Both sexes feed along the undersides of their hosts causing sores that gradually enlarge with continual attack year after year. Calves less than 5 to 6 months old are not attacked. Summary of Life History Adult females dart from their resting place on the cattle to lay their eggs in fresh cattle droppings. The maggots develop very quickly in the manure, and a life cycle may be completed in 2 weeks, resulting in several generations per season. Populations usually peak in late July and August.
Horn flies overwinter as pupae or larvae under manure pats.
They will not breed in feedlots. Horn fly resistance to pyrethroid-impregnated ear tags is becoming widespread in range pastures. Do not apply ivermectin pour-on to dairy cattle within 2 months of calving. Return to Livestock Pests Factsheet Copyright Province of British Columbia All Rights Reserved.