Team:Toulouse/project/context
Context
Content
Bees and pollination
Bees excel in pollination and thus play an essential role in maintaining ecosystems. By gathering pollen, they allow the reproduction of 84% of the plants that grow in Europe. Bees are also responsible for the production of 35% of edible fruits and vegetables. Without bees, tomatoes, apples, vines and many more would be as good as gone (see Figure 1 for some examples). Among all bees, the domestic bee, Apis mellifera, which builds its swarm in hives provided by man, is directly responsible for the production of honey, wax and propolis. Very often, the swarms are selected for their honey production yield.
Figure 1: Examples of products dependent of bee pollination to reproduce
According to the French INRA (Institut National de la Recherche Agronomique), the bee extinction would represent an economical worldwide loss of 163 billion USD. This huge number reflects the bees’ importance in the global ecosystem.
Domestic bees
There are three types of bees
living in a beehive: the worker
bees, the queen bee and the male
bees. In the colony, the work is
divided in such a way that each
caste is specialized in its activities.
The worker bees are infertile female
and represent the most numerous caste
in the beehive. Their main activities are
cleaning the beehive cells, ventilate the
beehive, cap the beehive cells containing
the bee eggs, feed the larvae, gather pollen and defend the colony.
The queen bee ensures the renewal of the bee colony. The male bees represent the infertilized eggs. Their main activity is the reproductive function with the queen bee outside of the beehive.
The Queen, a Worker and a male (coming soon)
Collapse of Bees
Over the last few decades, a decline in the honeybee population has been observed. In Europe 24%
of the domestic bee species are threatened of extinction. The more indicators you consider, the
more alarming appears the situation. The bee loss has a multifactorial origin. Among these factors,
the Colony Collapse Disorder (CCD) is one of the most important. CCD corresponds to a massive loss
of bee colonies. The causes of this syndrome are numerous: pesticides, viruses, parasites… among those,
a mite wisely named Varroa destructor is one of the bee’s fiercest enemies.
Varroa destructor
Varroa destructor is an obligatory ectoparasite of bees. This means it is an external parasite and it cannot survive without its host, the bee. The varroa development cycle takes place in the beehive cells in parallel of the bee development cycle. The key individual of the varroa development cycle is the adult female commonly named “founder”. The founder reproduces exclusively in a brood cell that represents the beehive cell containing a bee larva. The founder reproduction occurs during the phoretic phase where the varroa is moved from one bee colony to the other by an adult bee.
Figure 2: Synchronized life cycle of Honeybees and Varroa destructor.
The mite varroa is originally an ectoparasite of Apis cerana, the Asian honey bee. The first contact between varroa and Apis mellifera occurred in the 1950s in Asia. During the following years, the infestation spread to Europe, Africa and America. The first signs of this infestation were observed in Europe and North America in the early 1980s. The varroa mite is a particularly virulent parasite of Apis mellifera since the European honeybee did not have time to develop an adaptive response to it. In Asia on the contrary, Apis cerana and Varroa destructor co-evolved over the centuries and reached a certain balance. Interestingly, the Asian honeybee is capable of detecting the parasite’s presence and of getting rid of it.
Figure 3: Effects exerted by Varroa destructor on honeybee and their consequences.
Outbreak of Varroa destructor in the Western World
The mite varroa is originally an ectoparasite of Apis cerana, the Asian honey bee. The first contact between varroa and Apis mellifera occurred in the 1950s in Asia. During the following years, the infestation spread to Europe, Africa and America. The first signs of this infestation were observed in Europe and North America in the early 1980s. The varroa mite is a particularly virulent parasite of Apis mellifera since the European honeybee did not have time to develop an adaptive response to it.
In Asia on the contrary, Apis cerana and Varroa destructor co-evolved over the centuries and reached a certain balance. Interestingly, the Asian honeybee is capable of detecting the parasite’s presence and of getting rid of it.
Means to fight Varroa destructor
There are three types of methods usually implemented to fight against varroas:
♦ Search of tolerant or resistant bees through selection
♦ Use of “zoo-technic” or “bio-technic” means (Trapping of varroas in empty male broods or installation of fenced entries)
♦ Use of chemical treatments
This last type is the most effective however none
of the treatments has a 100% effectiveness and their
use is often limited since bees generate consumable
products. The lack of effectiveness of those treatments
(not to mention the resistance phenomena expected) induces
the presence in hives of enough varroa after treatment to allow
the population to grow beyond the dangerous threshold for a bee
colony. The use of such treatments (usually annual) can eliminate
up to 90% of the varroa population,
which is insufficient to maintain the epidemic at safe levels.
On top of that, such treatments may have serious disadvantages:
♦ It has been shown that some miticide or
degradation metabolites from these molecules accumulate in beeswax.
This long term accumulation could explain the emergence of resistant
parasites.
♦ Some treatments were shown to have detrimental effects on bees.
Those effects are usually increased if the treatments are misused.
♦ Some treatments can contaminate the production of bees and impact on the quality of the products.
Bees are probably one of the most valuable animal species to mankind through their unique role in pollination. The speed at which they disappear is alarming and considering the emergency of the situation our team decided to use the tools offered by synthetic biology to fight the Varroa destructor.
References
- [1] Yves Le Conte, Marion Ellis, Wolfgang RITTER. Varroa mites and honey bee health: can Varroa explain part of the colony losses? Apidologie, Springer Verlag (Germany), 2010, 41 (3), <10.1051/apido/2010017>.
- [2] Sammataro, D., Gerson, U., Needham, G., 2000. Parasitic mites of honey bees life history implications and impact. Annual Review of Entomology 45, 519-548
- [3] Peng, Y-S., Fang, Y., Xu, S., Ge, L., 1987. The resistance mechanism of the Asian honey bee, Apis cerana Fabr., to an ectoparasitic mite, Varroa jacobsoni Oudemans. Journal of invertebrate pathology 49, 54-60.
- [4] S, L, P Wendling. 2012. Varroa destructor (ANDERSON ET TRUEMAN, 2000), UN ACARIEN ECTOPARASITE DE L’ABEILLE DOMESTIQUE Apis mellifera LINNAEUS, 1758. REVUE BIBLIOGRAPHIQUE ET CONTRIBUTION À L’ÉTUDE DE SA REPRODUCTION.
