Material: a rectangular graduated container in which 300 bees fit. Three hundred bees occupy a volume of 100 ml water. Fill this volume of water in a container and mark a line at the water surface (Lee et al., 2010b, www.beelab.umn.edu). Given that bee sizes change with race, this volume should be verified and adapted for the particular bee under scrutiny.

1. Hold the frame at approximately 10 degrees from the vertical.

2. On the upwards facing side, slide the graduated container downwards on the back of the bees so that they tumble in it, making sure the queen is not one of them.

3. Rap the cup on a hard surface to be sure the bees are at the marked line; add or subtract bees as needed.

4. Collect 3x 300 workers from any three frames in the first brood box.

Sampling such a large number of bees takes into account variations among frames to obtain an average infestation rate, and does not damage the colony if a non-destructive method is used to loosen the mites from the bees (see section ‘Powdered sugar’). Strong colonies (>10,000 bees) are not dramatically affected by the removal of this amount of bees and will quickly recover. However, for analysing varroa population dynamics throughout the whole season with frequent and destructive sampling of bees (e.g. at 3-week intervals), lower numbers of individuals (300 bees per sampling date) should be used.