A keen awareness of the level of Varroa mites in the hive is a primary concern for most beekeepers. There are numerous ways to measure the mite load, with methods like the sugar and ether rolls being popular. In recent years, one of the more popular treatments is the application of oxalic acid. This treatment is also favored by many beekeepers.
When oxalic acid works – and when it doesn’t
A large percentage of Varroa in the mid-summer hive do their damage under the protective cover of the capping on a cell. As well as sucking the life out of the larvae resident in the same cell, the female Varroa that entered the cell initially will reproduce. Eventually, a single cell will be home to many Varroa.
Oxalic acid is not effective against Varroa in capped brood. This, of course, has important implications for how and when it can be used.
During the summer only about 15 – 20% of Varroa are in the phoretic state, meaning they are roaming the hive or attached to bees. The remainder are in a capped cell – out of the reach of oxalic acid. Therefore, the summer is often considered a less-than-optimal time to apply oxalic acid, since only a small percentage of the Varroa in the hive are “reachable”. But that doesn’t mean it cannot still be an effective treatment, as we will see below.
Treating Varroa is basically a numbers game, which should inform your use of oxalic acid. At a high level, there are two approaches to how it is applied.
But first, let’s take a quick refresher on the time from egg to emergence from the cell. Then we’ll consider how this relates to the application of oxalic acid.
The timeline from egg through pupa
From the day the queen starts laying eggs there’s an important clock that starts ticking (all beekeepers should be familiar with this, since it is important in many aspects of beekeeping).
Using rough numbers from when the egg is laid by the queen (there are many variables at play here, so this timeline will vary somewhat)…
1. Days 1 – 3 : During these initial days we have an egg in a cell, in an uncapped state.
2. Days 4 – 9 : Then the egg transforms to larva, a state that is maintained for six days. During this period the cell remains uncapped.
3. Days 10 – 21 : Finally, the larva transforms into a pupa. The cell is capped at the start of this period.
As a simple memory trick, start with three days (the time during which we have the egg) and then double twice – 6 then 12 days – for each subsequent stage. That gives us roughly 21 days, though see above for our qualifier on this timeline.
During steps 1 and 2 any Varroa in the cell would be easy targets for oxalic acid. The cell is uncapped, so Varroa there would be fully vulnerable – if they were in the cell.
In fact, Varroa are a little more clever than that! They don’t even enter the cell until just before the cell is capped. Therefore, for the first nine days (3 + 6), there are no Varroa in the cell. We’re good with those cells!
But when the Varroa creeps into the cell just before it is capped…then we have a problem. We have a female Varroa and her offspring-to-come safely cocooned in the cell and protected from oxalic acid.
