7 minute read

Bee diseases - a worldwide problem

Dr Wolfgang Ritter, Bees for the World, Germany

Bees are suffering increasingly from diseases. In addition to changing environmental conditions and application of pesticides, increasingly profit-focussed hive management systems contribute to this situation. The worldwide trade of bees and their products constantly confronts bees with the challenge of new diseases and parasites.

Protection against diseases within the bee colony

Honey bees are socially organised insects. Their colony consists of ten to fifty thousand infertile worker-bees, a number of males (drones) and one egg-laying female, the queen. However, a bee colony is far more than the sum of its individual bees. Like cells in higher organisms, they are able to cooperate via neuronal stimuli and hormones.

The individual bee owns a cellular and humoral defence system, similar to higher animals, and their social defence system is of major importance. Old and ill bees die on forage flights or are rejected from entering the hive on their return. Furthermore, the hygienic behaviour of the bees involves the removal of ill brood from the nest. This hygienic behaviour is genetically fixed and more or less obvious within the different breeding lines, bee races and species.

The negative effect of the pathogens and parasites is revealed by a shortened lifespan, a modified morphology, physiology or behaviour of the individual bee. For the colony, the consequences are a reduction of colony strength until it approaches collapse.

Diseases are spread within the honey bee colony, and between honey bee colonies. A horizontal transfer of diseases means the transfer of pathogens or parasites within the colony from bee to bee, and from colony to colony. A vertical transfer means that the pathogen is transferred ‘down through the generations’ from the queen, the drones or the workerbees to the brood. Between apiaries, diseases are spread by swarms and by beekeepers’ interventions.

Bee diseases have different causes

Like other animals, honey bees can be infested by different pathogens or parasites. In addition to fungi and bacteria, viruses have become increasingly important, because they are transferred by parasites. Most of the parasites found with bees are mites and other insects.

Foulbrood is spread worldwide

American foulbrood (AFB) and European foulbrood (EFB) have been spread nearly all over the world. However, AFB and EFB do not present a serious problem for honey bee colonies everywhere, and often colonies are able to heal themselves. In Africa foulbrood has importance only where bees are kept in the industrialised American/European way, using frame hives. The conventional African way of beekeeping allows bees to swarm. (Formation of artificial swarms is commonly used to control foulbrood in Europe.) In many countries, for example in USA, antibiotics are used for control of foulbrood. However, their use masks the infectious disease with the effect that after the end of the treatment, a relapse happens. Moreover, this introduced the risk of residues in honey.

The yellow remains of larvae killed by EFB
Photo © Wolfgang Ritter

Nosemosis has different kinds of pathogens

Nosemosis represents a diarrheal disease of the bee caused by a fungus. The pathogen destroys the bee’s midgut epithelium where it multiplies inside cells. As long as the infested bees are able to defecate outside the hive or die there, the bee colony is able to heal itself. With increasing infestation, the bees are more likely to defecate within the nest. When other bees pick up the faeces to remove them, mass infestation takes place. Finally, the bee colony dies.

Originally, the only parasite of Apis mellifera was Nosema apis, and the Asian hive bee Apis cerana was infested exclusively by Nosema ceranae. In 1973 Nosema ceranae was found for the first time in Apis mellifera in China. Since 2003, this parasite new for Apis mellifera has rapidly spread all over the world. Today it has nearly everywhere replaced the original parasite Nosema apis. However, the spores of this new parasite cannot survive frost, and reproduce more quickly in warm climates. Therefore, opposite to Nosema apis, Nosema ceranae causes more problems in warm climates than in cold ones.

Varroa mites transfer viruses

Regarding Varroosis, now spread all over the world, the situation is nearly the same. Originally, the mite Varroa destructor infested only Apis cerana. The introduction of Apis mellifera to Asia provoked its shift to the new host. In the meantime, Varroa destructor has spread nearly worldwide on Apis mellifera. This parasite only multiplies in the brood of honey bees. There it damages the developing bees: the colony becomes increasingly weak and also more susceptible to other diseases.

