Varroa resistance: what trait/s should you focus on?

We’re all worried about what varroa means for beekeeping in the future. There is no doubt that things will change.

Imagine living with varroa without putting chemicals into hives that can harm bees, honey, wax, people, and beekeepers’ wallets. Through breeding, this can be a reality.

Honey bees have natural defences to keep varroa populations below economic thresholds before chemical treatment is necessary. By breeding queens and drones from well performing colonies that keep mite populations low, varroa resistance in commercial stocks can be developed. But without focused selective breeding, beekeepers are locked on the ever dangerous ‘chemical treadmill’ which only creates more virulent mites and associated viruses, ultimately doing more harm than good.

Professional Beekeepers has provided a series of articles on varroa resistance traits. The tables below bring together all the options for consideration by beekeepers.

Breeding should focus on traits that result in decreased mite loads, increased colony survival and reduced use of chemicals. Proof of low infestation levels without treatment or with minimal treatment should be provided when calling stock ‘resistant’. Breeding is not a ‘set and forget’ varroa intervention, it will take time to develop resistant stock and selection must be ongoing to retain the traits in the population. One generation of outbreeding (queens mating with stock that doesn’t carry the traits) can result in traits being lost.

There are few proven varroa resistant stocks that are currently commercially available. There are several reasons for this including:

  • Poor quality of the starting stock – the bees don’t have traits that beekeepers want, making the program unviable
  • The difficulty of accurately measuring varroa resistance traits
  • Focusing on single traits that cannot confer high enough resistance on their own
  • Release of stock before it has fully developed resistance or appropriate commercial traits – this results in a loss of trust for the program
  • Breeding programs that don’t select for resistance in both queens and drones, meaning the trait doesn’t reach high levels in the population.

The Resilient beekeeping in the face of varroa report recommended focusing on two bee breeding strategies— Mite Population Growth (MPG) and Mite Non Reproduction/Varroa Sensitive Hygiene (MNR/VSH). Both strategies confer lower mite loads resulting in reduced or no chemical treatments, and increased colony survival.

Figure 1. Traits that contribute to varroa resistance.

Hygienic behaviour (HYB): Unhealthy brood signals are detected by workers and they remove the brood, along with varroa if that was the cause of the illness.
Uncapping/recapping (REC): Workers detect signals from the brood that either they are unhealthy or infested with varroa, and remove the pupae and varroa.
Varroa-sensitive hygiene (VSH): Workers detect that a pupae is infested and remove it, along with varroa.
Grooming and biting (mite biters; MB): Workers groom themselves or others to remove mites, which may cause damage or death of varroa.
Short development time (SDT): Bee brood have a shortened development time, meaning that varroa either doesn’t have enough time to produce viable offspring or produces fewer viable offspring. The mite non-reproduction (MNR) suite of traits includes VSH, REC, HYB and other traits that we have not yet found a way to select for; it includes any trait that prevents varroa from reproducing. Selecting for low infestation rates (LMP) or low mite population growth (MPG) will select for all of the above traits, and any we have not yet identified. Selecting queens to not pass on viruses to the eggs they lay (suppressed in ovo virus infection; SOV) is a tolerance trait that is thought to improve the ability of honey bees to live with varroa (tolerance) rather than resistance (keeping mite numbers low). Some populations have viruses but do not suffer from ill effects (virus tolerance; VT). Naturally resistant populations will have some of the above traits but also tend to have small colonies and small brood nests, which reduces the number of hosts, swarming or absconding and thus introduces breaks in the brood cycle and reduces mite population growth, or small populations, which reduces reinfestation.


Breeding groups should be supported by a breeding focussed extension and education program with dedicated staff that will assist the groups in setting up selection and breeding structures, and assist with selection assays, evaluation, data collection, and data interpretation.

