The Life Cycle of the Varroa Destructor Mite

Varroa destructor mites are widely recognised as one of the most prevalent parasites affecting honey bees (Apis mellifera) globally. Unlike the Asian honey bee (Apis cerana), with which it co-evolved, Apis mellifera lacks natural immunity or resistance to these mites. Consequently, uncontrolled Varroa populations can typically cause a bee colony to perish within one to two years. These mites are frequently associated with significant bee colony mortality events. Given their relatively recent spread to Apis mellifera, beekeepers do not possess extensive historical knowledge for combating this parasite.

The Varroa destructor mite is a parasitic arachnid that requires a bee host to survive independently. The life cycle of the female mite consists of two primary periods: a passive parasitic stage on adult bees and an active reproductive stage within the capped brood cells of bees or drones. Male mites are found exclusively within the capped brood. During the passive phase, female mites reside on adult bees, commonly concealing themselves under the bee's sternites, which is where the white fatty tissue is concentrated. Recent detailed studies have revealed that during this period on adult bees, the mites feed exclusively on the white fatty tissue, rather than primarily on haemolymph as was previously believed. This feeding supplies the mites with valuable nutrients essential for their survival and the ability to lay eggs later.

Adult bees serve as carriers, spreading mites through contact with other bees, during swarming, or while foraging for nectar and pollen. Reproduction of the mites takes place within the bee brood. A female mite enters a brood cell just before the bees cap it. To evade detection and removal, she hides between the larva and the base of the cell. While submerged in the larva's food, she breathes via a specially adapted respiratory tube. Roughly five hours after the cell is capped, the larva finishes its food. The mite mother then punctures the bee pupa, and both she and her hatching offspring feed on the internal fluids. During this reproductive period, the mites' diet shifts to approximately 75% haemolymph and 25% white fatty tissue, providing carbohydrates like trehalose from the haemolymph for rapid energy.

Mites exhibit a strong preference for drone brood, infesting it 8 to 10 times more frequently than worker brood. This preference may stem from the longer pupal development time of drones, allowing mites more time to reproduce, and potentially larger food reserves within drone cells and pupae. A female mite can raise 2-3 daughters in a drone cell, compared to only 1-2 in a worker cell. Mites select host larvae using chemical cues (pheromones); they can differentiate between the scents of larvae of various ages and prefer five-day-old larvae. Drone larvae emit a slightly higher amount of these pheromones, which could contribute to the mites' preference. Additionally, cell size, height, and age are correlated with infestation levels; older combs with accumulated cocoons and cells that are shorter (less space between the larva and the cell opening) are more susceptible to infestation.

The infestation of developing winter bees in the autumn months is particularly detrimental, weakening the bees before winter. Colonies weakened by mites often perish in late autumn, failing to survive until spring. Effective mite control strategies, implemented at the correct times corresponding to the mite's life cycle, are critical for maintaining colony health and preventing large-scale mortality.