Pasture Fly Resistance

Cow and calf on range
Regardless of fly control product used, plan for resistance by rotating products between insecticide Mode of Action (MoA) groups every fly season, and if possible, during the fly season. Photo credit T.L. Meyer.

Regardless of your choice of livestock fly control product and application method, plan for resistance. For example, many horn fly populations in Nebraska exhibit a level of resistance to synthetic pyrethroid insecticides.

Fly resistance to insecticides is defined by the Insecticide Resistance Action Committee (IRAC, a heritable change in the sensitivity of a pest population reflected in repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species. Simply put, flies can develop resistance to products if used repeatedly.

The recommended practice to manage resistance is to alternate insecticide Mode of Action (MoA) groups. This applies to dusts, insecticide ear tags, animal sprays, pour-ons, insect growth regulators (IGRs and also known as feed-throughs), and compressed air application devices.

Insecticide MoA groups are based on how the insecticide works against insects. Continual use of products from a single MoA group can lead to reduced control and resistance to all products in that MoA. To improve fly control and minimize resistance, do not apply insecticides within the same MoA repeatedly. Rotate between MoA groups every fly season and if possible, during the fly season. The table below contains information about the primary action site, the active ingredient, and a partial list of products from each MoA group currently labeled for use against pasture flies.


IRAC ( MoA Classification of Pasture Fly Control Products Available

Main Group,
Primary Action Site

Sub-group (if applicable), Active Ingredient

Partial List of Products

1 Acetylcholinesterase (AChE) Inhibitor

Nerve Action

1B, Organophosphates

Corathon™, MAX40™, Patriot™,

Prolate/Lintox-H-D™, Co-Ral®, Rabon®, Ravap®,


3 Sodium channel modulators

Nerve action




PYthon™, Cylence Ultra™, Saber™ Extra, Permethrin®

Evergreen® EC60-6


6 Glutamate-gated chloride channel (GluCI) allosteric modulators

Nerve and muscle action



Abacmectin, XP820™, Ivomec®,

Eprinex®, Dectomax®, Cydectin®

7 Juvenile hormone mimics

Growth regulation

Juvenile hormone analogues

Methoprene (Altosid®) IGR

15 Inhibitors of chitin biosynthesis affecting CHS1

Growth regulation



17 Molting disruptor




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