Varroa – know your enemy
Part 1: The problem – Varroa’s explosive reproduction
Already Alexander the Great is said to have coined the expression “to defeat your enemy you must get to know him”. Whether Alexander really said that is unclear, but it is definitely necessary to understand the life cycle of the varroa mite in order to defeat the mite, or at least keep it at bay. Varroa destructor originally came from the Asian bee (Apis Cerana) and jumped over to our European bee (Apis Mellifera) in the middle of the 20th century. The Asian bee is able to handle the mite which Mellifera does not and therefore the varroa has become the dominant cause, directly and indirectly, of the high winter losses we see. Without any form of treatment, most Colonies usually perish after 2-3 years (10). The varroa has 2 different phases in its life, first phase when they sit on adult bees and eat from the fat body (Phoretic phase) (1) and the reproduction phase which only takes place inside the brood cells, preferably in the drone cells (2). When the mites parasitize the adult bees, it is partly for the newly mated female to “mature” and partly to be transported to a new cell. This phase can last up to a week before they find a new suitable cell to initiate a new reproductive cycle. When the varroa mite has found a suitable cell, it crawls in just before the bees cap it and hides at the bottom of the cell in what is left of the larval sap. Three days after the cell is sealed, the female lays the first haploid egg that develops into a male (3). Then 4 (in a worker cell) or 5 (in a drone cell) diploid eggs are laid which develop into new females, see fig below. Just before the new bee crawls out of the cell, the male mates with his sisters. The male does not survive outside the cell and neither does the un-matured female mites, but the mother mite crawls out with one or two mated daughters (in a drone cell, up to three new mites can be fertilized).
Fig 4 the might family(10).
About 11 days after the cell is sealed, the varroa family looks like this. Upper row mites in different stages. Bottom row, newly molted female, the mother mite and the male. The varroa prefers to crawl into drone cells, the ratio is approximately 8:1 which is probably due to the fact that the drones have a longer spawning phase which increases reproduction as more daughters can be born, about 1.3-1.45 in worker cells versus 2.2-2.6 in a drone cell (4 ). Because the rate of reproduction is exponential – for every female that enters a drone cell, an average of 3.5 come out – in combination with the short reproductive cycle (takes about 30 days for two cycles), the development becomes explosive, especially in the spring when there are drone cells in abundance. With these facts, it is easy to understand that 50 mites at the beginning of May can turn into several thousand in two months, but luckily there are some slowing factors. The figure shows that the varroa lags behind the bees and where the maximum number of bees occurs at the beginning of June, the varroa reaches its maximum at the beginning of August. 2-3000 mites are not life threatening when the number of bees is 50000 or more. But when all these mites will be split on only 10-20,000 bees in the fall, the mite level becomes far too high and the community quickly succumbs.
Practical application: The mite level is often low in the spring and explodes during May-June without measures. When the bee population goes down to 10-20,000 individuals, the mite population must be distributed among fewer bees, causing colonies to crash.
As mentioned earlier, there are a couple of limiting factors for the mite reproduction; one is infertility in the mites and the other is that the number of cycles the female mite can carry out seems to be limited to 2-3 cycles (5). What causes infertility in varroa is not completely clear, but one reason may be the absence of a male. Since usually only one male is born, it becomes impossible to carry out the mating if he dies before impregnating his sisters. It turns out that it’s relatively common for that to occur, meaning unfertilized females come out of the cell. These can complete a phoretic phase and then invade a cell where they can lay a haploid egg which then develops into a male. In this case, the son can fertilize his mother in a so-called Oedipal fertilization, this mating is usually not as successful than when young females are fertilized and the number of new mites that are created via Oidiapal fertilization is limited (6). As stated, the rate of increase is exponential with the exponent 3.5 in the drone cell, causing the mite level to explode in May-June. If we have, for example, 10 mother mites that enter mature drone cells, approximately 35 female mites come out after about 14 days. The males and immature mites remain in the cells, and die, because they cannot feed themselves. After a phoretic phase of approximately 5-7 days, 35 mites are now ready to invade, which then gives 122 mites after the next reproduction cycle. The rate of increase is thus up to 12x per month as long as drone brood is present, but when only worker brood is present it is reduced to a maximum of 6x, in reality the rate is slightly less because not all mites are fertile (4) and that some are lost with foraging bees that die in the field. In untreated communities, the colonies die after a few years as the Varroa weakens the communities both at the individual level and at the colony level. The individual bees that have been parasitized as larvae lose weight, get more viral diseases (eg DWS) and have a shorter lifespan (7). At colony level they are affected in at least 2 ways; The drones decrease in weight and have a clearly poorer chance of mating (8) and the ability of the colonies to swarm decreases (9). Practical application: The rate of increase is at least twice as high when drone cells are available, which is the reason for the explosive rate of increase in May-June.
