Resistance to Disease and Pests

Breeding for disease and pest resistance is work that uses selection to strengthen the ability of honey bee colonies to cope with varroa, brood diseases and viruses through their own defensive behaviour. The main target traits are hygienic behaviour, varroa sensitive hygiene (VSH) and grooming behaviour. Resistance breeding can reduce the use of treatments, but it does not replace regular mite counts and integrated control.

Why does resistance breeding matter?

Chemical control has its limits: Varroa destructor has developed resistance to some groups of active ingredients, some substances leave residues in beeswax, and every treatment means labour and expense. A colony that slows mite growth through its own defences can stay healthy with less frequent intervention.

Two concepts need to be distinguished. Resistance means limiting the multiplication of a pest or disease agent (e.g. preventing mite reproduction). Tolerance, on the other hand, means the colony suffers less harm despite carrying the load (e.g. resilience to viruses). Breeding programmes mostly target resistance traits, which are easier to measure.

What can the Asian honey bee teach us?

The original host of varroa is the Asian honey bee (Apis cerana). This species has co-evolved with the mite over a very long time: workers groom mites off each other, find and remove mite-infested brood, and the mite can reproduce only in drone brood. As a result, the colony lives in balance with the mite. The Western honey bee (Apis mellifera) has weaker forms of the same defences; the aim of breeding is to strengthen them.

Which traits are selected for resistance?

Hygienic behaviour

The behaviour of finding and removing dead or diseased capped brood. It is very effective against chalkbrood, increases resistance to American foulbrood and gives partial benefit against varroa. How to measure it is described on the "Hygienic Behaviour Test" page.

Varroa sensitive hygiene (VSH)

VSH (Varroa Sensitive Hygiene) is the behaviour of workers specifically picking out and opening capped worker cells in which a mite is reproducing, and removing the pupa. As a result, the offspring the mite raises in that cell cannot mature. This trait was developed in the USA from colonies in which mite reproduction was observed to be suppressed; it was later understood that this suppression came from the selective removal of cells with reproducing mites. VSH lines are now used in many countries.

Recapping

In some colonies, workers open the capping of a sealed cell, look inside and then close it again. This behaviour can disrupt the mite's reproductive pattern. It has been seen at high rates in some populations that survive without treatment; studies on its use as a selection criterion are ongoing.

Grooming behaviour

Grooming is bees removing and biting mites on their own bodies or on each other. Missing legs or body damage in mites that fall to the hive floor are traces of this behaviour. It is generally weak in the Western honey bee and hard to measure; research results are mixed, but it has been reported to respond to selection in some lines.

Short capped brood period and virus tolerance

The shorter the time worker brood stays capped, the fewer offspring the mite can raise in one reproductive cycle. There are small differences between races, but this trait is of limited use in practical breeding. Tolerance to mite-borne viruses such as deformed wing virus is important, however; because it is hard to measure directly, it is mostly assessed indirectly through overwintering success and colony health.

How is resistance measured in the field?

For these measurements to be meaningful, the colonies must be kept in the same apiary and follow the same treatment schedule. If a single colony is treated separately, its results cannot be compared.

What has been learned from treatment-free surviving populations?

In Europe and North America, several bee populations that have continued to survive for many years without varroa treatment have been studied. The Swedish island of Gotland and some groups in France are the best-known examples. In these populations, reduced mite reproduction, increased recapping and smaller colony sizes have been observed. In the USA, resistant lines have also been developed from bees brought from Russia's Far East, where they have long lived with varroa.

These examples show that resistance is possible; but most of them emerged after years of heavy losses and under isolated conditions.

The challenges of resistance breeding

• The drone side: With open mating, a trait gained can be diluted within a generation; isolated mating or instrumental insemination gives a great advantage.
• Regional work is needed: Selection in a single apiary is diluted by the genetics of the surrounding area.
• Reinfestation: The mite load coming from collapsing colonies nearby can put pressure even on a resistant colony.
• Balance with other traits: A resistant line must also be acceptable in terms of gentleness, productivity and overwintering.
• Time: Reliable results need several generations and regular measurement.

What can a beekeeper do for resistance breeding?

1. Measure: Do mite counts and hygienic tests on all colonies on the same dates.
2. Treat fairly: When the threshold is exceeded, treat all colonies at the same time and with the same group of active ingredients; rotate active ingredient groups, and follow residue rules during the honey harvest.
3. Select: Make the colonies with the slowest mite increase and the highest hygienic scores your breeders, provided their other traits are also suitable.
4. Don't forget the drones: Give these colonies drone comb; limit drone brood in weak colonies.
5. Requeen: Requeen colonies with consistently high mite loads from the selected line.
6. Cooperate: If you want to buy a tested resistant line, buy from registered producers and try it under your local conditions.