There are a number of diseases that affect the adult bees. The acarine or tracheal mite (Acarapis woodii) is one of them. In some places it has had a serious economic impact while in others bees have developed a high degree of genetic resistance to the disease.
Tracheal mites are obligate parasites, feeding and reproducing in the breathing tubes (or trachea) of the adult bees. Once the young mites are mature they move out of the breathing tubes to look for a new host bee. Younger bees, hatched for less than three days, are more susceptible to infection than older bees and it was originally believed that the hairs covering the tracheal entrances were much stiffer in older bees preventing them from being infested with the young mites as they moved to new breeding sites. Now however, it is thought most likely that this phenomenon relates to the length of time the mites will have to breed -this will be much longer in a young bee potentially allowing the mites to produce many more offspring and giving them a selective advantage.
Despite the infestation being located in the parts that deliver oxygen to the bees' body tissues, there are frequently no outward signs of the disease even in heavily infested colonies. There are some reports of bees crawling or clustering in front of the hive, appearing confused and disorientated. Some bees may show 'K-wings' although this phenomenon can also be seen in other adult bee diseases. 'K wings' occur where the rows of hooks (or hamuli) holding the bee's wings together have become detached and the wings stick out in the form of a letter K.
The main consequence of infestation is to shorten the lifespan of the bees during periods of dearth. This may reach the point where the old bees are dying more quickly than they are being replaced and the young brood cannot be looked after sufficiently, both events potentially leading to the colony dying out. Because they are obligate parasites, tracheal mites can only move between colonies as a passenger on a drifting or robbing adult forager. This means that transfer between colonies in slow unless the beekeepers assists it by moving combs between colonies. A hive that is empty of bees will not harbour acarine mites.
The disease is best diagnosed by dissecting the main trachea and looking at it under a microscope (up to x40 magnification). In a healthy bee the trachea appears uniformly creamy-white while in infested bees it is discoloured or stained dark brown. At the higher magnifications (x40) the mites are visible in the breathing tubes. The best control measures are to maintain vigorous, healthy stocks better able to tolerate infestations. Some strains of bee are also more prone to acarine so it is helpful to select strains of bee that appear resistant.
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