Resistance is widespread to these chemicals, so why are they still used? Because appearances can be deceptive.
Organophosphate (OP) flystrike dressings are still commonly used because they often appear to get the job done. But for many years, widespread and severe resistance of maggots to organophosphates has been documented.
Such dressings are generally used by producers in one of two ways and the method chosen greatly affects whether a strike heals successfully.
Where a strike wound is shorn, including an unaffected strip around the wound, and an organophosphate dressing is applied, the strike is likely to resolve.
But where the dressing is simply applied to a strike wound without it being shorn, there is a much greater chance of the strike continuing.
In both situations, the organophosphate dressing will be survived by any OP-resistant maggots.
But in the first situation, the majority of maggots are removed by shearing and the area is made unfavourable for remaining maggots. Few remain to even need the services of the dressing.
In the second situation, the dressing is totally responsible for killing the maggots and if resistance is present, then sufficient maggots may survive to continue the strike.
Over time, the second practice would favour an increasing level of resistance to organophosphates in the local fly population, through the selection of resistant maggots that live through the dressing.
So what should you do? In the first instance, the best practice is to clean up the wound with mechanical shearing, which generally removes more wool and maggots than hand shearing. Lightly brushing the shorn wound with a piece of cloth is helpful to remove more maggots.
A few centimetres of unaffected skin should also be shorn around the wound to ensure that there are no missed channels of strike and so that the adjacent wool does not protect the wound and provide a favourable place for maggots to survive.
Collect the maggots and affected wool and place these in a plastic bag in the sun for some hours to kill them, which will help to prevent the build up of flies. If the sheep does need to be hand clipped in the paddock, a bag or piece of tarpaulin placed under it will allow the wool and maggots to be collected into a plastic bag.
If the wound has been well shorn and most maggots removed, a dressing may not be required to stop the current strike. If in doubt use a product that will quickly kill maggots—this generally means a product with either spinosyn or ivermectin, as products with cyromazine or dicyclanil may take a few days to kill maggots, in which time they can still be harming the sheep.
However, once a sheep has been struck, it has a much higher chance of being a repeat offender and the wound will attract more flies. If only a short length of protection is required until the sheep is sold, use a product with spinosad applied just on and next to the wound. If a lengthier period of protection is required, choose a product that will give sufficient length of protection for future strikes, but with a suitably short withholding period. And don’t just treat the wound. Ttreat the sheep as per directions on the label. For example, apply a full backline treatment if the sheep has shown it is susceptible to body strike.
Don’t be complacent with chemicals when treating just the odd struck sheep. Keep rubber gloves with your chemical, including any small amounts you keep on the ute or bike for paddock treatments.