Varroa mites are tiny external parasites that attach to honey bees, posing a significant threat to colonies. These mites feed on bees, weakening them and making them susceptible to various health issues.
The Varroa Mite Unveiled
The Varroa destructor mite is a small, reddish-brown external parasite, typically measuring about 1 to 1.8 millimeters wide and 1.1 to 1.7 millimeters long. Its flattened, oval body allows it to easily hide between bee segments.
This mite originated as a natural parasite of the Asian honey bee, Apis cerana, which developed natural resistance. The mite eventually jumped hosts to the European honey bee, Apis mellifera, which lacks similar natural defenses.
The varroa mite’s life cycle is closely intertwined with the honey bee colony. Female mites enter brood cells just before they are capped. Inside the capped cell, the female mite lays eggs: one unfertilized egg (male) and several fertilized eggs (females). These offspring mites feed on the developing bee pupa. Once the adult bee emerges from the cell, mature female mites also emerge and attach to adult bees, feeding on their hemolymph.
Devastating Effects on Honey Bees
Varroa mites directly harm individual bees by feeding on their hemolymph, depleting energy reserves and suppressing immune systems. This feeding can lead to reduced lifespans and physical deformities, such as underdeveloped or crumpled wings, making bees unable to fly and forage effectively.
Mites also act as vectors for various bee viruses, transmitting them within the colony. The Deformed Wing Virus (DWV) is frequently associated with varroa infestations, causing bees to emerge with malformed wings and shortened abdomens. Acute Bee Paralysis Virus (ABPV) is another common virus transmitted by mites, leading to rapid bee mortality.
The combined stress of mite feeding and viral infections compromises the colony’s overall health. Unchecked mite populations and rampant virus spread can lead to a drastic decline in the colony’s population. If left untreated, these stresses often result in colony collapse, particularly during colder months.
Identifying a Varroa Infestation
Beekeepers employ several methods to monitor varroa mite levels. Visual inspection can sometimes reveal mites on adult bees or in brood cells, but this method is generally not reliable for estimating infestation levels.
Sticky board checks involve placing a sticky board under a screened bottom board for a period, typically 24 to 72 hours. Mites that fall off bees are trapped on the board, and counting them provides an estimate of the mite drop rate. A powdered sugar roll test involves gently shaking a sample of approximately 300 bees with powdered sugar in a jar. The sugar causes mites to detach from the bees and fall through a screen, where they can be counted.
An alcohol wash is considered one of the most accurate methods. This involves collecting a sample of about 300 bees and immersing them in rubbing alcohol, which causes mites to detach. The mites are then sieved out and counted, providing a precise percentage of mites per bee. These monitoring methods are crucial for beekeepers to gauge the mite population and decide on appropriate management strategies.
Strategies for Varroa Mite Management
Managing varroa mite populations requires an integrated pest management (IPM) approach, combining various methods to keep mite levels below damaging thresholds. Breeding for mite-resistant bee strains is a non-chemical strategy. Traits like hygienic behavior, where worker bees detect and remove diseased or mite-infested pupae, and Varroa Sensitive Hygiene (VSH), which specifically targets mite-infested brood, are selected for in breeding programs.
Cultural practices also play a role in mite control. Drone brood removal, or trap comb, exploits the mites’ preference for drone brood cells, which have a longer development time. Beekeepers can introduce frames of drone comb, allow mites to infest them, and then remove or destroy the capped drone brood before the mites emerge. Queen manipulation, such as caging the queen for a period or requeening the colony, can disrupt the mite breeding cycle by creating a brood break. During this period without capped brood, mites are exposed on adult bees, making them more vulnerable to treatments.
Chemical treatments are broadly categorized into hard and soft chemicals. Hard chemicals, or synthetic acaricides, such as fluvalinate or amitraz, are highly effective but carry the risk of mites developing resistance over time. Soft chemicals, often naturally derived compounds, include organic acids like oxalic acid and formic acid, as well as essential oils like thymol. Oxalic acid can be applied as a dribble or by sublimation, while formic acid is used in various pads or strips. Thymol-based products release vapors that are toxic to mites.
These softer treatments generally have fewer resistance issues and are considered more “bee-friendly” when applied correctly. However, their efficacy can be dependent on ambient temperatures. Regardless of the chosen method, beekeepers should rotate different treatments to prevent resistance development and meticulously follow product instructions for bee safety.