How to prevent blowfly strikes

THE month of June is normally a high-risk period for blowfly strike, however with changing weather patterns it is not uncommon to hear of flystrike as early as March or as late as October.

So rather than waiting for the onset of a fly strike, preventative measures must be taken.

Sheep are most commonly struck around the tail where the wool becomes soiled by faeces and urine. Castration and tail wounds; clipping wounds; head wounds on fighting rams; and any open wound will attract flies, particularly if infected by bacteria.

The smell of foot-rot is particularly attractive to flies, especially blow flies, and often the fleece over the chest wall becomes contaminated from the infected foot when the sheep lies down.

Fly eggs, once laid, hatch within a few hours or days. The emerging larvae or maggots are very active and secrete enzymes which liquefy the skin and flesh of the sheep upon which they are feeding.

In suitable weather conditions in the summer, it takes less than a week for the fly to emerge, but often it is two or three weeks.

It’s vital to be aware of the symptoms of blowfly strike which include agitation, matted and discoloured wool and a distinctive smell.

With flies laying thousands of eggs, it is easy to see why the population of blow flies increases so rapidly under warm, humid weather conditions.

The main objective is not to allow sheep to become susceptible to attack before treatment. Shorn sheep and lambs with short fleeces are not usually attacked, but as the fleece grows so too does the risk of fly strike.

Good grassland management, where sheep and lambs are moved regularly and lambs dosed for Nematodirus Battus, will ensure good parasite control and reduce the incidence of blowfly strike.

The key to prevention of blow fly strike is to treat sheep and lambs early with pour on of choice. The main two pour-ons for prevention are Insect Growth Regulator and Pyrethroids.

Insect Growth Regulator (Active Ingredient: Dicyclanil) has a withdrawal period of seven to 40 days, which basically stop first-stage larvae from developing into the destructive second-stage larvae and, so, prevent the disease by breaking the blowfly life-cycle.

The second type of pour-on for the prevention of blow-fly strike in sheep are Insecticidal Cypermethrin-based products (Pyrethroids).

These basically treat and prevent new strike. Watch out for the withdrawal period if treating lambs close to their target weight, select IGR protection with a short withdrawal period of seven days to treat these lambs.

As these products are expensive, it is important to apply to clean sheep on a dry day with the correct application equipment and head.

Use a four-stroke procedure in applying the pour-on. The first stroke will be from the shoulders to halfway down the back; the second stroke will finish off the back down to the rump.

The third stroke will be up one side towards the tail, and the fourth and final stroke will be up the other side towards the tail.

Bring the third and fourth stroke upwards to allow waste product to land on the back as opposed to the ground.

Prevention is the best form of control and a long period of cover is more cost-effective.