some worm larvae not exposed to the drench chemical are carried into the new paddock to dilute any potentially resistant larvae that survive exposure to the drench. This may initially seem counter intuitive, but it has been demonstrated to be an effective means of preserving drench efficacy, especially in moderate to low rainfall areas that have a distinct dry summer.

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