Understanding flystrike risk
What makes sheep susceptible?
Recent research has shown that susceptibility to breechstrike is dependent on the following factors:
· Breech wrinkles
· Wet dags
· Breech cover
· Yellow wool
· Urine stain
The most important determinant of breechstrike susceptibility is breech wrinkle score. Score 5 animals (heavily wrinkled) are highly susceptible to breechstrike. Score 1 animals are plain and have good resistance to breechstrike.
In high rainfall areas dags can be as important as wrinkle in determining breechstrike susceptibility.
Other factors that will influence susceptibility to flystrike generally include:
· Body wrinkle
· Fleecerot, lumpy wool and footrot
· Physical injuries of some type (eg poll damage or marking wounds)
· Long moist wool
· Existing strike
Reducing the risk of flystrike involves an integrated approach of management and breeding to reduce the risk factors affecting a flock. The decision to stop mulesing requires each sheep producer to re-assess the integrated approach that will best suit their environment, enterprise and business situation.
Causes of flystrike
The advent of flystrike depends on the following factors:
- Presence of Lucilia cuprina flies
- Warm humid climatic conditions that persist for 3 days or more
- Susceptible sheep
The fly that causes most flystrike in Australia is Lucilia cuprina. It is generally found once the temperature is over 170 C and there has been sufficient rain to moisten the soil where fly pupae are located. The fly pupae hatch from the warm moist soil and become flies.
Lucilia take about 4-8 days before they are mature enough to lay eggs, then after a further 6-24 hours the eggs hatch and become maggots. Over 3-4 days the maggots grow and moult several times and feed on the sheep’s skin in the last day or 2.
The maggots then fall from the sheep and burrow into the ground where they stay as pupae until conditions are again right for them to hatch. This may be as little as 1 or 2 days under ideal conditions of temperature and moisture or several months through the winter period.
In most parts of Australia, Lucilia cuprina flies require sheep to complete their lifecycle and generally only travel a few kilometres and stay near sheep. Windy conditions of more than a stiff breeze (~30km/hr) limit their activity.
Early in the fly season the few flies present may not be successful in causing flystrike.
Other flies that cause strike are Calliphora (native brown flies) and Chrysomyia (secondary strike flies).
Download 'Triggers leading to blowfly strike' - see below.
CJ Hawkins Homestead, University of New England, Armidale NSW 2351