Week Three is not just what I consider to be the most pertinent in terms of information necessary to keep your bees alive, it is the longest of our course sections as well. So pour a cup of tea, get comfortable and read on!
and when you are ready, on to
Although we are only just at the end of July, it has been an unusually warm and dry since March here on the coast. The reproduction hives have been “nuc’d out” repeatedly, for spring nuc sales, raising two or three queens in succession. This of course means there were few baby bees being made, so by July the hives were healthy but small, with brand new queens, the last we will make them raise this season. The honey hives have been harvested, and to stock the student apiary, two were split and requeened with bought queens. As David Eyre of Orillia, Ontario’s The Bee Works advises, we are taking all young (2015) queens into the winter. Young queens have a higher rate of lay, and lay longer into the fall. This means you have a good, healthy, large cluster of the all-important long lived winter bees to take the colony through the winter:
“A “winter” bee is produced at the end of the summer. It is physiologically different than the summer bee, with a different hemolymph (blood) protein profile than the summer bee. Winter bees also have fatter bodies which they rely for nourishment during the non-foraging months. A winter bee will live much longer (4 to 6 months) than a summer bee (45 days). The sole purpose of the winter bee is to get the colony through ’til spring. In the fall as the hive prepares for the long winter months ahead, the bee population drops as the summer bees die off, replaced by the smaller winter cluster. Brood production stops. When the outside temperature is above 50°, bees take cleansing flights as they do not defecate inside the hive ” (West Mountain Apiaries)
This makes fall management, particularly of small hives like those in our student apiary, very similar to spring management. We want to support the colony in ways that make it grow at a time of year when there is little to grow on: so we will feed both white sugar syrup to replace nectars and protein to replace pollens. Note that brown sugar and some other sweeteners are toxic to bees. Use only white cane or beet sugar.
Bee quality is very much a function of nutrition. The more food the colony brings in, the more likely the bees are to get an optimal start in life. Bees take marvelous care of one another, given the essentials. The more they have, the more they give…well fed baby bees are swimming in royal jelly, are fat, round, white, and look juicy and wet:
If the larvae look discoloured, dead, rotted, dry, or are not curled up in a nice C shape, pull the alarm bell! (we will have a closer look to see what is causing the problem, hoping to catch any serious disease state in the earliest stages)
We will be feeding the colonies, and attempting to monitor just how much nectar and pollen they are bringing in themselves. We want to give the bees a bit of a boost, but not too much. It is expected that the queen will, given the approach of winter and the local nectar dearth, reduce her rate of lay, reducing the expansion of the hive. But by the end of September we want to see a good colony in need of two deep boxes, with good amounts of honey at the corners of most of the inner frames (the outer frames will be entirely capped honey), a good store of pollen on many frames, and brood still coming along. So we will give them feed: of syrup and a protein pattie or syrup plus protein powder (affectionately known as “Almond Juice” as this is how very early spring colonies are fed to get them to size to meet the California almond bloom…it saves work to be able to just top up the feeder, not open the hive, put on the protein patty, close it and then also top up the feeder).
Colonies can be started in a variety of ways:
1. a swarm might choose your empty equipment as their new home. Yay! Free bees!
2. you might be lucky and get a call to catch a swarm. Yay! Free bees!
3. you may buy a package of bees (a few pounds of bees, usually 2 or 3 lb. plus an unrelated, caged fertile queen). In 2015 these cost about CAN$220.
In Canada this is how we import bees in the spring, and we can only import them from New Zealand. Unfortunately, 60% of the NZ package queens are superceded in their first season, we think thanks to the disruption of the bee clock that occurs in being transferred from the season calendar in one hemisphere to another: the queens arrive at the end of the NZ summer, finding themselves now at the beginning of our spring, and facing another approaching summer.
Supercedure, if it happens, can be smooth, with the colony keeping the old queen laying until they are sure the new one has returned, mated, and is laying reliably. But if the old girl is disposed of and the new queen is lost on her mating flight, you have an emergency on your hands! There will be no eggs and no way to make a new queen, and beekeeper intervention will be required ASAP.
But even in the normal run of things, as the package gets established, builds comb, and the queen begins to lay, there will be no new baby bees for at least 3 weeks. During this time the package population drops as bees age out and die. It is critical that you check early to make sure new brood is coming along, and support the package by giving them clean drawn comb if you have any, and feeding lavishly, particularly if they arrive in early spring (March) when there is little feed out there for them and lots of cold and rain.