In the next section, we go through our Varroa management and what can be done to reduce the impact from Varroa. references
(1) Kuenen, L.P.S., Calderone, N.W., 1997. Transfers of Varroa mites from newly emerged bees: preferences for age- and function-specific adult bees. J. Insect Behav. 10, 213–228. (2) Boot, W.J., Schoenmaker, J., Calis, J.N.M., Beetsma, J., 1995b. Invasion of Varroa jacobsoni into drone brood cells of the honey bee, Apis mellifera. Apidologie 26, 109–118. (3) http://www.ask-force.org/web/Bees/Rosenkranz-Biology-Control-Varroa-2010.pdf (4) Martin, S.J., 1995b. Reproduction of Varroa jacobsoni in cells of Apis mellifera containing one or more mother mites and the distribution of these cells. J. Apicult. Res. 34, 187–196. (5) Fries, I., Rosenkranz, P., 1996. Number of reproductive cycles of Varroa jacobsoni in honey-bee (Apis mellifera) colonies. Exp. Appl. Acarol. 20, 103–112.
(6) https://link.springer.com/article/10.1007/s13592-019-00713-9
(7) Amdam, G.V., Hartfelder, K., Norberg, K., Hagen, A., Omholt, S.W., 2004. Altered physiology in worker honey bees (Hymenoptera: Apidae) infested with the mite Varroa destructor (Acari: Varroidae): a factor in colony loss during overwintering? J. Econ. Entomol. 97 (3), 741–747.
(8) Duay, P., de Jong, D., Engels, W., 2002. Decreased flight performance and sperm production in drones of the honey bee (Apis mellifera) slightly infested by Varroa destructor mites during pupal development. Genet. Mol. Res. 1, 227–232.
(9) Fries, I., Hansen, H., Imdorf, A., Rosenkranz, P., 2003. Swarming in honey bees (Apis mellifera) and Varroa destructor population development in Sweden. Apidologie 34, 389–398.
(10) http://www.ask-force.org/web/Bees/Rosenkranz-Biology-Control-Varroa-2010.pdf
Part 2- the solution
In the previous part, we went through how Varroa reproduction takes place and the reasons for the explosive development, which is the reason why untreated communities usually perish within 2-3 years. Here we will go through what our strategy looks like.
When I first saw the above figure, I spontaneously thought that something was wrong – why wait to treat until the Varroa is at its strongest? Since a “stitch in time saves nine” a better approach would be to treat the varroa before they had time to explode and grow strong (see the dashed line in fig 1.)? That idea became the starting point for our strategy, which is based on never allowing the Varroa to reach levels where they affect the colony to any great degree. In principle, it is a method that follows the IPM strategy (Integrated Pest Management) where we avoid using strong pesticides and chemicals as far as possible and where the basis of our treatment is drone brood cutout.
