In my previous post about almond pollination I mentioned how mild our winter was here in California this last year. That was a very good thing for our almond pollination contract as we easily met the 10 frame contract requirement. Not only that, strong hives early in the year means that we should be able to double our operation this year. However, all of the good news comes with a catch. Since the hives never shut down brood production over winter, neither did the varroa mite.
The overwintering period with a “brood break” is one of the natural varroa control processes that favors the honeybee. During this period of time the bees don’t raise any drones, which the varroa mite prefers to parasitize because they get more bang for their buck. The bees also raise worker brood in very small batches, so the varroa mite is just out of luck and declines in population. Unless… the winter is very mild and the bees never go into full overwintering mode. When this happens, the varroa mite population levels begin the year dangerously high. Couple that with an exponential growth model and the severity of the matter becomes obvious.
Let’s explore an example of exponential varroa growth and the implications that it will have on us this year. I don’t know if there’s an actual exponential growth model for varroa (I assume that researchers have some), but let’s keep it simple and use the following equation P = N * 2s. Where “P” is the final varroa population, “N” is the starting varroa count, and “s” is the varroa reproductive cycles. We’ll use a simple doubling of population every brood cycle and disregard any other factors that affect population size. On to the example: Let’s assume that in a regular year, we enter February with 10 varroa mites. If so, in 8 reproductive cycles (about 6 months) the varroa population will be 2,560 (P=10*28). Now let’s assume that the mild winter allowed 15 varroa mites to survive AND they start reproducing one brood cycle earlier. In this case the mite count would be 7,680 (P = 15*29). Clearly you can see how a mild winter, which allows the varroa mite to get a slight head start in the year could easily triple your mite count by fall of the following year. For this reason we are forecasting a varroa storm this fall for California beekeepers. It would be wise to have a varroa management plan in place early in the year. If you treat your bees for varroa, you may want to begin your treatments earlier than usual, at least one brood cycle earlier.
[If there are any biologists in the audience. The equation described above is oversimplified and used to attain a solution accurate enough for a rough order of magnitude – to illustrate an exponential growth rate. An actual Dynamic model of varroa population should and would include negative/positive reinforcing loops for input parameters such as honeybee brood rearing patters at different times of the year, natural mite fall, mite re-infestation, weather, etc.]
Here at Estrada Farms our plan is to closely monitor the situation and employ natural varroa control tactics whenever possible, such as making splits with a prolonged broodless period (preferably after they’ve made some honey). If the mite levels are too high going into the fall, this may be the year that we deploy oxalic acid as a last ditch effort (ill write about our results if we do). However, our first line of defense is the bees themselves. They haven’t failed us yet and even if we do end up losing some hives to varroa, it will certainly cull out the least varroa resistant bees from our operation. The reverse is also true, and this type of year should highlight which hives manage to thrive despite the high varroa infestations. This would allow us to select the best queens to use as breeders for the next few years.
Every beekeepers goal should be to be treatment free. If we ever get there, it would mean that we finally allowed the bees to evolve natural mechanisms to deal with varroa (much like the Asian honey bee). So even in a year like this, don’t be afraid to let your bees feel the varroa pressure a little bit. If you can afford to lose half of your bees, then lose half of your bees. At least you’ll know that your bees next year will be better because of it.
Knowing that the storm is underway is the first step to being prepared. Good luck and whatever you plan to do to control varroa, make sure to start early.