Sheep farmers are being warned about the over-use of some insecticides, with the news that lice in some areas appear to have become resistant to the active ingredients in Crovect and Ectofly.

Commonly used, these two synthetic pyrethroid products can be used to treat lice for up to 14 weeks, ticks for up to nine weeks and headfly for up to four weeks and blowfly strike for 6-8 weeks. However, following almost 30 years of use, it appears that lice in some parts of Shetland have become resistant to the products.

"We've been using Crovect to control lice on sheep since the early 1990s and never had a problem with it until the last few years," said Kenny Anderson who runs 900 breeding ewes plus followers mostly on common grazings.

"To begin with we'd apply the normal 10ml per ewe and 5ml per lamb and it would control the lice fine, but over the past couple of years we've had to double the dose rate and sometimes have to give them a double dose a week later," added Mr Anderson.

"We don't have any scab on the island and few tick to speak of, so in my mind it is the parasite that has changed and not the product."

Backing up these statements, James Jamieson who farms 270 ewes also on common grazings and in-bye parks, nearby said: "I'm having to give some sheep up to 30ml in one go which is more than double the dose rate recommended, for the product to work. I used to spray my sheep once in September/October and that was enough, but now, I'm having to use it twice and that is with a far bigger dose each time than what is recommended."

According to the Sustainable Control of Parasites in Sheep's (SCOPS) advisor Lesley Stubbings, this is not a first, with previous reports of resistance in lice to the cypermethrin pour-ons.

There are, however, other reasons why treatment may apparently fail.

"The problem is that when people use these products in fully fleeced sheep it is very difficult for the lice to be exposed fully to the product – this is why for years now, SCOPS have advised farmers not to use them in this way – the best time to use them is ‘off shears’ when the product can get down to the skin (and hence the lice) effectively," she said.

"Farmers should also make sure the presence of lice is diagnosed by a vet before applying the product."

However, she also pointed out that controlling any sort of disease or pest is difficult when the sheep are on common grazings and therefore encouraged farmers to plunge dip them in an organo phosphate product.

"My advise would be that the Shetland farmers think about getting a mobile dipper on the island for a period this summer to get everything dipped and nail the louse problem."

* The two products are sold with a moderate risk of resistance as biting lice developed high resistance to synthetic pyrethroids in Australia and New Zealand in the 1990s.

In Australia they were eventually withdrawn from the market for biting lice control on sheep. Several cases have been reported in the UK as well, but so far it seems to remain a limited problem. Nevertheless it is a warning, and the more synthetic pyrethroids are used against biting lice, the higher the risk for resistance to develop.

This also means that if this product does not achieve the expected efficacy against the mentioned parasites, it can be due to resistance and not to incorrect use, which is usually the most frequent cause of product failure.