Following on from my article about using brood breaks to help with varroa management, I’m going to get into ‘death by charts’ territory today. Regular readers will know that I’m a big fan of Randy Oliver’s spreadsheet model, which can be tweaked to reflect one’s colonies and treatment regimes. In the absence of any beekeeping, and as an alternative to tidying up my messy bee sheds, I’ve been trying out different scenarios to see what the model says. As it happens, I’m partial to a giant spreadsheet model, having lived by them for years when doing company budgets and forecasts.
The model, which no doubt took a lot of work to put together, is bound to be an illustration – a guide based on averages (perhaps ‘median’ would be more accurate). Individual colonies can be all over the place, which means that we must judge each on its merits. The model will hopefully cover the majority of hives, assuming that the correct inputs are selected. I find it helps me to look a ‘big picture’ stuff such as when to expect mite loads to become a threat to the wellbeing of the colony, and when to time different treatments. It’s an excellent strategic tool to help with planning, and it doesn’t involve trudging across a muddy field.
What I do now
The first charts, chart 1a and 1b show the changing numbers of bees, sealed brood and mites over a typical year in a regular hive. I use Apivar (amitraz) or thymol in August or early September, followed by oxalic acid treatment in late November/early December. It mostly works well, but not always. The only way to be sure is to take off and nuc the entire site from orbit, but that doesn’t just kill mites.
The difference between these two charts is just the timing of the autumn treatment, which seemingly makes little difference. In both cases, the mite population is 20 at the beginning of the year and 20 at the end. However, with the later treatment, the maximum level of mites is higher, due to the two-week delay in treatment. I aim to treat in mid-August to leave the brood that will become ‘winter bees’ as few mites as possible. The model assumes an ingress of mites in autumn from robbing/drifting; a setting which needs setting according to your local circumstances.
Doing Nothing is not an Option
Chart 2 shows what happens in the same colony with no attempt by the beekeeper to deal with varroa. They die at some point in the winter. The problem is not just the high mite levels, but the combination of this with a declining bee population and less brood.
Chart 3 is my attempt to model what would happen if I trapped the queen in a cage for 25 days, artificially creating a brood free period. I have assumed that the mites get clobbered by oxalic acid treatment once the brood is gone. Perhaps inserting a ‘bait comb’ of unsealed drone brood then removing it when capped would achieve something similar, I don’t know. For me, a quick oxalic treatment when the mites are at their most vulnerable is the most efficient, safe and effective course of action. I have also given them the usual winter treatment, bringing the ending mite count down to 20 again.
Chart 4 shows what might happen with a caged queen as in chart 3, but without the oxalic acid in July. According to the model, it ends in tears. The brood break alone may set the mites back for a time, but once the queen is laying again, the brood is infested and stands little chance. Using the queen cage to create a broodless colony needs to be combined with a mite treatment to be effective. With such treatment, all looks very good indeed; without it, they die. A bait comb to remove mites will undoubtedly help for those opposed to treating although it adds further delay and is probably not as effective as oxalic acid.
Brood Frame Removal
An alternative to caging the queen to create a brood break is to remove the brood and replace it with foundation. When combined with a bait comb, 85% of varroa mites likely get removed from the hive (see chart 5). The model shows that without a winter treatment, the mite level rebuilds to the point where the colony dies at some point over winter. You don’t want to be going into winter with 800+ mites on board.
Chart 6 takes things a step further; after removing all of the sealed brood, the hive receives a treatment of oxalic acid as well, which is incredibly powerful. With this plan, the model shows that the number of mites at the end of the year is 500 – high, but not fatal. However, going into the following season with 500 mites will require an intervention in the spring to prevent colony death. Chart 7 shows how this pans out, with an amitraz treatment in late March, followed by a repeat of the brood removal process in July. I’m not keen on relying on amitraz in spring, nor with going through winter with 500 mites.
Zap the Buggers
Finally, chart 8 shows the best way to work with the brood removal option, in my opinion. Zap the buggers twice a year when broodless, first in July and then later in November or December. If I were to pick between the two ‘brood break’ options; caging the queen or removing frames of sealed brood, my instinct is to go with the former. In fact, I hope to try this next season – the Chart 3 option. If I can manage mites without the use of amitraz, I am protecting my bees against future varroa resistance to the miticide.