The Varroa destructor mite is a major global threat to honey bee populations and beekeeping. This external parasitic mite targets both adult honey bees and their developing brood. Its widespread presence causes substantial economic losses for beekeeping worldwide. It is a primary factor in the decline and collapse of honey bee colonies.
Understanding the Varroa Mite
The Varroa destructor is a small, reddish-brown, oval-shaped mite, roughly the size of a pinhead, measuring about 1.0 to 1.8 mm wide and 1.1 to 1.7 mm long. Its flattened body allows it to easily maneuver between the segments of a bee’s exoskeleton. It originated from the Asian honey bee, Apis cerana, where it coexisted in a balanced host-parasite relationship. However, when it transferred to the European honey bee, Apis mellifera, the latter lacked natural defensive behaviors to manage infestations.
The Varroa mite’s life cycle has two primary stages: phoretic and reproductive. In the phoretic stage, adult female mites attach to adult bees, feeding on their fat bodies, which are crucial for immunity and metabolism. This allows mites to move throughout the colony and spread between hives. Mites typically spend several days to weeks in this phase on an adult bee.
The reproductive stage begins when a fertilized female mite enters a brood cell with a late-stage larva, just before it is capped. Inside the capped cell, the mite lays her first egg, which develops into a male, followed by several female eggs. These offspring mature within the sealed cell, mating with the male sibling before the bee emerges. This cycle is particularly efficient in drone brood cells due to drones’ longer development time, allowing more mite offspring to mature.
Devastating Effects on Bee Colonies
Varroa mites directly harm honey bees by feeding on their fat bodies, an organ critical for immune function, detoxification, and energy storage. This feeding weakens individual bees, reducing their lifespan and vitality. Damage to the fat body compromises the bee’s ability to withstand stressors and pathogens. This parasitic feeding contributes to a decline in the health and robustness of the colony.
Beyond direct feeding, the mite vectors various honey bee viruses, significantly amplifying their impact. Viruses like Deformed Wing Virus (DWV) and Acute Bee Paralysis Virus (ABPV) are transmitted and replicated by the mite. The mite’s feeding punctures create wounds, providing entry points for these viral pathogens into the bee’s hemolymph. This inoculation bypasses the bee’s immune defenses, leading to higher viral loads and more severe disease symptoms.
Combined mite feeding and viral infections suppress the bees’ immune systems. This makes bees more vulnerable to diseases and environmental challenges. A compromised immune system diminishes the colony’s ability to fight bacterial and fungal infections, exacerbating their decline. This weakened state contributes to a decline in colony health and resilience.
These cumulative stresses reduce individual bee lifespan and impair foraging ability. As the mite population grows, the colony’s population dwindles, impacting its capacity to gather resources and rear new brood. Ultimately, these factors can lead to the collapse of the honey bee colony, particularly during stress periods like winter. Colony collapse often occurs rapidly once mite and virus levels reach a critical threshold.
Strategies for Mite Management
Effective Varroa destructor management relies on an Integrated Pest Management (IPM) approach, combining multiple strategies to control mite populations. This approach aims to maintain mite levels below a damaging threshold, promoting long-term colony health. IPM integrates chemical, cultural, and genetic methods, adapting to local conditions and mite resistance patterns. Beekeepers regularly monitor mite levels to inform management decisions.
Miticides are a common component of Varroa management. These include synthetic compounds (e.g., fluvalinate, coumaphos) and natural-origin substances (e.g., oxalic acid, formic acid, thymol). Each miticide has specific application methods and environmental considerations, often applied as strips, dribbles, or vaporization. Challenges include mite resistance to certain synthetic compounds and potential residues in hive products.
Cultural and mechanical practices offer non-chemical control. Drone brood removal involves periodically removing frames of drone comb, which mites preferentially infest due to drones’ longer developmental time. Powdered sugar dusting can dislodge mites from adult bees, causing them to fall through a screened bottom board. Brood breaks, achieved through queen caging or temporary queen removal, interrupt the mite’s reproductive cycle by eliminating capped brood cells.
Breeding honey bee strains with natural resistance traits is a long-term strategy for mite control. Varroa Sensitive Hygiene (VSH) is a promising trait, where bees detect and remove mite-infested brood, preventing mite reproduction. Other resistant traits include grooming behavior, where bees remove mites from their own bodies or nestmates. These genetic approaches aim to develop bee populations that can naturally suppress mite growth.
Regular monitoring of mite levels is fundamental to effective management. Techniques like the sugar shake or alcohol wash allow beekeepers to assess the mite load within a colony. This information guides treatment decisions, ensuring timely and appropriate interventions. Consistent monitoring helps prevent mite populations from reaching damaging levels before colony decline appears.