When to Treat Bees For Mites?


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Deciding when to treat bees for varroa mites is one of the most important decisions in beekeeping. About the size of a pencil mark, varroa are one of the most serious pests of honey bee colonies. They are responsible for the deaths of millions of hives each year. Controlling the infestation of varroa mites in your hives is key to beekeeping success. 

Why You Need to Control Varroa Mites in Beehives

Seeing varroa mites in drone brood means it time to treat image.

Mites were not always a big problem for beekeepers. We can only imagine how wonderful beekeeping must have been before varroa mites arrived in the United States.

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Of course, there were still challenges faced by beekeepers but everyone agrees it was an easier time. If your colony had food and a good queen – the honey harvest was your biggest job.

But, that has all changed. Varroa destructor mites originated in Asia. There they used another type of honey bee, Apis Cerana as a host. The mites and bees are able to co-exist with few problems.

The honey bees found in the United States are of European origin – Apis mellifera. They had no previous history with varroa mites and therefore no resistance.

Once varroa arrived in force, modern beekeeping changed. Varroa feed on bees and serve as vectors for viruses and disease. They weaken colonies and result in sick non productive hives.

At least for now, any one interested in keeping bees must have a plan to deal with this external pest of the honey bee. Problems caused by mite infestations are devastating. Control must be a part of your hive management plan.

When do you need to worry about varroa mites? For most beekeepers, it is necessary to treat bees for varroa mites several times a year. It is not a one and done deal.

Varroa Treatment Schedule Varies

The exact timing of mite treatments depends on several factors: the genetics of your bees, the product used for mite control and where you live.

Some “genetics” of honey bees are more susceptible to infestations. In some regions of the country, controlling varroa mites in colonies is a bigger deal than others. They simply have a bigger problem with mites. 

Therefore, the beekeeper should learn how to monitor infestations in the hive. From there, you can decide what if any action to take.

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This is especially important to the new beekeeper deciding on the optimum number of hives to keep. Don’t have more than you can manage.

The No Treatment Method

Many beekeepers strive to move away from the use of chemical treatments. And, the future looks promising as bee breeders work to breed mite resistant bees

But for the vast majority of beekeepers, failure to control varroa mites in the hive will result in a mite crash. This means that the bee colony dies as a result of varroa.

The idea of not having to worry about infestations is great but the reality is that it doesn’t turn out well for most beekeepers. Many hives die needlessly.

Female varroa mite on bee larva image.

Varroa Mite Treatment Thresholds

There are several approved varroa mite treatments to consider. But, any treatment plan (even natural ones) causes some stress to our colonies. We want to avoid unnecessary treatments.

Also, beekeepers with a lot of hives find it expensive to treat their colonies. In the interest of saving money, work and stress for the bees, researchers have developed treatment thresholds.

These are general guidelines that a beekeeper can use to make a plan for when it is time to take action against varroa mites.

Economic Threshold

Often called the “economic threshold”, research says that when the mite infestation reaches a certain level, the colony will experience decline and be less productive. If mite growth is allowed to continue, the colony will die.

What is that number? Unfortunately, these numbers are “soft” and tend to vary somewhat from one region to another.

And, they vary from one expert to another and one year to another. We can still use them as a guide. But remember, acceptable mite levels are guidelines-not rules set in stone.

In my years of beekeeping, the “allowed” number for mite infestation keeps dropping. We are learning that it takes fewer mites than previously thought to harm bees.

In short, the honey bee colony can deal with a small number of varroa mites. It is not necessary (if even possible) to kill each and every varroa mite in the hive.

Mite Counts – How to Test Varroa Mite Levels

New beekeepers often say, can’t you see mites on adult bees? Yes, you can. But, you can not use visual inspection to judge your infestation levels. 

You will not be able to see the vast majority of mites that are reproducing in the brood cells. By the time you notice mites on your adult bees it may be too late to save the colony.

Instead, beekeepers use various methods to perform mite counts. Performing mite counts on your colonies is absolutely necessary.

The sugar shake and alcohol roll are two common testing methods that any beekeeper can complete. Another, though not as accurate is the bottom board drop count.

Once you have performed your counts of the estimated mite levels, it is time to analyze your findings. Is it time to treat or would it be best to continue to monitor the levels?

Is it Time to Treat?

One common way of identifying a possible mite problem is noticing Deformed Wing Virus (DWV) in your hive.

Colonies with high mite infestations tend to have more problems with this virus. However, this alone is not a true indicator of the severity of the infestation.

How many varroa mites are too many? Rather than going strictly by a calendar, use your mite counts as an indicator of colony status.

After performing your mite count, it is time to consider how big your mite problem is. Most researchers agree that a varroa infestation rate over 3% needs management.

Calculate Mite Percentage Using 300 Bees:

Using a standard ½ cup (measuring cup) of bees from frames in the brood nest yields roughly 300 bees. After conducting your sugar shake, and counting any varroa we are left with these numbers.

If we find 9 varroa in this number of bees, you have a 3% infestation level.  i.e. 9 mites/300 bees = 3 mites per/100 bees.  At this rate, I would definitely treat for varroa mites.

But, what if you find a 2% infestation or even 1%. Can a single mite count give true results? Some beekeepers promote multiple mite counts for accurate numbers. However, few beekeepers do multiple counts.

Should You Treat Bees for Mites With Infestation Levels Below 3%?

In regions where mites are known to be bad, treatment is always necessary. Finding no mites or a 1% level may encourage you to watch and wait for a bit. 

Depending on the time of year you may be able to skip mite treatments for a while-but it is a risk.

If the infestation level is 2% or greater, you have a decision to make. Depending on your schedule and the time of year, the beekeeper may go ahead and implement a treatment to keep the mite population low.

Fall Absconds or Heavy Mite Loads?

