Farm Tech

Getting on top of lice

Options for mid-micron and fine wool growers

By Stuart Edwards, Meat & Wool Innovation senior environmental technologist

The most effective way to control lice is still by chemical treatment off-shears. But the treatment is only as effective as the chemicals you use and how you apply them.

For several years, many growers have successfully used SP (synthetic pyrethroid) pour-ons off-shears. However, a number are now reporting that these treatments are no longer effective.

It is hard to know whether this is due to under-dosing, or whether lice have become more tolerant of SPs. Either way, the extent of the problem is of concern.

Fortunately, two new louse pour-ons (Magnum and Zapp) are now available. Both earned a good reputation last season. As IGRs (Insect Growth Regulators), they are from different chemical families to the SPs used in the past.

When using effective chemicals like these, it is realistic for farmers on developed properties with secure boundaries to plan to become louse-free. Already, a number of New Zealand and Australian farms have achieved this goal.

If you think this can be done on your farm, check for lice at pre-tup crutching and if possible, later in the season. If no lice are found, consider moving to treating every second year.

Fig 1. Sheep body louse life cycle.

Pour-on tips

When using a pour-on:

• Ensure the correct dose is applied. Where the dose rate is based on animal weight, set the application to the weight of the heaviest animal in the mob.
• Split the dose in half. Apply the first half starting from the top of the neck and work towards the mid-shoulder. Apply the second half starting from the rump and work toward the shoulder, overlapping at the shoulder.
  OR
  Apply half to either side of the backbone starting halfway up the neck and working back to the rump.

These application techniques will improve the distribution of the chemical around the body of the sheep.

IGR pour-ons

IGR pour-ons only work on the developing insect as it goes through its larval stages, and are not effective against adult lice.

When used off-shears on pre-lamb shorn ewes, the treatment should be applied at least six weeks before lambing. By allowing enough time for adult lice to die of old age, the risk of transferring lice to the unprotected lambs will be greatly reduced.

If you can’t dip six weeks before lambing, then still treat the ewes off-shears, and apply an IGR pour-on on the replacement lambs at weaning as a combined fly-louse treatment. IGR pour-ons have excellent fly control properties as well.

Note: Zapp is registered for off-shears lice and fly treatment in all breeds. It is also registered for fly treatment on crossbred sheep with up to 3 months wool growth.

Make sure you observe chemical withholding periods for works lambs.

Louse control using a modern jetting race

Recent work carried out by Novartis on flat comb shorn Merinos has shown that, if used correctly, a modern jetting race can be used effectively to control lice off-shears, even on fine wool sheep. Here’s how to get optimum results:

• Set the side nozzles of the jetting race so a coarse liquid spray, rather than a fine droplet spray, is delivered. This will aid penetration into the fleece.
• Do not turn up the pressure, as this tends to reduce penetration. Higher pressure generates fine droplets that sit on the wool surface and are shaken off when the sheep leaves the race.
• Run the sheep through the race twice, so a wet-on-wet application is made. This again aids penetration to skin level.
• Don’t recycle the dip wash as this leads to a reduction in the treatment efficacy.
• Use a mixture of Zenith (or Fleececare) and Seraphos at the manufacturer’s recommended rates. The OP (Seraphos) in the mixture kills adult lice, while the more persistent IGR (Zenith) kills any emerging nymphs and louse larvae.
• Control the speed of the sheep through the race to ensure you get a consistent even application. As with other application methods, the treatment is only as effective as the amount of chemical delivered to skin level.

As this treatment is being applied off-shears, the residue implications are minimal.

Louse control using shower or plunge dips

We recommend off-shears shower dipping for dry stock only. The stress associated with the process makes it unsuitable for treating mid to late term pregnant ewes.

Off-shears shower dipping can be an effective way to control lice on unmated hoggets, wethers and maiden 2-tooths, especially if late spring, summer or early autumn shearing. For an effective treatment to be applied you must ensure that the chemical concentration in the dip wash is maintained at a constant level.

Chemical stripping does occur with OP dips resulting in some sheep being treated with dirty water rather than effective dip chemical. This is likely to result in a louse population being maintained in your flock.