Tag Archives: mite

Mite Monitoring – it does make a difference

by Dewey M. Caron

Varroa mites continue to change the beekeeping landscape. In the ‘good ‘ol days’ it was far easier to start a bee colony and keep them alive than today. After introduction of the varroa mites into the U.S. (1987), there was an unprecedented rapid approval of tau-fluvalinate pesticide by EPA for in-hive mite control. And the resulting miticide Apistan® worked very well in killing mites.

Unlike the tracheal mite, varroa mite did seem to be controllable at first. Beekeepers changed from a chemophobic attitude about pesticides (chemicals around bees were to be avoided – ‘feared’) to chemophilic (chemicals were tolerated/useful – ‘liked’). But all too soon we began to understand the ‘unintended consequences’ to the hive environment and our honey bees when we began putting a pesticide inside a bee colony. We seriously contaminated our beeswax with the miticide and mites developed resistance to this chemical within several years meaning it became ineffective.

The subsequent generation of pesticides (Coumaphos – an organo-phosphate compound, developed to kill enemy soldiers in war-time) and Hivestan (fenpyroximate pyrazole) were even more detrimental in the bee environment and mites even more rapidly acquired resistance. Current sole synthetic miticide Apivar (amitraz), although generally not as harmful to bees or their environment, is showing signs of mites developing resistance to it as well.

A mite management concept has begun to creep into our language – IPM, Integrated Pest Management. IPM fits with the concept that it is not practical (impossible) to kill every last mite in our colonies (nor possible to keep them out) but rather we should seek to manage the mite population at a level that is minimally damaging. As we manage the honey bee population for gain, we need to incorporate and integrate mite management – an integrated control program.

The question beekeepers need ask is NOT does my colony(ies) have mites? but rather HOW MANY mites does my bee colony(ies) have? To determine how many means varroa mite monitoring, a crucial first step of an IPM management program. When we determine HOW MANY we become better informed bee colony stewards. HOW MANY helps us know the risk of doing nothing and helps us target seasonally appropriate control(s) to slow mite population growth with goal of keeping mite populations at less damaging, more tolerable levels.

One early (alternative) non-chemical mite control treatment was use of Screen Bottom Boards. With widespread adoption of screen bottom boards in place of solid boards, a whole industry developed incorporating mite monitoring sticky boards beneath the bottom screen (one example shown at right). There were attempts in Georgia, Washington State, and by myself in the Mid-Atlantic States, to determine how best to count fallen mites with a sticky board and how to interpret the numbers – a threshold was developed.

This threshold number has been modified as we have learned that mites vector and enhance viruses. Initially 50 mites per days was a threshold. The number has been lowered, initially to one per hour (24/day) and now one per every 2 hours the mite board is in use (12/day). If boards collect more mites the risk to the colony increases with higher mite fall.

Currently 79% of beekeepers responding to the PNW winter colony loss survey indicate they use a screen bottom board as a mite control technique. (see Screen Bottom Boards – a 4 year perspective  Report posted to www.pnwhoneybeesurvey.com). However after 4 years of survey data for Oregon and Washington small-scale beekeepers, a clear picture emerges that screen bottoms aren’t very useful in reducing mites:

The four year average – 41.3% winter loss level of those using SBB  vs 43.4% loss level for those not using SBB – represents a mere 5% gain (i.e. improved winter colony survival) with screen bottom boards.

A constant discussion has been should the colony bottom be left exposed during winter or should the screen be closed as a winterizing management? Closure was often achieved by using a mite monitoring board with the screen bottom. Four years of survey data tallying the always and sometimes left open response compared with the closed in winter response reveals:

10 percentage point difference in favor of closing the SBB over the winter period (i.e. less winter loss if the board is closed in the fall.)

