Why do honeybees matter?
Honeybees form an integral part of the human food chain. In addition to producing both honey and beeswax, honeybees provide essential pollination services for hundreds of crops, including apples, almonds, avocados, beets, berries, cabbage, canola seeds, carrots, cherries, cotton, cucumbers, kiwi, melons, peaches, pears, plums, soybeans, squash, and tomatoes. It is estimated that 1/3 of the human diet comes from insect-pollinated plants, and honeybees provide 80% of that pollination. Various studies have suggested that bees pollinate over $15 billion in crops in the United States every year.
Unfortunately, honeybee colonies have also suffered much devastation in recent years. Repeated parasitic infestations, bacterial, viral, and fungal infections, and increased environmental stresses on the honeybee population have contributed to huge losses. Mite infestations in the late 1980s and 1990s practically eliminated wild honeybee colonies and caused the loss of 50-70% of managed bee colonies (Peterson). More recently, in late 2006 and early 2007, large numbers of bee colonies have been lost for no apparent reason. This phenomenon is known as Colony Collapse Disorder and represents a great threat to the honeybee population and their essential agricultural functions in the US.
This Background section will focus on two of the more serious parasites threatening honeybee colonies today: varroa mites and nosema. Colony Collapse Disorder will be discussed in the Analysis section of this website.
The Honeybee is a subset of the wider category of flying insects known as bees. A honeybee colony consists of one queen (fertile female), about 500-1000 drone bees (fertile males), and about 50,000-60,000 worker bees (sterile females). There are four common species of honeybees (De Jong):
Apis mellifera: The European (or Western) Honeybee. This species is native to Europe and North Africa. It has been transplanted by humans to every continent and is responsible for almost all commercial pollination. In general, when people refer to honeybees, they are referring to A. mellifera.
Apis cerana: The Eastern Honeybee. This species is native to southern and southeastern Asia. It is sometimes used in commercial pollination, especially in parts of Asia where A. mellifera colonies have been devastated by mite infestations. A. cerana is the original host of the Varroa mite and is less adversely affected by it than the transplanted European honeybee.
Apis florea: The Dwarf Honeybee. This species is also native to southern and southeastern Asia.
Apis dorsata: The Giant Honeybee. Also native to southern and southeastern Asia, this species is generally wild and relatively ferocious.
There are several types of parasitic mites. The two mites that are responsible for most damage to honeybee colonies are the Varroa mite and the Tracheal mite.
Varroa mites: Varroa destructor, V. jacobsoni
Varroa could be considered the scourge of the honeybee. It has a worldwide distribution. In the US, varroa nearly wiped out wild colonies and destroyed over 50% of commercial colonies in the 1990s (De Jong). When Colony Collapse Disorder became a major concern, varroa mites were one of the first suspected causes. However, the evidence has suggested that they are not responsible.
It was originally thought that the species Varroa jacobsoni was responsible for much of the devastation caused by parasitic mite disease. However, in 2000, Anderson and Trueman used genetic analysis to show that V. jacobsoni was really two different species. The original parasite, V. jacobsoni, is a relatively non-harmful parasite of the Eastern honeybee, A. cerana. Most parasitic mite disease is actually caused by the closely related species Varroa destructor.
The transportation of the Western honeybee A. mellifera around the world for use in agricultural pollination brought it into contact with A. cerana, the original host of varroa. Contamination between the two species occurred in Southeast Asia, and V. destructor mites became capable of infecting A. mellifera (De Jong). The global spread of A. mellifera allowed for the global spread of the varroa. Varroa mites were discovered in A. mellifera colonies in the Philippines and the Soviet Union in the 1960s, in Europe and South America in the 1970s, and in the US in 1987.
The varroa mite is a flat, oval, reddish-brown mite. It is visible to the naked eye, though it can often be difficult to find. Adult female mites live on the adult bees, usually between the abdomen and the thorax or underneath the abdominal sclerites (hardened portions of the exoskeleton of the bees). The varroa mite appears very similar to and is often confused with the relatively harmless bee lice, Braula coeca.
Life Cycle of the Varroa Mite
The life cycle of the varroa mite is demonstrated in the diagram below. It is important to note that while adult mites live on the outside of the bees, mite reproduction occurs within the cells of the bee brood. Varroa mites have shown a preference for drone brood over worker brood (De Jong). Destruction of the drone (fertile male) brood can result in an inability to fertilize the queen and the death of the colony.
The mites, both in adult stages on the bee and in their larval stages in the brood, feed off hemolymph. When larval mites feed off the hemolymph of the larval bees, this can have detrimental consequences for the health of the adult bees.
Symptoms and Diagnosis of Varroa Infestation
Symptoms of parasitic mite infestation include: large numbers of pupal bees that do not develop into adults and emerging adult bees with shortened abdomens, misshapen wings, or deformed legs (Shimanuki and Knox). Infested bees usually weigh less than healthy bees and have problems flying. The varroa mite can also act as a vector of other diseases, such as the deformed wing virus.
Diagnosis of varroa infection is usually achieved by finding the presence of mites in the hive. It is easier to find the mites in the drone brood cells than on the adult bees. The number of mites in a colony is lowest in the spring, increases in the summer, and is highest in the fall (Shimanuki and Knox).
Treatment and Control of Varroa
It is difficult to treat varroa mite infestations. There are many chemical treatments available, but they are of limited effectiveness. The standard treatment is to use acaricides, but these can harm the bees and contaminate honey and wax (De Jong).
