‘Refugia’ refers to the part of a worm population that have not been exposed to drenching when other animals are treated (in refuge from the drench).
The population of worms on a property is composed of stages within the cattle and stages on the pasture. Keeping a significant population of larval worms on the pasture that originates from worms susceptible to the drench can slow the build-up of parasites resistant to drenches.
As cattle graze, they ingest a mixture of larvae of resistant and susceptible types, and if sufficient larvae of susceptible origin are present, the genes for resistance will be diluted, and the build-up of drench resistance slowed.
The proportion of larval worms on pasture compared to adult worms inside the animal varies with environmental conditions. For example, few worm eggs or larvae survive under hot and dry condition (such as in a feedlot or in dry paddocks). In contrast, small paddocks with high stocking rates will often be heavily contaminated with worm larvae, meaning the proportion of the worm population living as adults in the cattle will be relatively small. A high refugia relates to a higher proportion of susceptible to resistant worms on the pasture. It does not relate to the number of worms.
The location of worms at any given time, and the relative favourability of the external environment (pasture) for worm larval development, largely determines the extent of refugia. If the majority of worms are in the animals, and conditions on the pasture are hostile to larvae, drenching the mob will reduce the refugia, as any resistant worms surviving the drench will not be diluted with susceptible ones.
Understanding the seasonal environmental patterns that influence worm larval development and survival in your region should be used to guide your refugia management.