Western Australia worms, flies and lice update - January 2016

Albany: Brown Besier, DAFWA (brown.besier@agric.wa.gov.au)

Worm problems are usually far from sheep producers’ minds at this time of year, but what a difference the rain makes!

The recent downpours in the southern agricultural areas, especially, will significantly increase the risk that this will be a wormy year, with more severe and earlier worm problems than usual. However the likelihood will vary greatly between districts, mobs and farms.

Worm risk factors

Several factors indicate the worm risk:

• Pasture germination: if the rain is sufficient to bring on a sustained germination, worm eggs can quickly develop to the larval stage, and grazing sheep will begin to pick up worm burdens. However, if only a few weeds come up, or any germination dies back quickly, there will be little effect on worm populations.
• Worm egg output by sheep: if sheep don’t have significant worm burdens, they won’t be putting out many worm eggs in their dung, and pasture germination will have little effect. The main risk is where summer drenches (recommended for weaners and hoggets) were not as effective as they should have been. Also, ewes due to be drenched in autumn (as now recommended) will be carrying some worms, and drenching may need to brought forward.

Action now recommended

Firstly, assess whether there has been any change to the worm risk. In many situations there will be no change to the usual advice: drench ewes in early-mid autumn, and check worm egg counts of summer-drenched sheep towards the end of summer.

However, where a general germination occurs, there are some revised recommendations:

Sheep given summer drenches: Resistance now affects all but the newest drench types, and if, for instance, abamectin, moxidectin or even a triple combination type have been used, you can only be sure they were fully effective if you’ve tested for resistance.

Action: if not sure the drench worked fully, take dung samples for worm egg counts on some representative mobs. If the average count is more than about 100 eggs per gram, a re-drench within a week or two is recommended. If it’s lower but still over about 50 eggs per gram, re-check in 4–6 weeks, and again, re-drench if over 100 eggs per gram.

Ewes not drenched in summer: In most years, autumn drenches should be given in late March or early April, before conditions change to allow general worm egg development on the pasture. If this occurs earlier due to the rain, this drench should be brought forward, and given by about mid-February. (You can take worm egg counts to check the need, but in the great majority of ewe mobs that have not been drenched since winter or spring, a drench will be warranted.)

Barber’s pole worm areas: Green pasture in summer favours barber’s pole worm development, and we’ll certainly see more of this than in most years. Worm burdens will take 5–6 weeks after the rains to develop, and it’s difficult to predict whether they will be significant in any particular mob. The best guide is whether green pasture has persisted, as is especially likely on perennial pastures. Worm egg counts on a couple of representative mobs at that time will indicate whether a drench is needed, and whether a product with a long-acting effect against barber’s pole is justified.

In summary, where rains have been unseasonably heavy there is a higher risk of worm problems, whether now or later in the year. Timely action now will determine whether 2016 unfolds as a severe worm year on your farm, or whether worms can be managed by normal tactics.