How Best to monitor Mites

Beekeepers generally have available 5 methods to assess the mite population in their colonies. Sticky boards were initially considered a good tool to do whole colony mite number assessment. However there are problems in use of this tool. Foremost with sticky boards, when used to asses mite colony population, was and remains how to quickly and accurately count the mites on the boards. Mites are tiny, sticky boards collect lots of debris and ants and other scavengers may remove fallen mites.

Natural mite fall (i.e. mite drop not enhanced with use of a control), can vary a great deal for a variety of reasons. Size of colony makes for variation, time of season, degree of auto-grooming behavior by bees and also how the boards are inserted and how long they are left inside adds to more variation. Efforts to reduce counting by sampling only a portion of the drop proved impractical as a large portion of a sticky board still needs to be counted to obtain an accurate assessment.

Sticky bottom boards are useful in providing a quick ‘snapshot’ as to whether there are few or many mites in a colony. It can help validate the effectiveness of a control that was used if utilized post-treatment. They are not adequate to provide a precise estimate of mite population without accurate, time-consuming counting and multiple, repeated sampling.

BOTTOM LINE: Screen Bottom Boards are not an effective mite monitoring method

More problematic as monitoring tool is using a visual method of counting number of mites. This is used for colony worker adults or number of mites within drone brood cells. The phoretic mites, (i.e. those on the adults), are highly mobile and some of the mites are deeply embedded between body segments to enable them to feed on adult hemolymph and fat body. Their deep reddish color and compact flattened body means they blend into the adult color patterns.

The counting of reproductive mites within drone cells has proved to be highly variable. The reproductive female mites are easier to see on the white drone pupae (compared to adult bodies) and sampling of drone pupae within capped cells with use of a cappings scratcher is easy. However numbers are not easily correlated to the actual population level within the colony.  One serious drawback with this method is during the fall months, when mite population assessment is most critical, there are often no or few capped drone brood cells available to assess.

BOTTOM LINE: Visual mite monitoring methods are not effective enough

Thus today we believe counting of the mites on the adults by washing the adults to remove their mites (the feeding mites as well as those ‘riding’ the adult) to be the “best” method. Our two recommended methods are the use of powdered sugar or using alcohol (or a non-sudsing soap). Use of powder sugar allows return of adult bees back into their colony whereas the alcohol/soap kills the sampled bees. See Tools for Varroa Guide on website www.honeybeehealthcoalition.org

Generally a sample size of 300 adults, collected from a brood frame, is recommended. The adult bees can be collected directly into a wide-mouthed sampling jar, previously marked (by sample counting) to indicate 300 bees or the brood frame can be shook into a bucket or plastic container and a ½ cup size measuring cup filled with the bees to dump into the sampling jar. The solid lid of the sample jar is replaced by a 5-mesh screen.

If using powder sugar, a generous amount of powdered sugar (confectioner’s sugar) is sifted onto the sample and the jar vigorously shaken for a minute to thoroughly distribute the sugar and then left to stand for a minute or more to allow heat to build up in the 300 bees. If alcohol is used, add enough iso-propyl (rubbing) alcohol to cover the bees. The liquid is swirled and the bees thoroughly washed for at least a minute. DO NOT RUSH SAMPLING. The mites between the body segments of bees need to separate from their host’s body to get an accurate count. See accompanying photos.

USING powder sugar mite monitoring           Using alcohol wash mite monitoring

The sample jar with powdered sugar is then inverted and excess powdered sugar shaken out, along with the mites that had been on the adults. With the alcohol/soap, the mites will be in the liquid portion which can be poured from the sample through a filter, such as a coffee filter, for counting or simply counted in the bottom of liquid with adult bees held above the liquid at base.

The number of mites washed/sugared from the bee bodies is then converted to per cent number of mites for 100 bees (6 mites on 300 adults = 2% for example). The ‘threshold’ for decisions on continuation or initiation of control is currently 2-3% – this number too has been steadily decreasing. Some beekeepers like to keep the number below 1%. Repeat sampling can be done to confirm accuracy. When the sample is below 2-3% monitoring should be repeated to confirm mite population remains at a low level.