Drone cutting takes place in a three-part frame that is inserted at the end of April – the first week we cut out two parts and the second week one part. Then we have wax and cells in three different stages and can start cutting out one third of the frame with covered drone cells every week. After 6 weeks we have removed a large portion of all mites, a theoretical calculation (1) shows that two weeks of excision can reduce the amount by 50%, a practical study showed similar results (2 ). We can do up to 6 cutouts until the drone period is over and theorethically reduce the amount up to 85%. This means that we usually have less than 50 mites left, which is also supported by the mite fall being close to zero at the end of June.
Practical application: 2-3 cutouts reduce the amount of Varroa by up to 50%. With 6 excisions over 6 weeks, the Varroa level can be pushed down to harmless levels.
After the drone period is over, we continue to keep track of the mite fall and in many cases the it remains at such low levels that no other treatments are needed – our threshold value is about 15 mites a week, which corresponds to a couple of hundred mites. Should the level increase and exceed 25 mites a week, then we resort to the mildest acid in our arsenal, i.e. lactic acid. The advantage of lactic acid is that it is uncontroversial with food and gives no aftertaste to the honey, only a slight sourness. Since the honey is acidic in itself, this is not a problem, and the bees are minimally affected by the lactic acid. The disadvantage is of course that you only get to the bees that are outside the cells, 30-40% of the mites are outside the cells and these are the ones we can kill. It may seem like a small percentage, but since the goal is not necessarily to eradicate the mites but only to ensure that they never become too numerous, lactic acid works just fine. We take an example; assume that the mite fall has increased so that we begin to approach 500 mites (5% if we have 10,000 bees) and then we do a lactic acid treatment and get rid of approx. 30%, i.e. 100-150 mites. Then we have 350-400 left, which is still too much. Then, if the mite fall is, indeed, too high, we repeat the treatment after one week and access another 100-150 mites and then the number is down to a harmless level. In practice, we do any lactic acid treatment much earlier (over 25 mites/week) so usually one treatment is enough to get below the threshold. Someone might remark that the method seems tedious, but we always have a spray bottle with lactic acid with us and if treatment is needed, we do it at the same time as we do the weekly review, which means about 1m of extra time.
Our experience is that with this strategy (green and yellow level in the IPM pyramid) we can keep the varroa level below our threshold throughout the season and thus avoid all other acids and chemicals. Only exceptionally do we end up with such high levels in August that formic acid has to be used (1 colony out of 13 this year received formic acid, no colonies needed oxalic acid). In the 8 seasons we have developed and used our method, we have never experienced any winter losses (colonies that did an early queen change not included).
Practical application: Drone cutout + lactic acid treatment when necessary drastically reduces the need for stronger acids and other chemicals.
This year we also did a follow-up test to confirm the results; a colony that was found to be brood-free received an oxalic acid treatment (we used the drip method, which is highly effective on brood-free colonies) on November 15, and we measured the mite fall to confirm how many mites the hive really have. We counted about 50 during one week and in a control colony (untreated) in same location we saw during the same week about 5 mites, which indicate that the strategy gives the desired result.
In conclusion, some advantages of our strategy:
1. All chemicals affect the bees negatively – the stronger the acids, the more side effects in the form of dead bees, less brood and a higher risk of the queen being balled up and dying. 2. The mites tend to become resistant to pesticides and increasingly stronger agents must be used. We do not contribute to the negative spiral with our strategy.
3. With our method, you get minimal impact on the colony because the number of mites is always low, it maximizes the number of foragers bees and provides a prerequisite for a larger honey harvest.
4. You avoid handling strong acids, that require protective equipment, which most of us are not trained to handle.
5. You don’t have to disturb the bees in the middle of winter because oxalic acid is not needed.
6. You have full control of the colonies and treat when necessary and not casually because “you have to do it that way”. In this way, we minimize the stress on the bees and get stronger bees that overwinter better.
references:
(1). https://etd.ohiolink.edu/apexprod/rws_etd/send_file/send?accession=osu1481534982440449&disposition=inline
(2). https://www.academia.edu/19046309/Strat%C3%A9gie_de_lutte_alternative_contre_Varroa_destructor_en_Europe_centrale?email_work_card=view-paper
DoD's bisyssla 