Beekeeping,  Bees

Oxalic Acid Fogging for Varroa Mites

With varroa mites so prevalent among beehives, it is only a matter of time before a beekeeper will need to think about mite control. Proper mite detection and treatment is critical in preventing infestation and colony collapse. 

There are a few different methods for treating varroa mites, and it is best to change up the treatment every so often to prevent resistance. Oxalic acid fogging for varroa mites is an effective method for treating these pests. 

What are Varroa Mites?

Varroa mites are tiny external parasites that feed and live on honey bees, their larvae, and pupae. These mites cause malformation and transmit viruses, resulting in weakened honey bees. 

While small amounts of varroa mites are not usually noticeable, a large mite population will result in parasitic mite syndrome. Symptoms of a varroa mite infestation are scattered brood, impaired flight, crawling bees, and chewed cappings. 

If not addressed, a colony infested with varroa mites will face a large decrease in it’s population, supersedure, and eventually, the death of the colony. That’s why it’s important to treat your colony for mites as soon as you notice a problem.

How do Varroa Mites Affect Bees?

Varroa mites affect honey bees by feeding on both adult bees and their larvae and pupae. Many larvae and pupae that become infested by varroa die before emerging from their cells. The ones that do survive show signs of physical damage. 

When varroa mites feed on brood, the weight of the hatching bee significantly decreases. Because of this, the bee’s flight will be impaired, as well as the gland that makes royal jelly. Less royal jelly being produced compromises brood feeding. 

Bees parasitised by varroa mites have a much shorter life span than healthy bees.  They leave the hive for long periods of time, and also have a lower rate of return. This is attributed to problems with their ability to navigate. 

Varroa mites are a vector for many viruses.  When varroa mites feed on honey bees, the bees immune system weakens, which makes them more susceptible to viruses carried by the mites or other pests.

A small number of mites in a hive is not a big deal, however, as the mite load increases, the entire colony is affected.  If the colony is not properly managed, a severe infestation will occur, interrupting foraging, reproduction, and brood rearing.  This is called colony collapse. 

Signs of Parasitic Mite Syndrome

Parasitic Mite Syndrome, or PMS, occurs when varroa reaches a high population, causing the colony to deteriorate and eventually die.  Signs of Parasitic Mite Syndrome are: 

  • Scattered brood pattern
  • Sunken and chewed cappings
  • Yellow or brown larvae
  • Larvae slumped in the bottom or side of cell
  • Crawling or crippled adult bees
  • Dead bees that were unable to fully break free from their cell after the brood cycle
  • Supersedure of queens
  • Unexplainable reduction in colony population

Colonies showing signs of Parasitic Mite Syndrome must be treated or the colony will weaken to the point of colony collapse. 

Oxalic Acid Fogging for Varroa Mites

To combat Parasitic Mite Syndrome, many beekeepers administer oxalic acid fogging for varroa mites. You don’t need to open the hive to do it, it can be done even in cold temperatures, and it is quicker than the dribble method. Foggers allow the beekeeper to quickly move from one hive to the next, making it preferable for beekeepers with multiple hives. 

What is Oxalic Acid?

Oxalic acid is a natural organic compound found in plants, vegetables, nuts, and seeds. Oxalic acid is repellent to herbivores, so it is useful to prevent plants from getting eaten. Furthermore, oxalic acid has proven to be an effective treatment for varroa mites. 

Oxalic acid fogging for varroa mites has been widely used by beekeepers in the US and Europe for many years. With proper application, It is deadly to the mites, while being harmless to the bees. It isn’t entirely known how oxalic acid affects varroa mites, however it is thought that it enters their bloodstream through their feet. 

It is important that oxalic acid is used only as needed.  Overuse carries the risk that the mites build a resistance to it, decreasing its effectiveness. This is why most beekeepers like to vary the types of treatment used. 

How to Treat Varroa Mites with Oxalic Acid Fogging

Protective Gear

Although it’s natural, oxalic acid must be handled with care.  It can cause irritation to the mouth, eyes, and nose.  Gloves and face protection like an oxalic acid respirator are commonly used when administering the treatment. 

When to Treat Your Hives

You must decide when to treat your hive.  The reason is because oxalic acid is not effective on varroa inside of capped cells.  And that is exactly where they like to hide. 

In the summer, only about 15% of varroa are attached to bees. The rest of them are inside of capped cells. This is not the optimal time for oxalic acid fogging. However, in the fall and winter, there is little capped brood, so the mites have nowhere to hide.  This is an excellent time for oxalic acid fogging. 

If you need to treat varroa with lots of capped brood present, there is a different method. After the initial treatment, apply oxalic acid again in 5-7 days. The first treatment addresses the mites outside of the cells, while the second treatment should get the mites that leave the capped brood cells. With this method, it can be difficult to get all of the mites, however it is still effective in controlling the mite load. 

How to Use an Oxalic Acid Fogger

To treat varroa with oxalic acid fogging, you will need an oxalic acid fogger, oxalic acid powder, and distilled water or ethyl alcohol.  The oxalic acid powder must be mixed with distilled water or ethyl alcohol to form a solution to go into the fogger. The standard is 25 grams of oxalic acid powder in 100ml of water or alcohol. 

Once you have mixed the solution, pour it into the fogger’s oxalic acid tank. Attach the tank to the fogger.  If your fogger uses propane, ignite it, but if it uses electricity, simply switch it on. 

Allow the fogger to heat and press the trigger to release a puff of oxalic acid fog. Now you are ready to treat your hives. Stick the nozzle in the hive and release 1-2 puffs of fog per beehive. 

After 24 hours, inspect your hives to check for fogging efficiency. You will be able to see dead mites on the bottom board. Remove the dead mites to prevent ants from being attracted to the hive. A screened bottom board or sticky board can help with this. 


Varroa mites are external parasites that live on honeybees and their brood. Without proper treatment, the mite population will grow until colony collapse.  Oxalic acid fogging is a quick and effective way to combat varroa mites.