The proportion of worms that are not exposed to selection by drench, i.e. those “in refuge” from drenching which escape selection for resistance and contribute to the next generation.

There are two major populations of “in refuge” worms:

1. Eggs and larvae on the ground
Normally in New Zealand, most (~ 85 – 95%) of the worm population are found as larvae on pasture (refer below). So when animals are drenched, only a small proportion of the worms are exposed.

Roundworm life cycle

2. Worms in stock that are not drenched
If all stock classes are drenched, this increases the population of worms exposed to anthelmintic. More importantly, it does not leave any animals for the susceptible parasites to cycle through.

Reducing the proportion of the total worm population on a farm that is exposed to drench will slow down drench resistance.