Chrysomyia spp: Screwworm fly, tertiary sheep blowflies, carrion flies

General Description: Flies are 8 to 10 mm. Long. Chrysomyia adults are bluish-green. Larvae of most species are 10 to 14 mm. Long with thorn-like spines.

Life Cycle: Typical blowfly life cycle. Chrysomyia are "tertiary" blowflies, which strike sheep after primary infestations are established by Lucilia and Calliphora blowflies.

Location: Areas of skin with constant moisture; the breech and tail areas are most commonly affected, after lesions have been established by primary blowflies.

Geographical Distribution: Australia, New Zealand, Asia, Africa, United States.

Significance: Chrysomyia blowflies may weaken and kill sheep, reducing the return from sheep raising. If sheep are struck by Chrysomyia, death will usually follow.

Effect on Host: Blowflies cause parasite worry, weakness, and extensive lesions with bacterial infection. Death may occur. Larvae feed on damaged skin infested with Lucilia or Calliphora larvae, which become established on areas with skin disrupted by parasite worry from lice or mites, castration, tail docking, or bacterial infection of chronically wet skin. Myiasis develops, and expansion of the lesion produces a large wound in which bacteria multiply. Lack of feeding by sheep weakens them, and death may occur from bacterial toxins in the blood of the host. Depression, loss of appetite, stamping of legs, jerking of tail, and foul-smelling, expanding areas of loose skin under discoloured wool are characteristic signs.

Diagnostic Information: Presence of larvae in wounds.

Control: Spraying or dipping of sheep in insecticide preparations may protect against blowfly strikes. In Australia a localised high-pressure application of insecticide, known as "jetting" is used. Resistance of Chrysomyiato several insecticides has been reported. Crutching (shearing the breech area) and the Mules' operation are very helpful in preventing myiasis.

Adult fly