American foul brood is recognised by larval remains that can be drawn out by a match stick
Photo © Wolfgang Ritter

Because of the structure of their epidermis, honey bees are well protected against many kinds of viruses. The Varroa mite overcomes this natural defence mechanism when sucking, and thus infests the brood with viruses directly or via the adult bees. Especially the Deformed Wing Virus, and also the Acute Bee Paralysis Virus, accelerate the collapse of the colonies. In most parts of the world the bee colony dies within a short time. Bee colonies of Apis cerana native to Asia are able to survive an infestation of Varroa mites without any problems. In sub-Saharan Africa and other regions of the world, for example Brazil, bee colonies of the western honey bee Apis mellifera survive without any treatment when allowed to develop in a natural way.

Tropilaelaps mites not yet spread everywhere

Different types of the Tropilaelaps mite infest the giant honey bee species Apis dorsata and Apis laboriosa. Some have switched to Apis mellifera that have been introduced into Asia. Only a few types of Tropilaelaps damage the European honey bee species, including Tropilaelaps clareae and Tropilaelaps mercedesae. Though the outer appearances of Tropilaelaps and Varroa mites are different, the ways they live parasitically are similar: both mite species infest honey bee brood and multiply within it. When brood is infested by both kinds of the mite, the Tropilaelaps mite dominates, because its offspring develop faster.

However, Tropilaelaps mites cannot survive on adult bees: they use adult bees only as their carrier for spreading.

Generally, Tropilaelaps mites spread in ways similar to Varroa, although we still do not know why Tropilaelaps mites have only partially left their original range on their new host Apis mellifera. The difficult way of transfer via adult bees, and the mites’ susceptibility to brood-free phases in a bee colony, could be reasons.

In Asia, bee brood is often infested with both mites: Varroa (right) and Tropilaelaps (left)
Photo © Denis Anderson

In comparison with an adult Apis mellifera bee, the wandering Small hive beetle larvae is the same size, while the adult beetle is only two thirds of the length
Photo © Rau/Oberursel

Small hive beetle damages only weak colonies and honey stores

Another pest is not so easy to classify into the general scheme of diseases. The Small hive beetle, Aethina tumida, is often classified as a pest but it can undoubtedly be regarded also as a predator.

Originally this beetle was exclusively spread in sub-Saharan Africa. African bees actively defend themselves against the beetle. Since at least 1996 the Small hive beetle has been carried to new continents by global trade. There it has spread via bees, or independently on its own wings. It is difficult to diagnose the beetle in the bee colony. After opening the hive, the beetle immediately runs to the dark. Special traps have been developed in which the beetle can hide from the bees: these make it easy to identify a newly-beginning infestation. In most cases, the beetle is detected only when its larvae appear in huge numbers and ferment the food, which makes honey leak out of the cells. However, this only happens in very weak colonies or in honeycomb stores. Therefore, this beetle will cause only limited damage to colonies, but is a real pest in honey storage. The native host of this beetle, the African honey bee races, are able to keep the beetle population at a low level: one important defence mechanism is the frequent migration, swarming and absconding of African honey bee races. Moreover, in African beekeeping methods, honeycombs, which beetles can damage, are not stockpiled. Therefore, the Small hive beetle is a problem in Africa only when bees are kept in oversized hives - like most frame hives.

Spreading bee diseases must be prevented

Bee health problems have increased mainly because of the constant import of new diseases. The introduction of diseases via bees can be inhibited by strict import controls or bans on importation of bees. Despite this, there is still the risk of importing diseases by land, because of the movement and drifting of bees and swarms, and by sea due to wild swarms, for example in containers. The biggest problem worldwide, is caused by the fact that in veterinary medicine, honey bees are not everywhere regarded as livestock, and consequently are given insufficient attention at national frontiers. In addition to illegal imports, there are also uncontrolled imports. This is a worldwide problem.

Conclusion

Honey bee disease control has to consider management methods and environmental conditions. Local honey bee races and local management systems cause fewer disease problems. To have healthy honey bees in the future, we must end this global spreading of diseases.

Dr Wolfgang Ritter Bees for the World Germany wolfgang.ritter@beesfortheworld.de