Fly Strike in Calves

Calving presents the opportunity for fly strikes. Fly strike begins when a female blow fly, usually metallic green bottle fly, lays a small batch of oblong white eggs on an attractive site. These scavenger flies do not deposit eggs on clean dry tissue; they select lingering wounds, accumulations of soft manure around the anus, or tissue around a healing umbilicus. Exudates from irritated and inflamed tissue attract more flies, compounding the problem. Although these maggots do not feed on healthy tissue, their activity can cause damaged tissue in the area to die. Animals become restless and may stop feeding. Consequences include extreme discomfort, infection, and dehydration, and in extreme cases, death of the animal.

Green Bottle Flies

Green bottle flies (Figure 1) are common insects in Kentucky. They lay their eggs in carcasses, feces, and garbage. Large numbers of them are around by the end of summer and they constantly move in search of places to deposit eggs. The eggs hatch in less than a day and the maggots feed on bacteria in these sites for 3 to 4 days before entering the soil to pupate. Adults emerge in about 10 days to start the cycle again.

Figure 1.  The green bottle fly Phaenicia sericata, is commonly involved in fly strike cases (Photo: Lee Townsend, Extension Entomologist).


Removing manure accumulations and cleaning wounds before strike can occur is the key to preventing problems. This is the way to approach established strikes as well.  Several fly strike products (screw worm aerosols) containing 0.5% permethrin can be used to protect areas. Read and follow the label directions. Contact your veterinarian in cases of severe strike.


By Lee Townsend, Extension Entomologist



Posted in Livestock Pests
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