Dynamics of permethrin resistance in a colony of horn flies (Diptera: Muscidae)

A colony of horn fly, Haematobia irritans (L.), was established from field-collected, resistant flies. Changes in resistance levels were monitored under a regimen with and without insecticide pressure. The colony remained closed with no introduction of susceptible flies. In the absence of permethrin pressure, resistance levels dropped to 2.5 times from an initial level of 17 times at LC50 and remained at that level from generation 12 through 33. In the presence of various insecticide pressure regimens, resistance increased to 500 times at LC90 by generation 86. In a separate study, susceptible colony flies were subjected to permethrin pressure, with first resistance evident in generation 16 and increasing to greater than 100 times at LC90 in generation 88.