BOTTOM LINE: Washing adults collected from a brood frame using powdered sugar or alcohol/soap is the faster and most accurate mite monitoring method for assessment of mite population in colony

In 4 survey years the percent of OR/WA individuals using sticky boards as their monitoring tools has steadily decreased (from 37% of individuals using them in 2015 to currently 25% of respondents using them in 2018 survey response) while alcohol wash as a monitoring method has increased from 5% of reported use in 2015 survey to 14% in most recent survey.  Use of powdered sugar to monitor mites has also steadily increased although more modestly. These trends are shown in graph below for 4 survey years.

With our pnwhoneybeesurvey we ask percentage of OR hives monitored for mites, whether sampling was pre- or post-treatment or both and, of the 5 possible mite sampling methods, what method was used and what month(s) was it employed.  184 individual respondents (63%) said they monitored all their hives. When those individuals monitoring all their colonies was equated to the loss they reported, the loss rate was 38. Sixty-one (22%) individuals reported no monitoring and when paired with their loss had a loss level of 49% , 9 percentage points higher [43 individuals reported monitoring some of their colonies; they had a 26% loss]. Numbers are shown in Figure below.

Comparing the last 2 survey years there was a lower loss level, better winter survival difference of 8 percentage point lower losses (48.5% no monitoring vs 40.5% loss total monitoring all colonies.) This means a 17% advantage.

In the 2017-18 season, 68% of Oregon individuals used more than one sampling method. 32% of individuals used a single monitoring method (23 individuals used alcohol wash, 19 used sticky board and 18 powder sugar). Of those using multiple sampling methods, 39% used 2 methods, 24% used 3, 5% (10 individuals) used 4 and 1 individual used all 5. Sticky boards were used 108 times, at least once in each month. Alcohol wash was used the fewest times, 51 total and only 11 months with little utilization in an additional 3 months. Most sampling to monitor mites was done in July – September, as might be expected since mite numbers change most quickly during these months and results of sampling can most readily be used for control decisions. See graphic below illustrating monthly use for each of the 5 sampling methods.

Summary: Monitoring colonies for mites helps inform the beekeeper on HOW MANY mites (size of mite population) and with repeated sampling if the number is increasing or remaining level. This can key a decision on risk of doing nothing about mites (i.e. not using a mite control) and also can help confirm that a treatment (when sampled post-treatment) was effective in reducing the mite population. Of the several sampling methods the recommendation is to wash adult bee bodies, collected from the brood area of the colony, with powdered sugar or alcohol/soap. Mite number in sample should be below the 2/3% level to confirm there is a low risk of mite damage to the colony. Consult the Tools for Varroa Guide information on the website www.honeybeehealthcoalition.org for further information on sampling, the meaning of mite sample numbers and for information on mite control.

Download the PDF report here

DWV – deformed wings

I did not have visual confirmation of mites, but did find a few deformed wings on dead bees in October.

RESPONSE: We do not believe seeing mites on bees or deformed wing bees are useful for predicting mite infestation. We know our bees have mites. When we see a mite on an adult bee and see bees with Deformed wings –we are seeing the “tip” of the iceberg – that colony has mites at a high enough level to be at extreme risk of NOT surviving winter.


I’m sure my bee left their hive in the fall because of the Varroa mite, I will be treating this year, I had two hive that made it through winter but died off when we went through three weeks of rain. I should have feed them more. I already had one of my hive come back and I plan that more will follow.

RESPONSE: High varroa mite populations are apparently one of the reasons bees abscond (leave) their hive late in the fall. I say apparent because there is no experimentation to “prove” that assumption. Colonies do have a tough time in the spring and can perish when the weather takes a turn for the worst. Sorry you both of them. Better spring weather helps them recover.