Table 1. Varroa resistance traits selected for in breeding programs

What is selected forHow it worksHow the trait is measured
Hygienic behaviour (HYB)Detection and removal of dead brood.Workers remove dead or sick brood. If sick due to varroa the foundress likely lives but loses a brood cycle.Killing brood with either pins or liquid nitrogen, or using hydrocarbons associated with unhealthy brood, and seeing how many are removed after a period.
Uncapping and Recapping (REC)Detection of cells that may be infested with varroa; an indication of VSH and MNR.Holes in the wax caps change the temperature and humidity of the cell, killing immature mites. Foundress likely lives but loses a brood cycle.Carefully cut wax caps off pink-purple eyed pupae, use microscope to count the number that have had part of the wax cap replaced.
Varroa sensitive hygiene (VSH)The detection and removal of pupae infested with varroa; the major contributor to MNRWorkers remove brood infested with varroa. The foundress likely lives but loses a brood cycle.Best performed by inoculating brood with varroa and counting the number removed after a period of time. Alternatively, place a comb with a known infestation level in a colony and remeasure infestation after a period of time.
Short development time (SDT)Shortened bee developmental time.Fewer mite offspring reach maturity.Inspecting frames at regular periods to determine the length of time taken for brood to emerge from the time they are capped.
Grooming/Mite biting (MB)Removal of mites from adults.Incapacitation or death of varroa outside brood cells.Collecting mites from bottom boards and inspecting for bee-caused damage under a microscope.
Suppressed in ovo virus infection (SOV)Reduced propensity for queens to pass viruses on to eggs.Reduces viral prevalence (virus resistance) thus reducing vectoring by varroa.Viruses present in a pool of 10 drone eggs.
Mite non reproduction (MNR) also referred to as suppressed mite reproduction (SMR)Mite reproduction is reduced, either by the workers, the brood, or the mites themselves.Workers remove brood infested with varroa. Increased signalling by brood that they are infested. Changes in molecules that varroa needs to activate their ovaries. Changed signals by brood that reduce infestation at appropriate age. Age of mites – either too young or too old. Quality of bee that mites spend the dispersal period on.Infestation levels in brood (brood mite load; BML) and ratio of reproducing to non-reproducing mites.
Low mite population (LMP) or mite population growth (MPG)Any trait that limits varroa population growth.Some or all of the traits above or ones that we do not know of.LMP through an alcohol wash, but also brood infestation or mite fall on bottom board. MPG is the same but measured over time.

Table 2. Advantages and disadvantages of different selection methods

AdvantagesDisadvantagesHeritabilityEvidence for reduced population growthEvidence for increased colony survival
Hygienic behaviour (HYB)Assists with American foulbrood, and may assist with chalkbrood.
Can be selected for in the absence of varroa.
Multiple methods in use.
Requires specialist equipment.
Single mechanism.
Does not keep MPG below treatment level.
0.06 — 0.65Variable
Poor
Uncapping and recapping (REC)Doesn’t result in death of brood.
Can be selected for in the absence of varroa.
Time intensive.
Single mechanism.
Requires a microscope.
0.04 — 0.57NAVariable
Varroa sensitive hygiene (VSH)Main mechanism of MNR.Time intensive.
Single mechanism.
Results in death of brood.
0.18Variable
Variable
Grooming/Mite biting (MB)Doesn’t result in death of brood.Multiple methods in use.
Mite fall and damage to mites may not be a result of actions by bees.
Requires microscope.
Single mechanism.
0.00 — 0.71Variable
Variable
Short development time (SDT)Low measurement accuracy.
Difficult to implement.
Affected by temperature.
There is a limit to how fast bees can develop.
0.22 — 0.89Poor
Variable
Suppressed in ovo virus infection (SOV)Reduces the prevalence of viruses that can be vectored by varroa, the main cause of colony death.Requires genetic testing
Unclear if and how transmission of viruses via varroa plays into viral landscapes in adults.
0.21 — 0.53NANA
Mite non reproduction (MNR)Selects for any trait that prevents mite reproduction.Time intensive at low infestation levels.
May require having highly infested colonies to provide mites to challenge colonies with.
0.06 — 0.63Variable
Variable
Low mite population (LMP) and mite population growth (MPG)Selects for any trait that keeps mite populations low.
Quick and cheap.
Needed for monitoring mite load to determine if/when to treat.
LMP:
0.00 —1.24;
MPG:
0.04 — 0.32
Strong
Strong
Natural resistanceNo treatment cost.
No chemical treadmill.
Only bees with resistance mechanisms will survive.
Any trait that contributes to resistance is selected for.
Short-term colony losses.
Populations tend to be small.
Traits are variable and often not conducive to beekeeping.
NANANA
Heritability gives the proportion of variability that is due to genetics. High values indicate the trait will be easy to select for in a population, low values indicated it is hard to select for in a population. When a trait is marked variable, this means it has proven useful in some populations, and not in others. Evidence for reduced mite population growth and increased colony survival (Guichard et al. 2020).

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