A mitey problem

Whilst I am interested in all things bees, my main work is looking at the ecto-parasitic mite of honey bees known by the Latin name Varroa destructor. I would imagine that most beekeepers know of Varroa and will have had to deal with the mite themselves. This is because since it jumped species from eastern honey bees (Apis cerana) to western honey bees (Apis mellifera) in the 1950s it has spread almost worldwide.

Varroa adult female (Gilles San Martin)

Why is Varroa a problem?

Varroa mites are a problem for colonies because they feed off the fat bodies of individual bees. This feeding can cause some nutritional deficiency and water loss however the main reason it is so detrimental

is because it spreads viruses. The most important virus is known as deformed wing virus (DWV) which drastically reduces the lifespan of bees that are infected with the virus as pupae. This premature loss of workers can cause a colony to collapse, particularly over winter in temperate climates. Those infected as pupae also emerge as adults with a shortened abdomen, a reduced body weight and sometimes the characteristic crippled wings (not all infected individuals have crippled wings). If bees are infected as adults, they experience less severe effects but still have a shortened lifespan. In fact, some studies have shown that it causes these adults to begin foraging at an earlier age. Foraging, because it is risky, is the last job a worker bee performs and marks the end of the workers life. When a worker becomes a forager, they experience body changes similar to aging in humans this includes reduced body fat and vitellogenin stores.

***side note: vitellogenin is thought to prevent the onset of aging in bees, correspondingly the long-lived winter bees have high stores of vitellogenin

Varroa life cycle

Varroa has two life phases the phoretic and reproductive phase. During the phoretic phase an adult female mite lives on an adult bee, hiding under the sternites to prevent detection. Once a female is ready to reproduce, she leaves the adult bee and is attracted toward worker or drone brood cells by chemical signals including brood hydrocarbons and brood food constituents. Within the brood cell the foundress mite hides from hygienic bees within the brood food until the cell is capped. She then escapes and lays her first egg out of harms way on the upper wall of the cell. This first egg is not fertilised and is always a male egg. The foundress then lays fertilised female eggs in 30-hour intervals. The offspring feed from a hole the mother mite has created in the pupa and go through a number of moults from protonymph, to deutonymph and then adult. After the final moult they become sexually active and the son mates with the daughters.

In worker brood mites can produce around 1.3 to 1.45 mature mated daughters and in drone brood cells 2 to 2.5. Drone brood has a longer post capping period which allows mites to produce more offspring, which is potentially one of the reasons why Varroa mites prefer reproducing in drone brood. However, drone brood is produced in much low numbers than worker brood and is also seasonal. Thus, the majority of the mite’s population growth comes from reproduction in worker brood. In the original host, the eastern honey bee, mites are unable to effectively reproduce within worker brood which means that the eastern honey bee is able to control the mite and they don’t suffer major losses.

Ultimately, the more mites there are in a colony, the more the virus spreads and the more bees are infected. If the number of mites can be managed, then colonies can survive the infestation. If mite numbers are not controlled a colony will typically die within 1 to 3 years.

In western honey bees controlling the number of mites is usually done with chemical controls and in temperate climates it is particularly important to use these to reduce the number of mites before autumn when the winter bees are produced. Mites can also be controlled by removing drone brood as when drone brood is present mites will naturally over infest this brood and so many mites can be removed at once. This practice is particularly popular in Cuba, where chemical control is not used. In fact the honey bee populations of Cuba are believed to be resistant to the mite. Varroa resistant colonies are those that can control the mite and survive without chemical control. There have been increasing reports of resistant colonies across many regions including Europe (the UK, France, Norway and Sweden), South America and South Africa.

A goal of my research is to understand this natural resistance to the mite in the hopes that we can help to encourage all colonies to develop resistance. If we can do this then we could remove the need to rely on chemicals to keep colonies alive. However, I will talk more about resistance in another post.