Disease Resistance Across Subspecies

Race-level disease resistance means that some honey bee subspecies, ecotypes and selected lines suffer less damage from diseases and pests thanks to inherited defensive behaviours. This resistance mostly comes from hygienic behaviour, grooming and traits that limit Varroa reproduction. Resistance is not immunity: resistant lines also need regular monitoring and intervention when necessary.

Where does disease resistance in bees come from?

The honey bee's defence consists less of the immune systems of individual bees than of colony-level behaviours. Researchers call this social immunity. These behaviours are heritable; in other words, they can be strengthened through queen selection or weakened when neglected.

• Hygienic behaviour: Workers detecting sick or dead brood inside a sealed cell, uncapping it and throwing it out. The best-known defence against American foulbrood and chalkbrood.
• Varroa-sensitive hygiene (VSH, Varroa Sensitive Hygiene): Specifically targeting and cleaning out sealed cells in which mites are reproducing.
• Grooming (allogrooming): Bees picking mites off their own bodies or off their nestmates and damaging them by biting.
• Uncapping and recapping (recapping): Workers opening a sealed cell, checking inside and sealing it again; this can disrupt the mite's reproductive cycle.
• Short sealed brood period: The shorter the time a cell stays sealed, the fewer offspring the mite can raise inside.

A natural example: the Asian honey bee and varroa

The original host of Varroa destructor is the Asian honey bee (Apis cerana). This species has co-evolved with the mite for a very long time: it grooms strongly, cleans out mite-infested brood, and in its colonies the mite can essentially reproduce only in drone brood. As a result, it lives in balance with the mite.

The western honey bee (Apis mellifera) does not have this history. When the mite switched hosts to the western honey bee in the twentieth century, it became devastating because it could also reproduce in worker brood. The resistance seen in the western honey bee today is a partial resistance formed by natural or artificial selection within this short shared history.

Which bee races and populations are more resistant?

The strongest evidence on resistance comes from populations that have had to live with varroa for a long time without treatment or with little intervention:

• African subspecies and Africanised bees: Populations in South Africa and Brazil can often live with the mite without treatment; a short development time and strong hygiene play a part in this. However, these bees are very defensive.
• Russian (Primorsky) bee: Selected from a population that lived alongside varroa for a long time in the Russian Far East; mite increase has been shown to be slower.
• Surviving populations in Europe: Colonies left untreated for years that survived in places such as the Swedish island of Gotland, France and Norway have been studied; suppressed mite reproduction and increased recapping behaviour have been reported.
• Selected lines: Lines selected specifically for hygienic behaviour or VSH are produced through breeding programmes in many countries.
• Syrian bee: Tolerance to varroa is also reported in this eastern Mediterranean subspecies.

Why does resistance vary by region?

A resistant line that does well in one region may not perform the same in another. Resistance behaviours are affected by climate, forage conditions, colony strength and mite pressure from surrounding colonies. In areas with many apiaries, mites arriving through robbing from weak or collapsing neighbouring colonies (reinvasion) can put pressure on even the most resistant colony.

In addition, the daughters of a resistant queen mate openly with the susceptible drones in the area. That is why the resistance trait weakens within a few generations unless it is maintained by ongoing selection in the region.

Trade-offs of resistant bees

Resistance traits sometimes come together with other, undesirable traits:

• Some tolerant populations are more defensive.
• Some swarm more often; some lines, such as the Russian bee, keep queen cups in the hive all the time.
• Some build smaller colonies, and honey yield can stay low.
• In hygienic lines the brood area may look a little more patchy in the first years; this is due to the cells being cleaned out.

How can a beekeeper increase resistance in their own apiary? Step by step

1. Count mites: In spring, late summer and autumn, measure the mite rate in about 300 bees using an alcohol wash or the icing sugar method; record the differences between colonies.
2. Measure hygienic behaviour: Kill a set area of sealed worker brood (e.g. 100 cells) by piercing or freezing; check the proportion of cells cleaned out after 24 hours. By the standard criterion, a colony that cleans out at least 95% in 24 hours is considered hygienic.
3. Monitor the bottom board: The proportion of damaged (bitten) mites is an indicator of grooming.
4. Rear queens from good colonies: Select colonies with low mite increase and high hygiene scores as breeders.
5. Select the drones too: Leave room for drone brood in your selected colonies; limit drone production in weak colonies.
6. Requeen poorly performing colonies: Colonies that constantly carry a high mite load or fall ill often should not be used for breeding.

These steps can make a clear difference in the apiary within a few years without buying a special line. Beekeepers in the area selecting together magnifies the effect.

Racial resistance is not enough against notifiable diseases

Hygienic behaviour increases a colony's chances against American foulbrood; but this disease is notifiable. If you suspect it, do not attempt treatment on your own: isolate the hive, do not move frames or tools, and report it to the official veterinary service / local agriculture authority. A definite diagnosis is made in the laboratory.