This multi-stage process is shown in the illustration below (click 'next >' at the end of each stage to move on).

How fast does drench resistance develop?

Drench resistance is a matter of when not if. However the rate is incredibly variable, ranging from a few years to more than 30 years.

Whether resistance takes a few years or a few decades to develop, once a critical proportion of resistant worms is reached the rate is very rapid. (See Figure 1.)

Figure 1: Growth in drench resistance over time

The rate of resistance development will depend on:

• the period of time drench-resistant worms lay eggs without competition from drench-susceptible worms (the period resistant worms have a reproductive advantage)
• the proportion of the total worm population exposed to drenching (how many suceptible worms are in refugia).