By Meghan Milbrath, Michigan State University Extension, March 8, 2016

Guest Blog

Beekeepers in northern climates have already lost a lot of colonies this winter.  While official counts won’t be recorded for a few months, some trends are starting to emerge.  One of these trends is a specific type of colony death.  In Michigan, I’ve received so many calls describing the scenario below, that I can describe the deadout before opening the hive, or before the beekeeper describes it over the phone.  While I may impress some with these predictive powers, the frequency of these types of losses indicates a real epidemic that is affecting honey bee colonies in northern states.

Characteristics of the common early winter death in northern states:

1. The colony was big and looked healthy in the fall

2. A lot of honey is left in the top supers

3. The cluster is now small, maybe the size of a softball

4. There are hardly any bees on the bottom board

5. Near or just below the cluster is a patch of spotty brood – some fully capped, and some with bees dying on emergence (heads facing out, tongues sticking out).

6. If you look closely in the cells around the brood, you will see white crystals stuck to the cell walls, looking like someone sprinkled coarse salt in the brood nest.


7. You don’t have records showing that varroa was under control.

Sound familiar?

We see this classic set of symptoms over and over in the states with a proper winter.   A big colony that seems to just shrink down and disappear.  Many people want to use the term colony collapse for this type of death, and while collapse is a good descriptor of what happens, this is not true colony collapse disorder.   This is death by varroa associated viruses.

How does it happen?

1. The big colonies –While beekeepers are often surprised that their big colonies are the ones that are gone first, it makes perfect sense in terms of varroa growth.  Since varroa mites reproduce in capped brood, the colonies that made the most brood (i.e. got the biggest) are the ones most at risk of having a high population of varroa.  Colonies that swarmed, or didn’t take off, or even fought a disease like chalk brood are less at risk from high varroa populations, because they didn’t consistently have large amounts    You should have good notes indicating cluster size going into winter, but even if you don’t, you can see the large circle of food eaten by a large cluster.


This colony had a large brood nest (indicated by the dark comb in this frame from the top deep box), and a large cluster going into winter (indicated by the large amount of honey that is eaten away where the winter cluster started).  Varroa were never monitored or managed in this colony, and it was dead by February, if not sooner. (Photo by Meghan Milbrath)

2. Lots of Honey – Lots of honey means that the colony died fairly early. Colonies with high levels of varroa, they tend to die fairly early in the season (before February), leaving lots of honey behind.  Once the bees are stressed and in cluster, the viruses take their toll very quickly.  In some cases the colony will even abscond in fall, or be dead before winter really hits.

The colony shown above had a third deep box that was filled with capped honey, indicating that the bees died early, and starvation was not the culprit.

3. Small cluster – Varroa levels peak right when the winter bees are getting formed. The bees that emerge from varroa infested cells are weakened, and more importantly, are riddled with viruses.  Varroa mites are notorious for carrying deformed wing viruses (DWV), but are known to transmit many more.  When bees are close tight in a winter cluster, the viruses can spread very quickly.

In our colony, the cluster was only the size of our hand – some bees had their heads stuck in the cells, trying to stay warm, others had fallen between the frames.

4. No bees on the bottom board – When a colony starves, the bees just drop to the bottom board, and you end up with a pile of dead bees in the hive. When bees get sick with viruses and other pathogens, however, they often will fly away.   Sick bees by nature leave the colony to die in the field, an act designed to prevent pathogen transmission in the colony.  When most bees are sick, they either fly away, or are too weak to return after cleansing flights.  An early fall illness means that a lot of the bodies probably got removed by workers too.

The colony we examined had only a few bees left on the bottom board (1-2 cups).   We didn’t see a lot of varroa, but there had been some robbing, so wax cappings covered a lot of the board.

5. Patch of spotty brood/ Bees dying on emergence – When a colony succumbs to varroa associated viruses or parasitic mite syndrome (PMS), we see a lot of effects in the brood. Unlike American Foulbrood (AFB), which attacks the larvae at one particular stage, PMS will affect developing bees at many stages of development.  It is one of the only diseases where you see bees dying right as they emerge.


