The Genetics of Mite Resistance in Honey Bees: Why It Takes Persistence and Ongoing Selection

Beekeeping has always involved selective breeding. For generations, beekeepers have chosen queens from colonies that excel in gentleness, high honey production, and overall hardiness. These traits don't appear by accident—they result from methodical selection over multiple generations. The same principle applies to breeding for resistance to Varroa destructor, the devastating parasitic mite that weakens colonies and vectors deadly viruses.

Yet, when it comes to mite resistance, many beekeepers expect quick fixes. Introducing genetics from mite-resistant stock—like those exhibiting strong Varroa Sensitive Hygiene (VSH) or response to Unhealthy Brood Odor (UBO)—into a colony once or twice won't lock in the trait permanently. Genetics in honey bees is complex, influenced by unique inheritance patterns, polygenic traits, and open mating. Without persistent selection, desired traits can dilute or disappear in just a few generations.

Honey Bee Genetics: A Unique and Variable System

Honey bees follow a haplodiploid sex determination system, which creates tremendous genetic diversity within colonies:

• Queens and worker bees are diploid females, inheriting genes from both the queen (mother) and multiple drones (fathers).

• Drones are haploid males, developing from unfertilized eggs and carrying only the queen's genetics.

• Queens typically mate with 10–20+ drones, resulting in workers that are half-sisters or super-sisters (sharing 75% genetics if from the same drone father).

This multiple mating (polyandry) promotes colony vigor but complicates trait consistency. Genetic recombination in the queen's egg production, combined with diverse drone contributions, means each new queen and her workers are genetically unique—even from a highly selected mother queen.

Many important traits, including gentleness, honey production, and mite resistance behaviors, are polygenic (controlled by multiple genes) and quantitative. Expression varies widely due to environmental factors, gene interactions, and the "lottery" of inheritance. As explained in Penn State's Honey Bee Genetics Basics, traits must be continually tested and selected because open mating introduces variability, and undesired genetics can quickly dominate without ongoing pressure.

PennState Extension

Mechanisms of Mite Resistance: VSH, UBO, and Hygienic Behavior

Varroa mites reproduce in capped brood cells, feeding on developing pupae and transmitting viruses like Deformed Wing Virus (DWV). Resistant bees disrupt this cycle through hygienic behaviors:

• Varroa Sensitive Hygiene (VSH) — Bees detect reproducing mites, uncap cells, and remove infested pupae, interrupting mite reproduction. Developed by USDA researchers, VSH stock significantly reduces mite populations.

• Unhealthy Brood Odor (UBO) Response — A newer assay and trait involving sensitivity to specific cuticular hydrocarbons released by stressed or parasitized brood. Bees with strong UBO response uncap (and often recap) cells mimicking unhealthy brood, disrupting mite cycles without always killing the pupa. Research shows colonies scoring >60% on UBO assays (e.g., using synthetic UBeeO) maintain low mite levels (<3 mites/100 bees) and higher survival rates.

These behaviors overlap with general hygienic traits that combat chalkbrood, American foulbrood, and other issues. Studies confirm that selecting for UBO or VSH leads to broader disease resistance, lower viral loads, and better overwintering.

At our operation, we specialize in breeding and providing bees selected for strong UBO expression. These bees exhibit heightened sensitivity to unhealthy brood odors, enabling proactive removal or disruption of mite-infested cells. Our goal is straightforward: flood local bee populations with UBO-like genetics through queen and nuclei sales, encouraging open mating that spreads resistance traits area-wide.

The Challenge: Traits Fade Without Continued Selection

Introducing resistant genetics is like adding a few winning lottery tickets to a big draw—the odds improve slightly, but you need many more for consistent wins.

• In open-mated colonies, drones from non-resistant hives dilute the trait.

• Polygenic resistance means expression requires accumulation of favorable alleles over generations.

• Without selection pressure (e.g., no treatments forcing survival of the fittest), weak colonies persist and contribute drones.

Research on bidirectional selection shows progress in 2–3 generations, but reversion occurs quickly if selection stops. Customers sometimes report: "The bees I bought from you didn't show strong mite resistance." That's expected after one or two introductions—it's not a "one-shot" guarantee. Success requires grafting from strong performers, raising daughters, and repeating over multiple cycles. Sometimes luck strikes early; often, it takes persistence.

Our Mission: Building a Resilient Local Bee Pool

We're committed to long-term improvement. By providing UBO-selected queens and packages, we aim to increase the prevalence of these genetics in local drone congregations. When your open-mated queens produce drones carrying UBO traits, resistance spreads naturally.

Encourage this by:

• Testing your colonies (UBO assays are becoming available commercially).

• Breeding only from top performers.

• Sharing strong drones through drone flooding or isolated mating yards.

For more basics on honey bee genetics and why continual selection matters, check this excellent resource: Honey Bee Genetics Basics.

Genetics isn't magic—it's methodical persistence. Together, we can shift the odds toward healthier, treatment-free bees that thrive against Varroa. If you're ready to add UBO genetics to your apiary and commit to the process, reach out—we're here to help build that resilient future.