Varroa 5
Monitoring for Varroa in August
August 2012
Starting the last week in August immediately after the honey supers were taken off a varroa screen was placed under each OMF for a total of 13 days (hives: S, T, U, V, W, X, Y, Z were full colonies of similar strength).

The 8 hives were treated with formic acid similarly to the March treatment. (As described in Varroa 3 in the menu above.) 3 doses of 20 ml of 60% formic acid placed on a pad at 3 days intervals. But this time the monitoring was slightly different. A count of the natural mite drop was taken 4 days before the treatment, and then again after each 20 ml dose; making 4 counts in all. The results are summarized in the tables.
varroa mite count
What conclusions can be drawn from these two experiments?

1. Looking at the mite drop before treatment with formic acid generally gives little indication of the mite infestation. Unless of course there is a significate count (see hive E in March).

2. It goes without saying that once formic acid is applied there is an immediate mite fall. The first count after 3 days is usually the greatest.

3. The mite drop in each 3 day period tends to decrease with each dose. In March for example there were no further mite falls in the final two days of the treatment.

4. The August mite count is significantly greater than that in March as would be expected.

5. All these colonies were of a similar strength and history but there is a significant difference in the mite drop between colonies.

6. The mite drop count should have been continued for a further 9 days, after the initial 9 days, in order to take into account those mites which were in the sealed brood and would only drop when the brood emerged.

7. No detrimental effect was notice on the bees. Only when the formic acid was first applied did the bees set up a vigorous fanning action which quickly dispersed the fumes throughout the brood box. It was surprising to see that the bees did not avoid the pad as might be expected if they found the acid not to their liking!

8. It was surprising easy to apply the 20 ml formic acid doses from the syringe. Each application only took about 20 seconds so that smoking was not necessary. The application was over before the bees realized what was happening!
varroa mite count
Next Page: Varroa 6
varroa mite count
August 2013
The results for 13 colonies treated with formic acid in August 2013 are shown on the bar chart on the right.

The results for both the 2012 and 2013 experiments are shown on the same scale.
varroa mite count
August 2014
The results for 14 colonies treated with formic acid in August 2014 are shown on the bar chart on the right (and some of the dead mites in the photo below).
What was very surprising this year was the relatively very high mite count compared to the two previous years. Note that the 2014 bar chart is not to the same scale as those shown for the two previous years.
The natural mite drop before the formic acid treatment gives little indication of the number of mites present per colony.
It is somewhat difficult to find an explanation for the sudden increase in the mite population. Perhaps the very good summer weather might be the cause allowing the mites to have more brood cycles.
In hindsight perhaps the colonies should have been treated with formic acid in the early Spring before the mite population had a chance to take off.
varroa mites
coloured fimamentous debris
coloured fimamentous debris
coloured fimamentous debris
One observation which is sometimes seen on the examination of the varroa screen after a 3-day application of formic acid is a series of colours carried by the filamentous debris. Pink, yellow, and a blue/green colours are shown below. Each colour is associated with a particular hive i.e. where a colour appears only one colour is seen. At present no explanation is available.
August 2015
The mite drop count shown in the bar charts to the right is not to the same scale as those above.
What is surprising is the relative low mite count this year compared to 2014. The weather perhaps provides the answer. Whereas 2014 was a very good summer, 2015 was quite poor with the mean temperature about 1.5 ℃ below the mean.
The relatively low mite count for some colonies (below 100) was very pleasing.
varroa mite count
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