Note the bee in the upper left is fully formed, and died on emergence.  You can often see frozen/melted larvae along with dead pupae.  Many beekeepers instantly suspect AFB, but AFB infected colonies usually will not be large and have produced a lot of honey going into the winter. (Photo by Meghan Milbrath)

6. White crystals in the brood – Around the cells where the brood died (the last place of the brood nest), you will often see white crystals stuck to the walls of the cells. These are dry (not suspended in liquid like crystalized honey), and are the crystalized pee of varroa.   Varroa mites defecate in the cells, and the resulting guanine crystals are left behind, and visible to the naked eye.


Cells on the right hand side of this photo contain small crystals of guanine acid, indicating varroa defecation.   Notice the dry, irregular shape, and that they appear stuck to the walls on the cells.  Some cells on the left hand side of this photo contain crystalized sugar.  Note the wet/liquid appearance, and that it is largely in the bottom of the cell. (Photo by Meghan Milbrath)

7. No records that varroa was under control. Notice that this says ‘varroa was under control’, and not that ‘the colony was treated’.  You may have applied a treatment, but it may have been too little, or (more likely) too late.  This year was a particularly difficult year for this, because in Michigan we had a really late summer – it stayed warm enough for beekeepers to go into their hives well into October.   Many beekeepers took the extra time to put on a varroa treatment, thinking that they were lucky to get one in.  While that treatment could help the bees for next season, it was too late for this winter.  September and October treatments would have been applied after varroa had gotten to their winter bees.  Winter bees are born in the fall, and with their special fat deposits that allow them to live through the winter months, they are the one who carry the colony to the next season.  If the winter bees have already been infected with viruses, the damage is done.  No amount of treatment or varroa drop would bring the colony back.

The only way to know that you have varroa under control is to monitor using a sugar roll or an alcohol wash.  Just looking at the bees does not work; varroa mites are so sneaky, that you rarely ever see them, unless the infestation is out of control, and it is too late.   Many beekeepers say that they never see varroa in their hives, so they don’t think that they have a problem.   In fact, a varroa infested hive can actually look like it is thriving.

Underneath the lovely brood cappings, and away from our view, the mites are reproducing and biting the developing bees.  The colony can look fairly healthy until the mites reach a threshold, and the colony succumbs to disease.  By the time you see parasitic mite syndrome, or see varroa crawling on bees, it is often too late for that colony (especially if winter is just around the corner).   Getting on a schedule of monitoring and managing mites will give you peace of mind that your healthy looking colony is indeed healthy.

The silver lining

If the above scenario is familiar, don’t despair.   First, you are not alone.  Many beekeepers got caught off guard with varroa this year.  They didn’t realize how bad it was, or got thrown off by odd weather patterns.  Second, when the bees die, the varroa mites die too.   We don’t yet have evidence that the viruses would stay in the equipment, so you can reuse your old frames.  The honey that is left can be extracted to enjoy (if you didn’t feed or medicate), and frames of drawn comb can be given to new colonies.   Most importantly, if you recognize the above scenario in your colonies, you now have more knowledge as to what is harming your bees, and you can take positive action.   You have time for this season to develop a strategy. Monitor your varroa mite levels using a sugar roll kit (available at pollinators.msu.edu/mite-check/ or at Mann Lake), read about integrated pest management for varroa, and make a commitment to prevent high mite levels this year before your winter bees are developing.   This is going to be the year!

