4.7. Brood attractiveness
Varroa mites prefer drone over worker brood (Fuchs, 1990; Rosenkranz, 1993; Boot et al., 1995). Honey bee lineages also vary in the attractiveness of their brood for the mites (Büchler, 1990; De Guzman et al. 1995; Guzman-Novoa et al., 1996). We here present a bioassay destined to compare brood attractiveness in comparable condition, i.e. in the same colony. This method is adapted from those of Ellis and Delaplane (2001) and Aumeier et al. (2002).
Procedure to test brood attractiveness
1. Queens from four different lines (Fig. 22, Step 1) tested are placed individually on a drawn, empty comb contained in a queen excluder cage.
2. Allow queens to oviposit for 24 h.
3. Twenty four hours later, the queens are removed from the comb, but the comb is left in the cages.
This limits further queen oviposition in the target combs.
43. Leave each comb with eggs is in its respective line colony for 6 or 7 days (for worker / drone brood respectively).
5. After this, cut out of the comb squares or circles of comb containing L4 larvae (see the section on obtaining brood and adults of known age in the BEEBOOK paper on miscellaneous methods (Human et al., 2013)) using a sharp knife or a metal spatula (Fig. 22, Step 2).
6. Take one section of larvae from each of the lines to be tested and randomly combine the sections of different lines in the centre of a fully drawn comb in which squares or circles of fitting sizes have been removed.
Each frame now contains one brood section from each of the four genetic line colonies (Fig. 22, Step 3).
7. Prepare a varroa receiver colony (Fig. 22, Step 4) by removing the open brood in the colony to decrease target brood (the frame just created having the various queen lines represented on one comb) competition with the colony’s own brood for the varroa present in the colony.
Alternatively, the queen in the receiver colony could be caged 22 or 25 days earlier to let all worker and drone brood, respectively, emerge.
7. Introduce each reconstituted comb holding eggs from each genetic line into the receiver varroa infested colony (Fig. 22, Step 4).
The colony can receive up to four frames generated from the procedure if the varroa populations are high enough. Mites invade worker cells from 15–50 h preceding cell capping (depending on the sex of the larvae).
8. Fifty hours post cell capping, remove the sections of capped pupae from the colonies.
9. If necessary chill in a freezer.
10. Assess varroa infestation rates (see section 4.2.3. ‘Measuring the infestation rate of brood and adult bees’).
Pros: brood attractiveness is assessed in comparable condition, i.e. in the same mite provider colony.