Final answer:
The number of horn flies on cattle to become economically important can range from 200 to 500 per animal. These flies can cause economic losses due to decreased weight gain, reduced milk production, and damage to the animal's hide.
Step-by-step explanation:
The number of horn flies on cattle to become economically important varies depending on several factors, including the breed of cattle, the size of the herd, the location, and the management practices. Generally, it is estimated that a population of 200 to 500 horn flies per animal can cause economic losses in terms of decreased weight gain, reduced milk production, and damage to the animal's hide. Horn flies are blood-sucking insects that feed on cattle and cause irritation and discomfort.