Meghan Milbrath, Ph.D.

mpi@msu.edu /517-884-9518

Meghan Milbrath is a beekeeper and the coordinator of the Michigan Pollinator Initiative at Michigan State University.   She performs pollinator related research and extension work, and works with beekeepers and stakeholders around the country. She started keeping bees over 20 years ago, and currently owns and manages a The Sand Hill apiaries, where she manages 150-200 colonies for queen rearing and nuc production

2015 Survey Q & A – Mite Problems

Q -In August I discovered severe mite problems in my 9 hives. I used powdered sugar and it seemed to generally improve the hives. I re-queened 5 hives. Those survived. The other 4 had a mite setback and died by late October. One hive had no bees. The others had dead adults. Two hives may have lost queens and I did not know it till it was too late.

A – Sorry to hear of your health issues. Mites can quickly overwhelm a colony and your discovery of a severe problem after not giving them as much attention as normal is not unfortunately, an unusual occurrence. I would have recommended a more ‘aggressive’ mite treatment than powdered sugar – powder sugaring is not very effective under the best of circumstances and seems to offer limited relief to the bees only under very low mite pressure.
Let’s hope this year is a better one for our bees and the mites do not get an upper hand over our management.

2015 Survey Q & A – Powder Sugar Dusting

Q – I dusted powder sugar to try and reduce the mites….the questions asked don’t make it clear what type of method this is

A – You had comment that our question does not make it clear what type of method powder sugar dusting is (nor does it reveal if such a method works or not). There simply is no space to explain some of these things we do to our bees – and frankly there is no ONE METHOD of sugar dusting and no clear evidence that it works better than doing nothing (i.e. not dusting). This is a survey instrument (and like all surveys it has some problems getting at the information we desire. I am after only some expression of how many people do this (no matter how they do it). If on analysis a good number of beekeepers are doing this and they experience lower losses, it will be something to further investigate (and better define) in the near future.

2015 Survey Q & A – Monitor for mites on a regular basis

Q-Some additional information that does not come up in the questionnaire: I lost 2 colonies last summer (around end of July, August). At first I thought it was due to AFB, but I have since come to the conclusion that it was due to mite infestation. An experienced beekeeper examined my hives and said he didn’t believe it was AFB. I didn’t treat for mites last spring, and I never did a mite count. But 2 of my colonies gradually got smaller and weaker and eventually just sort of disappeared. After that, I have since determined that I need to monitor for mites on a regular basis and did my first one a week ago – a powdered sugar roll.

A – You had a comment on your colony losses. It does sound like the situation (syndrome) we call Bee PMS – ugly (termed snot or cruddy) brood that can mimic some of the symptoms of AFB and population going downhill – it is indeed due to mites. You had a good read on the condition it seems. Hope this next season is a better one.

2015 Survey Q & A – Inspections

Q-It’s clear to me that we need to do more inspections and better protect our hives over winter.

A – Thanks for sending a survey. You had commented that you think more inspections are needed and that better protection is needed over winter.I do agree – however we find that the number one issue is varroa mites and those who are more pro-active in mite control, according to our surveys, have fewer losses. Our winters are not the issue as much as is our control of varroa.

2015 Survey Q & A – Supersedure

Q – I know that one of my colonies superceeded in 2014. Is that relevant?

A – We do ask about queens – supersedure means that the colony did replace their queen….the bees did it not you the beekeeper. It does mean a brood break and that does reduce mite numbers in the hive so It could have made the difference.

2015 Survey Q & A – Question for Treatment Results

Q- You may want to design a question that addresses treatments that worked, but worked too late. I treated with OA, after I was down to 1 hive, having watched 4 others die out even before winter. I didn’t recognize how bad the mite problem was until too late, and then finally got a vaporizer to treat, but only had one hive remaining. That one became very strong going into the winter, even though it was late fall.

A-Thank you for completing a survey. You had a comment on our adding a question on treatments that worked but were used too late.  That is a good idea. Unfortunately we have a constraint on the number of questions we feel we can cover and yet get data. We do have the question on what was used and when it was used –that question will get to your comment – partially at least. As we look at our full data base we will certainly take your comment under consideration – for another year. Appreciate your interest and comment.