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Western Australia worms, flies and lice update - November 2015

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides

Albany: Brown Besier, DAFWA (

Across most parts of the state, the season has dried off rapidly and this will reduce the number of worm larvae present on pastures ahead of the normal schedule. The blowfly risk will have reduced recently, although we are still hearing of occasional strikes in the lower Great Southern.

Worms and summer drenches

It has been a relatively wormy year, due mostly to the early rains in most districts, and we saw some high worm egg counts in lambs in October. If lambs have not had a drench within the last few weeks, and summer drenches are not due for a while, it will be worth checking their worm burdens now.

However, in many situations it will be possible to give summer drenches earlier than usual, which is preferable to giving a drench in both late-spring and then later in summer.

  • If drenching sheep onto crop stubble, seasonal conditions are not a factor as there are no worm larvae present.
  • On pasture, a general rule is that once a paddock is totally dry, and there have been a few days at least with temperatures over 30°C, almost all worm larvae will die and no further eggs will hatch. If in doubt, wait to mid-December, or longer along the south coast (but be aware that worm burdens may remain significant).

What sheep: In all except dry inland (wheatbelt) areas of WA, weaners and hoggets (blue and red tags) should receive a summer drench. Lambs are still developing their worm immunity and in most parts of WA, they usually have significant worm burdens by early summer.

Some sheep from the previous year’s drop will also not have developed a complete immunity, and unless a worm egg count shows they have low counts, a drench is recommended.

In low-rainfall cropping zones, there may be no need for summer drenches to lambs if they had a drench at weaning. Hoggets often also have low worm counts in areas of lower worm intake. A worm egg count will quickly show the need for treatment.

Adult sheep: in the mid to high rainfall regions in WA, it is recommended that adult sheep receive a drench in autumn (March or early April, and before green pasture growth occurs). Treatment is rarely justified in summer on worm burden grounds, and it markedly increases the development of drench resistance.

However, if it is considered most convenient to give drenches in summer rather than autumn, it’s important to leave at least 10% (and ideally 15­–20% or more) undrenched—those in the best body condition score. This ensures some non-resistant worms are carried through summer and can dilute out resistant ones—as with autumn drenching. (If barber’s pole worm is a major risk, the program should be discussed with an adviser.) (See the DAFWA webnote under the section “Livestock Parasites”, or WA worm control programs at

Drench types: Summer drenches should be as close to 100% effective as possible. Recent drench resistance tests indicate that the commonly-used abamectin or moxidectin are no longer always fully-effective: resistance was present in about 50% of tests for abamectin, and 30% for moxidectin.

However, these types are still an option if known to be fully-effective, or when used in a combination that is fully effective. Other options are the pre-mixed “triple combinations” (abamectin with two of the other older drench types: white, clear, or naphthalophos); monepantel (Zolvix®); or derqantel-abamectin (Startect®). The combination of a white and clear drench with naphthalophos (Rametin®) can also be fully-effective, but a follow-up worm egg count is recommended to check.

In all cases, taking a worm egg count from at least one mob after a drench confirms the effect, and is a guide to the future use of the drench group involved. 


Esperance: Nicole Swan, Swan’s Veterinary Services (

After an early start to harvest, things were put on hold for a lot of farmers as we had intermittent rain. Now the weather is hotting up and harvest will be underway again, it won’t be long before stubbles are available to drench sheep onto.

Recent counts have been varied. Lambs that received an effective weaning drench in September have had low counts (15 epg). Those that have not been drenched have had high counts of up to 470 epg average. Some adult sheep have also had reasonably high counts.

Now is the time to consider a strategic summer drench. Lambs, weaners and hoggets should receive an effective drench onto stubbles if available. It is worth waiting for stubbles to become available prior to drenching.

Older sheep should be monitored prior to drenching. Mobs with an average of 200 epg or greater should receive an effective summer drench. It is important that some sheep on the property are left undrenched to provide refugia. If all sheep are drenched then any worms left behind will be resistant ones. If some sheep are left undrenched then there will be a mixed worm population left behind (refugia). This will prolong the effectiveness of the drenches used.

Refugia can be achieved by leaving around 10% (or could be more where barber’s pole worm is not a major risk) of the mob undrenched or by not drenching some of the mobs and ensuring that these sheep at some time graze the pasture where the drenched sheep have been. Then, once it rains in autumn, the population of eggs that survive (hopefully not many) and then hatch out will be a mixed population.

Sheep should be monitored late summer (February/ March) to determine if drenching prior to the April/May break of the season is required.

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