4. you may buy a nuc (short for nucleus hive) of bees. These will be bought from a Canadian supplier as we cannot ship live bees or used bee equipment across the USA/Canada border. A nuc of bees is a mini colony in every detail. It contains, sometimes with minor variations: a laying queen, brood in all stages, worker bees of all ages, and frames of nectar/honey and pollen stores. Nucs can be different sizes, from two frames to five, and their price varies accordingly. But in general it is best to buy a five frame nuc: two frames of stores, three frames of brood in all stages, lots of bees (a good nuc seller will add an extra shake of bees, attempting to shake in a good number of nurse bees), and a laying queen no more than a year old. The equipment should be in good condition, the frames should have the date of their first year of use on them, and the bees and brood should be healthy and well fed. The population of a nuc will expand rapidly and should not experience the sag in numbers that a package will. Here is a great link to first year care for your nuc. Nucs usually cost what packages do, although they offer more than packages as they expand steadily from day one of acquistion.
5. you may buy an entire, established colony. Instant apiary! Expect to pay the price of a nuc plus about CAN$200 for the extra equipment and bees. You will need to have all the necessaries for transport yourself.
In general, you want:
~a site that is accessible by vehicle (beekeeping involves moving in and out of the apiary a lot of heavy things like honey boxes and water, and lots of equipment and tools…it gets very old, trudging down a path into a beeyard, carrying all that stuff)
~has good exposure to sunlight, especially early in the morning,
~is sheltered from prevailing winds (especially in winter)
~is in a rich forage area (bees preferentially forage from between 200′ and 1/2 mile from the hive, but will fly up to two miles for a meal…note that the further they have to fly for nectars, the less they can store when they return..ie the less honey they will put up….it is like driving to fill your gas tank, makes sense if it is nearby, not so much if you have to drive three hundred miles for a fill)
~has access to nearby, clean water, and is not exposed to pesticides and other bee-harmful chemicals
~is close to the beekeeper…if it is nearby you can visit more easily and more often and respond more quickly to emergencies, like swarming or robbing
~and has few other honey bee colonies nearby, as they will compete for resources, trade disease and pests, and will rob one another, given a chance (common in late summer nectar dearths).
Locally we get New World Carniolans from New Zealand, but you can with some effort source other sorts of bees. The NZ bees are great bees, but in addition to the new packages’ propensity to supercede in their first season, they have also demonstrated very poor resistance to Varroa destructor mites. Most beekeepers try to bring in varied genetics when they source new queens or colonies, to deepen their local gene pool. We are very fortunate that in being right on the USA/Canada border, our queens mate with the more diverse (generally Italian honey bees) populations of nearby USA colonies. This gives us a nice, diverse roll of the genetic dice. Locally, breeders are attempting to incorporate good overwintering ability, gentleness (because so many of our bees are now in urban locations), good honey production, and hygienic behaviour/ability to weather Varroa infestation.
In a nutshell, bee health is affected by pests, diseases, nutritional status, and the ability to find or be provided with good housing. Bees are struggling, thanks to the Three Horsemen of the Bee Apocalypse: the Varroa destructor mite, degraded forage habitat, and pesticides.
(second, excellent resource, here)
Parasites of note are: Varroa destructor mites, tracheal mites (Acarine mites), small hive beetles (recently found in Abbotsford BC, so bag all beetles found in hives for identification by the Provincial Apiculture department!! Note that these SHB have reproduced in Abbotsford, with larvae reported in at least one colony…let us hope the winter does them in), Braula. The pest we must know all about is Varroa destructor, which has spread worldwide after an accidental exposure of its native bee host to the European honey bee population. Although we have adopted anti-varroa strategies and treatments, the continued infestation of honey bee populations with Varroa may well spell the end of the honey bee…which at this point is barely surviving. A measure of the desperation of the situation is that, in Canada, there are no more feral colonies of honey bees. Mites make overwintering in the wild impossible. Swarms continue to fly out every spring to seek new, feral homes. But these colonies will not survive the winter, as they will be too weakened by unchecked mite growth.
This means, at least in Canada, the survival of the honey bee is now a function of having an attentive and skilled beekeeper.
Varroa not only reproduce in honey bee brood, they vector (carry to the bees) a host of diseases, including Deformed Wing Virus and EFB and AFB. For the honey enthusiast they are a disaster: even small mite loads cause hives to put up less honey. Varroa present a significant challenge to colony survival. The fertile female mite, after sucking some bee blood, hops into a brood cell onto a bee larva just as it reaches the stage where it is capped and spins its cocoon. She hides in the food jelly at the bottom of the cell, and feeds off the pupating larva. She prefers to infest drone brood, as drones are in the cell for 3 more days than workers are…giving the female Varroa critical extra time to produce fertile daughters. Her first task is to lay a male Varroa, who then inseminates the females she lays after. They emerge, mated and fertile, when the bee hatches. Note that this reproductive style causes exponential mite population growth.
