Varroa destructor, commonly known as the Varroa mite, is one of the most significant threats to Apis mellifera (the Western honey bee). This ectoparasitic mite reproduces within the sealed brood cells of honey bees and is a major contributor to colony collapse due to its parasitic feeding and vectoring of deadly viruses.
Understanding the reproductive behavior of Varroa mites is critical for beekeepers, researchers, and policymakers to develop effective management strategies.
Overview of Varroa Mite Biology
Species and Host
- Species: Varroa destructor (formerly thought to be Varroa jacobsoni)
- Primary host: Apis mellifera, though it originally parasitized Apis cerana
- Life cycle stages: Egg → Larva → Protonymph → Deutonymph → Adult
Parasitic Lifestyle
Varroa mites are obligate parasites that feed on the fat bodies and hemolymph of both adult and developing bees. Their parasitism weakens bees and transmits viruses such as Deformed Wing Virus (DWV).
Reproductive Cycle of Varroa Mites
Mite Invasion of Brood Cells
- Target cells: Primarily drone brood (preferred due to longer development time) and worker brood
- The foundress mite (a mature female) invades a bee brood cell just before capping.
- Timing: About 15–20 hours before the cell is sealed.
Reproduction Conditions
- Mites do not reproduce on adult bees.
- Reproduction occurs only inside capped brood cells, where conditions (temperature, humidity, food access) are favorable.
Mating and Reproduction Within Brood Cells
Laying of Eggs
Once the cell is capped:
- The foundress mite begins oviposition 60–70 hours post-capping.
- She lays her first egg (male), followed by multiple female eggs at regular 30-hour intervals.
Development of Offspring
- Male mite develops from the first (haploid) egg.
- Subsequent female mites (diploid) mate with the male inside the cell.
- Development time:
- Male: ~5.8 days
- Female: ~6–7 days
- Only female offspring survive to leave the cell, as males die shortly after mating.
Inbreeding
- Inbreeding is the norm: one male mates with several sister mites.
- This reproductive strategy increases mite numbers but reduces genetic variability.
Emergence and Dispersal
Mite Emergence
- When the bee emerges from the cell, the foundress and mature mated daughters exit with it.
- These mites attach to adult bees and begin phoretic phase (non-reproductive, dispersal phase).
Phoretic Phase
- Mites feed on adult bees, especially nurse bees.
- They use this phase to locate new brood cells for reproduction.
- Duration: Typically 4–11 days.
Factors Influencing Reproductive Success
Brood Availability
- More drone brood = higher mite reproduction, due to longer capping time (24 days for drones vs. 21 for workers).
Bee Defense Mechanisms
- Hygienic behavior: Bees detect and remove infested brood.
- Varroa Sensitive Hygiene (VSH): Specific trait where bees detect reproductive mites.
- Grooming behavior: Bees remove mites from themselves and each other.
Environmental Conditions
- Temperature and humidity in the brood cell affect mite development.
- Colony strength and seasonality influence mite population dynamics.
Reproductive Rate and Population Dynamics
- Reproductive rate: ~1.1–1.7 viable daughters per foundress per worker brood cycle; higher in drone brood.
- Without intervention, mite populations double every 3–4 weeks during brood-rearing seasons.
- High mite loads lead to colony weakening and collapse, especially in late summer/fall.
Summary of Life Cycle Timing in Worker Brood
| Event | Time (post-capping) |
|---|---|
| Cell capping | 0 hours |
| Foundress egg laying begins | ~60 hours |
| Male egg laid | 0 hour post-oviposition |
| First female egg | +30 hours |
| Mating of siblings | After female mites reach deutonymph stage (~Day 8–9) |
| Bee emergence | Day 21 |
Implications for Management
Monitoring
- Drone brood trapping: Remove and destroy capped drone brood to reduce mite load.
- Mite counts: Alcohol washes, sugar shakes, and sticky boards.
Control Methods
- Chemical: Miticides like formic acid, oxalic acid, thymol.
- Biotechnical: Queen caging, brood interruption.
- Breeding: Selecting for VSH or mite-resistant strains.
Conclusion
The Varroa mite’s reproductive behavior—synchronized with the honey bee’s brood cycle, highly efficient, and dependent on in-brood mating—makes it a formidable parasite. Understanding its life cycle allows for strategic interventions that can suppress mite populations and help protect bee colonies from decline.