Control of varroa infestations is also difficult. The infestation is not usually discovered until the mites have been in the colony for 2-6 years because the mites are “hidden” and low infestation numbers do not cause much noticeable damage (De Jong). Chemical control, such as acaricides, can reduce mite infestations, but not eliminate them. Changes in the management of commercial hives can help reduce the spread of mites. Recent research into the genetic manipulation of the bees has also shown promise. There are attempts to select for certain behaviors (with a genetic component) that will help keep a hive free of parasites (Royce and Rossigal).
Varroa mites remain a major threat to the honeybee today. In 2005, a varroa infestation in the Eastern part of the United States killed 50-70% of all hives in many regions. In April 2007, varroa mites were found in Hawaii, which had previously been free of the parasite (Godvin). Perhaps an even greater concern regarding the control of varroa is the fact that some mites have begun to show signs of resistance to commonly used acaricides and other chemicals.
Tracheal mites: Acarapis woodi
There are three different species of Acarapis mites. They are generally identified by their location on the bee. Acarapis woodi (the Tracheal mite) is the only species of acarine mites known to harm bees (Shimanuki and Knox). It lives exclusively within the prothoracic tracheae of the bee. In general, tracheal mites are less of a threat than varroa mites because they have a more limited geographic distribution and are more easily controlled.
Tracheal mites are small and difficult to find. Symptoms of tracheal infection include disjointed wings and distended abdomens (De Jong).
Control of the tracheal mite is easier than control of varroa. It is feasible to disrupt the life cycle of the acarine mite and thus prevent its transmission. The mites must invade a bee that is younger than 9 days old or the spiracles (air openings) will close and the mite will be unable to enter the tracheae. Various methods can prevent tracheal mites from identifying and invading young bees, leading to the death of the mites.
There are several protozoa that cause disease in honeybees. The most important ones are those that cause nosemosis: Nosema apis and Nosema ceranae.
Nosemosis: Nosema apis & N. ceranae
Both Nosema apis and N. ceranae are classified as microsporidia. This phylum was first discovered in the 19th century, after it was determined that N. bombycis was the cause behind pébrine disease, which affects silkworms (Keeling and McFadden). The vast majority of these parasites only affect animals; N. apis is associated with Apis mellifera, while N. ceranae has been associated with the eastern honeybee, A. cerana (Keeling and McFadden). These parasites cause nosemosis, or nosema disease, in honeybees. The disease is widespread, and many hypothesize it is one of the main reasons for the recent Colony Collapse Disorder (CCD).
A German scientist identified N. apis as the cause of nosemosis at the beginning of the 20th century (Paxton). In 1995, Dr. Ingemar Fries discovered N. ceranae in A. cerana colonies in China. The key difference between the two species described by Dr. Fries was in their ultrastructure and genetic material. In 2005, the parasite appeared in Vietnamese and Taiwanese colonies of A. mellifera (Paxton). In 2006, a team led by Dr. Mariano Higes identified N. ceranae as the cause of a nosema outbreak in Spain. The Spain study also showed that N. ceranae can wipe out a colony in eight days (Russell). Recent work by Dr. Joe DeRisi of the University of California, San Francisco shows that N. ceranae has spread to the United States as well.
It was always believed that N. apis and N. ceranae were unicellular organisms. But according to Dr. Eric Mussen of the University of California, Davis, recent work by taxonomists has put that view in doubt. Study of the genetic material of Nosema species shows that they could be a fungus. If this is true, then it could change the way scientists think about the disease and approach its treatment.
The life cycle of N. apis is shown in the diagram below, along with a picture of Nosema spores. N. apis is spread by spores that are ingested by adult honeybees. The spores then germinate in the honeybee’s gut. Once germinated, the organism inserts itself into the digestive cells. The Nosema produce new spores in the cells, which then burst and travel to the rectum. The spores are then passed in the honeybee’s excreta, and the cycle begins again. The spores can survive for several months.
It should be noted here that the life cycle of N. ceranae is virtually identical. In fact, according to Keeling and McFadden, most microsporidian life cycles are remarkably similar.
Symptoms and Diagnosis
Unfortunately, there is no one symptom that typifies nosema disease (Shimanuki and Knox). Some warning signs to look for, including: inability to fly, excreta on combs (pictured below), piles of dead or dying bees and the failure of a colony to build up in the spring. However, these signs could also be indicative of other ailments, such as mites. The disease is more prevalent in bees that have been confined, such as package bees (Shimanuki and Knox).
According to Shimanuki and Knox, removing and examining the digestive tracts of dead bees can lead to diagnosis. In healthy bees, the digestive tract is straw brown and translucent, while it is chalky white in diseased ones (see below). Honeybee feces can also be examined under a microscope for the presence of spores.
Treatment and Control
The antibiotic fumagillin has been shown to kill the infective phase of N. apis (Katznelson and Jamieson). Protofil, a natural product in the form of syrup that prevents the Nosema development cycle, can also be used as a preventative measure (Chioveau, et al.). The U.S.D.A. cites another control method. Even though spores can survive for months, outbreaks are often short-lived because the bees remove the infected feces before spores are ingested. Thus, cleaning out colonies suspected of infection can help reduce the disease.
Nosema disease remains as one of the major threats to honeybee populations around the world. Two key developments have been the discovery and spread of N. ceranae and the hypothesis that Nosema might actually be a fungus. The understanding of this disease and the parasite that causes it is especially important today, since it is suspected as one of the factors in the recent Colony Collapse Disorder.