It is not uncommon to hear of late Summer or Fall absconds. An “abscond” is the term used when all the bees in the hive are gone. This differs from a swarm of bees .

Reproductive swarms leave part of the bee population behind. In absconding, none or very few bees remain in the hive.

Perhaps some of these empty Fall hives are not true absconds but colonies collapsing from large numbers of varroa mites.

We don’t know exactly why this would happen other than the bees trying to leave a losing situation.

Late Season Varroa Mite Bombs

Why do many beekeepers have low mite infestations in July and dead bee hives by late Fall? These large, robust colonies seem to dwindle or disappear very quickly.

Large numbers of dead varroa mites are often found among the dead bees and hive debris on the hive floor. To begin to understand why this happens, we must look at the life cycle of each of the players in our game.

Beehive debris with dead varroa mites on hive floor image.

Bee Life Cycle

The life cycle of honey bees is not the same as that of the mites. The journey of the individual worker honey bee begins with an egg that reaches adulthood in 21 days.

Queen bees reach adulthood in only 16 days. But the drones, or male honey bees, are the favorites for varroa mites.

Drones emerge from their cells on day 24. They spend 3 more days in the sealed cell. The total population of the colony and the number of workers vs drones cycles throughout the year.

Bees in hive with mites visible on drone bee image.

Life Cycle of Varroa Mites

Varroa mites also go through several stages of development. The mated female mite (foundress mite) rides around on an adult bee. She feeds on the bee by biting through the bee’s exoskeleton.

Phoretic Stage

This is called the “phoretic stage”. You might see one on your bees-but usually the mites are on the underside of the abdomen.

This stage of life lasts 5-11 days when brood is in the colony. During Winter months with no brood, the phoretic stage can last for months!

Reproductive Phase of Varroa Mites

When the foundress mite is near a bee larva (almost ready to pupate), she drops off the adult bee and enters the brood cell. Hiding under the brood food in the bottom of the cell, she is capped inside with the bee larva.

Varroa mites can only reproduce inside the brood cell. The female mite produces a male mite first and then a daughter or daughters inside the cell.

Many mite treatments only kill the phoretic mites – not the ones hidden inside brood cells. Treating for varroa mites may need to be done in phases to catch mites out of the cells.

Mite Reproduction in Worker Brood

A worker bee cell is capped for 11 days giving the varroa time to produce 1.5 females. (Yea, I know you can’t have half a mite! – It’s a science average thing.)

The new female mite mates with her brother inside the cell. They feed on the developing bee, weakening it, and possibly spreading disease.

When the new bee emerges (assuming it is able to do so), the mother mite and her mated daughter emerge as well. The male mite dies inside the cell.

So, 1 varroa mite went in and about 11 days later – 2 came out. These 2 fertile female mites enter a new brood cell and both produce a viable daughter.

The original mother will be nearing the end of her life cycle. But, we still have 3 females inside the hive that originated from the first female or “foundress mite”.

Varroa mite numbers triple each month – by reproduction in worker bee cells.

Female mite in white brood food.

Impact of Drone Brood on Varroa Mite Population

Drone bees have a longer brood cycle – 24 days. And, varroa can identify the type of brood in a cell through pheromones. (They “smell” different).

On average, we can expect the mother mite and 2 viable daughters to emerge from a drone cell. This gives us 3 mated females from 1 drone brood cell (instead of 2).

These 3 mites find another drone cell. 14 days later, 8 viable mites emerge (because mom is at the end of her life cycle).

In just a little over a month (36-38 days) 1 foundress mite, became 8. Imagine how fast this reproduction can grow with hundreds of mites in hundreds of drone brood cells.

Varroa Mite Reproductive Explosion

The Varroa mite population in a colony will triple in a month when reproducing on only worker brood. But with drone brood inside, the mite population can double every 2 weeks.

Studies indicate that for every phoretic mite on a bee, there are 2-3 more mites under the brood cappings.

In July, if you find 100 mites on the house bees, there are about 300 under the caps for a total of 400 mites.

If the mite numbers triple in a month, that means 400 mites (July), 1200 (August) and 2400 (September). 

And that is when worker brood is used for mite reproduction not drones! Drone brood will allow even more mite reproduction.

When is it Too Late for Varroa Mite Treatments?

By mid July, many colonies have already started to slow down egg laying. But, you will still see a lot of bees inside the hive

Most colonies will have more older bees dying than are emerging. This results in a dropping population. (Your mite population is still tripling each month.)

As varroa mite numbers rise and bee brood numbers are reduced, a point is reached where every brood cell has a mite.

Emerging bees that take the place of old foragers are not healthy. Foraging is less effective and resources scarce. We have unhealthy adult honey bees trying to rear strong bees for winter.

Dead bees from parasitic mite syndrome image.

Best Time of Year to Treat Hives

In most cases, an early Spring treatment may be necessary. This gets mite loads down before the honey flow begins.

Most treatments for mite control can not be used when honey supers are on the hive – read your labels.

Monitor mite levels during the season until mid Summer. If not required before then, a mid-late season treatments lowers the number of mites and allows the colony to raise healthy bees for Winter.

Stressed colonies are more likely to suffer from problems such as European Foulbrood or Nosema.

Don’t wait until Fall, it may be too late. It does not matter whether you use oxalic acid vaporization, formic acid or another method – do something if needed. Then, check to see that they worked!

Perform mite counts, listen to other beekeepers and learn what the latest recommendations are for acceptable levels of varroa infestation. These change from time to time.

Mite resistant bee breeding is the hope for the future.  I have not found a bee that can exist treatment free in my region, but I remain hopeful. 

When is the right time to treat bees for varroa mites? Before, it is too late – that’s the honest truth – your hive will reach a point of no return.



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