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South Australia worms, flies and lice update - November 2018

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





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

Summer is well upon us with harvest in full swing in all areas under 500 mm annual rainfall, that is, where there is grain to reap. The tips of the peninsulas, Adelaide Hills and the mid and lower south east still have green parts, but they are definitely the exception this year. Worm larvae would still be surviving where there is ground cover, but as daily temperatures reach 30°C plus on consecutive days, they will be dehydrating and dying at a faster rate.

Worm egg count (WEC) monitoring indicates an inevitable reduction in worm burdens with the onset of summer across South Australia with only one in three mobs having significant counts and these being mostly in lambs/weaners. It’s not too late to do a drench trial using lambs or weaners to simultaneously check the effectiveness of multiple drench groups, but only where worm egg counts exceed 250 eggs per gram of faeces.

Most of the recent 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 expected in lower rainfall areas due to the earlier haying off of pastures or lack of pasture generally this year.

While larvae survival on dry pastures is much reduced, some larvae do remain protected within the faecal pellet. The real reduction in larval numbers comes about because fewer eggs are passed into the environment, as some larvae consumed by sheep may undergo a further dormancy in the abomasum rather than developing into adult worms. 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 infections. Recent rainfall events in some areas could easily predispose to barber’s pole worm infestations in a few weeks plus the more immediate risk of flystrike.

Monitoring WECs during November in all age groups is advised to gauge worm burdens across the flock and the 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 where drenching is done. This involves encouraging refugia (survival of worm larvae not exposed to worm drench chemicals) 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 up to 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 2018 state outlooks, please follow the links below:
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