Engel said most outbreaks are because of existing low-level lice populations building quickly in number and not from a stray sheep from next door infesting clean sheep.
‘Lice live right on the skin, so good saturation of the products is required in order to achieve good control.’
Blaming the neighbours can be tempting but not usually correct.
However, resistance can start to occur from ongoing repeat use of chemicals, particularly if doses are sub-lethal.
There is potential for resistance to the chemical families organochlorines, organophosphates, synthetic pyrethroids, and insect development inhibitors and other insecticides used to treat the biting sheep body louse (Bovicola ovis).
Synthetic pyrethroids in some parts of New Zealand are showing signs of lice resistance, she said. This has not been seen in Marlborough however.
The answer is to treat lice at the right time and the right way, and to alternate chemicals – the best option is shower or plunge dip two weeks after shearing.
“Farmers’ have gone away from this for labour reasons but it is effective.
“Lice live right on the skin, so good saturation of the products is required in order to achieve good control.”
It is important not to treat straight off shears as lanolin is needed to make the products bind.
Jetting can have mixed results as saturation can be difficult with low volumes of wash. Each sheep needs at least four litres of wash over their bodies so running sheep twice through a jetter is the best way to achieve this.
“Lice like to congregate on the flanks so jet nozzles must spray towards this area, not just from top and bottom.”
Problems can result from treating sheep with a combination lice and fly-control product at tailing time in long wool and expecting it to control lice.
Lice transfer from ewes to lambs within 24 hours of birth. Pre-lamb shearing can remove up to 80% of lice, followed by a dip or pour-on or spray-on – hand jetting will reduce the risk of lambs getting lice.
Some products have an immediate knock-down effect while insect growth regulator products have more long-term action (up to six weeks).
The damage done
Dipping long wool sheep by any method leads to high insecticide residues which could potentially result in product being rejected from international markets.
Wool quality declines with lice infestations. Itchiness causes the sheep to rub or chew their wool. This results in pulled wool – breakage and reduced fibre length – and colour changes and staining.
Cockle in pelts prevents colour uptake and causes leather quality issues.
Shear hard work
Sheep occasionally enter the shearing shed carrying lice, but the pests do not have a huge impact on the job of removing wool.
Northland shearing contractor Tom Stilwell said he saw the odd ewe with lice during pre-lamb shearing. They could only see the lice if the infestation was particularly bad.
He could tell those with lice immediately after shearing though, as they would rub against the catching pens once their fleece had come off.
Lice could make the wool a bit drier or stickier to cut, Stilwell said.
Punga Mullins, of Paewai Mullins Shearing in Southern Hawke’s Bay, said they only occasionally came across lice at shearing time and he had not seen any so far this year.
Mullins said it was not pleasant or ideal and could make shearing a bit harder because of the crust lice left in the wool but it didn’t stop them from shearing the sheep.
Central Otago-based shearing contractor Peter Lyon only saw lice infestations occasionally.
When he did it was more often when the sheep were handled at crutching in autumn.
Lyon said lice did not have a big effect on their job but it was not nice to see it having an effect on farmers’ production and most farmers were vigilant against the problem.
