One of the honey bee’s worst enemies is a tiny mite called Varroa destructor.
It is small and yet highly dangerous: the Varroa destructor mite is the most destructive enemy of the Western honey bee (Apis mellifera). The parasite has now spread to almost all parts of the world – except for Australia – and is a serious threat to bee health. Without human intervention, a bee colony infested with mites will typically die off in these regions within three years. In addition to the threat posed by the Varroa mite itself, there is also the danger of secondary infection from various mite-vectored diseases, which have also become more widespread and additionally weaken the bee colonies. The parasitic Varroa mites – much like ticks – transmit diseases that often prove fatal to adult honey bees and their brood. Combating the mite is a difficult task for researchers. This is because – despite a number of promising ideas – they have not yet managed to develop simple and long-lasting treatments for fighting the bee parasite, nor have they yet managed to breed a Varroa-resistant strain of the Western honey bee.
The Varroa mite is originally native to Asia, where it was first discovered on the island of Java in Indonesia over 100 years ago. The mite initially preyed on the Asian honey bee (Apis cerana). But over thousands of years the bee successfully adapted its behavior to the parasite. The bees fend off the mites through their intensive cleaning habits in the hive, thus minimizing harm to the colony. When European settlers brought the Western honey bee (Apis mellifera) to Asia, it also fell prey to the Varroa mite. Through these infested colonies the parasite was then introduced to Europe, where since the 1970s it has continued to spread. Recent genetic investigations have revealed that Varroa jacobsoni comprises 18 different genetic variants with two main groups: Varroa jacobsoni and Varroa destructor. Varroa destructor, the newly identified type, inflicts a great deal of harm in Europe, North America and elsewhere because the Western honey bee lacks sufficient defense mechanisms. Clearly, the equilibrium between Varroa destructor and the Western honey bee has not yet been established. The mite is now found in many areas of the world: it is common not only in China and Russia but also in Central Europe and North and South America. Even New Zealand and Hawaii reported cases of infestation in the first decade of the 21st century. Australia is the only part of the world where the mite has not yet spread, mainly as a result of intensive biosafety protocols at the borders.
Varroa destructor literally means “destructive mite.” And although the parasite’s name more or less says it all, this tiny arachnid is not much larger than a millimeter and lacks hearing and sight. The body of the mite has four pairs of legs and piercing and sucking mouthparts. It uses the numerous sensory hairs all over its body as receptors to sense its environment. The Varroa mite’s flattened shape and the suckers on its feet enable it to optimally grip the bee’s body. It uses its mouthparts to pierce the bee’s exoskeleton and feed on its hemolymph, a circulatory fluid similar to blood.
The parasite preys on both adult honey bees and their brood. Varroa females can also survive outside the brood cells by attaching themselves to adult bees. However, the parasite only reproduces in the sealed brood cells of the honey bee. Shortly before the brood cells are capped, the Varroa female mites enter and crawl to the bottom of these cells – they protect themselves from the bees that tend to the brood by hiding under the larvae. Here they first immerse themselves in the liquid brood food. Once this is depleted, the Varroa mite feeds directly on the bee larvae. The parasite has strongly adapted to its host in terms of habitat and food.