LATE FALL MITE CONTROL by beekeeper Fred

 The following is how I have been keeping my bees.  These methods may or may not work for you. 

Miticides are not 100% effective in killing off the varroa mites in a hive.  Most are quoted to have a 90 to 95% effectiveness.  NOTE: Powdered sugar is NOT considered to be a miticide and has VERY low effectiveness in controlling mites.  If a particular hive had a high mite infestation the 90 to 95% mite knockdown could still leave hundreds of mites in the hive and require a second treatment.  The mites will continue to reproduce as long as brood is present in the hive.   

Most northern beekeepers do an initial treatment for mites in mid to late summer after harvesting their honey.   Applying the miticide at this time has the benefit of allowing the bees to raise the “winter or fat” bees in a relatively mite-free environment.  But it also allows the mites a longer time to rebuild their population prior to the shutdown in bee brood rearing and coincident shutdown in varroa mite rearing.

For the last few falls the weather has been warmer to normal.  Whether this is due to normal weather fluctuations or global warming is anybody’s guess.  But for northern beekeepers the warmer fall allows the bees to continue raising young for an extended period.  I was inspecting several nucs a week ago and saw eggs, brood and capped brood.   With bee brood in the hive the varroa mites will also continue to multiply thus lessening the effectiveness of the mite control you had applied earlier. 

For the above two reasons some beekeepers apply a second or third mite treatment in September or October.  However, as the temperature declines the beekeeper may need to select a different mite control product.  For example, the instructions for Apiguard state to use it only when outside temperatures are above 59F.  For MAQS the minimum is 50F.  The beekeeper must also take into consideration that the applicators (pads, strips, trays) must be removed after a given time period; thus requiring the beekeeper to re-open the hive several weeks later.   For these reasons many beekeepers tend to use oxalic acid drip or vapor for late fall applications. 

Personally, I think that using oxalic drip or dribble is not ideal.  I don’t like the idea of wetting down the bees with the cold water solution of oxalic acid.   This could cause hypothermia and kill the bees in addition to the mites.   The upper and lower boxes must also be separated to get to the cluster in the lower box forcing the bees to re-propilyze the joint.  But again, that’s just my opinion and you know the saying that “10 beekeepers will give you 11 different opinions”. 

Therefore, for a secondary mite knockdown in the fall I use oxalic vapor as the agent.   I have used both an electrically heated vaporizer and a propane powered vaporizer.  Electrically heated vaporizers are available commercially and have received USDA approval.   The propane powered vaporizers have not yet received USDA approval.   Oxalic acid has a lower effectiveness in controlling mites; around 90%.  It also only kills phoretic (those on the bees and outside the brood cell) mites.  For those reasons it is a better secondary mite control that can be used when temperatures are cooler than your primary mite control. 

Remember the goal is to cut down on mite population thus minimizing the spread of viruses when the bees are in their winter cluster.   Also remember, all mite controls are also hard on the bees so over application of miticides could kill your bees in addition to the mites.  Follow miticide application instructions and monitor mite levels to avoid over application.