Varroa destructor

Varroa adult female  (Gilles San Martin)

Varroa destructor

The most damaging disease is caused by a mite: Varroa destructor and is considered the largest contributor to (winter) losses. Untreated colonies often collapse within 2 years from the consequences of the fast growing Varroa population.

Varroa mites were first discovered in Southeast Asia in Western Honey Bee colonies in about 1904, but are now present on all continents except Australia. During the 1980’s the mite quickly expanded in Europe, the USA and Southern America.

The Western Honey Bee is not able to deal with this mite (yet) as it comes from another Asian bee species, Apis cerana. The Apis cerana has been living with the Varroa mite since thousands of years and has learned to live with the mite.

Protonymph, deutonymph, adult male  (Giles San Martin)

Protonymph, deutonymph, adult male (Giles San Martin)

 
Damage to the bee, winter losses

The mite directly weakens the bee by sucking hemolymfe (“insect blood”), but also causes severe damage to the bees by spreading viruses and bacteria. Especially the Deformed Wing Virus proliferates with an increasing infestation of Varroa. Infected bees have a short life or are not able to function at all because of the deformed wings. But also the other viruses and bacteria cause the bee to live significantly shorter. As the colony is only capable in raising a certain amount of new young bees, total numbers go down with a total collapse of the colony as final result. Colonies can collapse during the year, but the shorter life of the bees is especially a problem during the winter and early spring when the weakened bees are not able to cope with the low temperatures or with the extra efforts needed to start the brood nest in early spring. During these periods many weakened bees die without new offspring being born. Varroa is the strongest contributor to winter losses as low Varroa infested colonies have very low winter losses.

Winter and Deformed Wing Virus  (Riad Abara, Wikipedia)

Winter and Deformed Wing Virus (Riad Abara, Wikipedia)

Treatments

Several treatments are being practiced to reduce the number of mites in a colony.

The oldest method of treatment of Varroa is the use of miticides: anti-parasitic compounds (also used for livestock). With the use of these compounds over the years, the mites have built up resistance against these compounds and its use is not allowed anymore in many countries. Furthermore these compounds can leave residues in the wax and honey.

Another treatment method is the capping of drone (male bee)-brood. As the mites have a preference of the (larger) drone-brood cells, capping or removing the drone-brood will also limit the proliferation of the mites. Positive feature of this method is that no chemicals are used and no damage is done to the worker bees in the colony. However, it is a quite some of work, not effective enough as only treatment and costs energy from the colony. One might also argue that we select a mite with a preference for workers brood which is exactly the opposite of what we want (the Asian Honey bee “allows” the mite to only breed in the drone brood and has for that reason not a real problem with the mite).

More recently is the use of organic acids & certain oils. These compounds are currently recommended in most countries as biological acceptable treatment methods. The applications can be efficacious, but heavily depend on the weather, the dosage used, the bee population, the hive-type and timing – making efficacy very variable. This makes it important to do multiple treatments per year. However, the treatments damage not only the Varroa but also the bee-larvae, worker bees and queens.  Furthermore there are human safety issues when not properly used.

The lack of a very effective and efficient treatment causes continued winter losses by Varroa.


Life Cycle of the Honeybee and Varroa Mite
Jeffrey Harris of the USDA-Agriculture Research Service

The solution from nature

This leads to the question “What can we do???”.

For a possible answer we can look how nature solved this. Parasite-host relations are very common in nature. In this non-mutual relationship, the parasite benefits at the expense of the other, the host. The host develops mechanisms to reduce the parasitic load. This creates an equilibrium, which is, in the end, a mutual benefit as a dead host is not in the interest of the parasite. In the case of the Varroa and the bee it has become clear that also here the honey bee can develop a natural response: Varroa Resistance. Varroa resistant honey bees will keep the Varroa infestation on a low level, will have healthy bees without disease making virus or bacterial infections and do not have to be treated by the beekeepers. On the following pages we will go in more detail on Varroa Resistance and will describe the Projects that are needed to get this natural solution in our western honey bees.