Read all you can on the Varroa mite!
Varroa Control (note this link is probably the single best resource on Varroa I have seen!!)
There are a lot of things we can do to keep bees alive and healthy in the face of Varroa infestation. But ultimately, we must eradicate the Varroa mite, world-wide. The simple fact is, the Varroa can evolve much more quickly than the honey bee can. In any race to evolve compensatory survival mechanisms, the Varroa will win every time. Please read up on anti Varroa strategies; they range from wishful thinking to “soft” treatments to “hard” treatments. I have seen bee club meetings devolve into shouting matches, thanks to disagreements on Varroa management.
We all hoped, after realizing the blinding speed with which Varroa evolve resistance to mite treatments and mite treatment strategies, that breeding survivor bees would be the answer. But recent research indicates that even survivor stock, when moved to new biozones, loses its ability to survive Varroa. Research to figure out why is under way now. This is an example of how quickly Varroa can evolve as compared to the honey bee: left isolated and to themselves, the colonies that survive with Varroa are not those whose bees have evolved, but whose Varroa have evolved…such that they do not kill their host colony. Returned to the world, the colonies are then invaded via drift with the more virulent Varroa that we have inadvertently produced in our efforts to keep bees Varroa free. And in some cases, bees in isolated zones, who are not reinfested by nearby colonies, may manage (with or without the assistance of their beekeeper and treatments) to weed the Varroa out of their population…and flight range. They can then remain Varroa free.
Alas most of us live in bee dense areas through which flows a river of heavily treated pollination bees, always on the move. We are constantly facing reinfestation by drift bees carrying Varroa hitchikers. And the local drone congregation areas are filled with non-Varroa-resistant drones.
One area in Canada, Thunder Bay, Ontario, recognized early that they could, in their remote location, keep Varroa out of their honey bee colonies. They worked with extraordinary cooperation to keep infested stock out of Thunder Bay, and aided beekeepers who found Varroa in their hives by destroying the colony infested but providing each other free replacement bees. They have struggled, but remain Varroa free. I think this cooperative model of Varroa defense could be adopted by many North American communities, in particular those not hosting migratory pollination operations and bees.
You will rarely, if ever, actually see a Varroa mite on your bees. DO NOT be lulled into a false sense of security by telling yourself “well, I haven’t seen any mites in MY hive!”
Not only are they tiny, and not on every bee, but they slide between the segments in the bees’ abdomens, and only a sliver of the mite may show. And that sliver is a similar colour to the bee itself, so is very hard to spot. By the time you do see them, or catch one crawling across a frame, you usually have catastrophic infestation levels.
***Let me pull you up short here and examine just what “catastrophic infestation levels” means in real life. If you see Varroa mites on your bees or your frames, that means one thing: mite levels are so high, so catastrophically high, that your hive is in imminent danger of complete collapse and death.***
I kid you not. These are very small parasites, smaller than the dot left by the head of a pin. By the time there are so many they are visible, crawling around, that means they have filled every available larval bee cell, and are wedged between every available bee segment, and are now running around trying to find somewhere to latch on and suck bee blood.
The next step is complete colony collapse.
To check your mite load quickly, you can spray a mite counting sheet with oil (ie. Pam) and slide it into the bottom of the hive. It will catch and trap all Varroa that fall off the bees in the normal course of things. Check it after 24 hours. If you have 10 or more mites, you should treat for Varroa in the near future. Do this regularly between treatments, particularly when you are new to the beekeeping craft!!!
Comparing the mite fall between hives can be interesting, and reveal which hives have larger Varroa populations. Then you can speculate as to why!
You can adopt a flexible array of anti Varroa tactics. You can use screened bottom boards, which are thought to reduce mite loads by up to an optimistic and perhaps unrealistic 30% (when mites lose their grip on adult bees and fall all the way to the bottom of the hive, and out of the hive). The price for screened bottom boards may be reduced brood amounts and honey stores, perhaps due to the reduction in brood nest size due to the cooler areas above the draughty screen.
This spring I used Apivar (Amitraz) strips, a harder treatment than is my preference, but I wanted to be sure that my nuc sales went out as mite free as possible. Apivar can be used in the cold early spring, unlike the more organic and (we think) less toxic formic acid pads (Mite Away Quick Strips), which are used when temperatures rise above 15C/59F, and are typically applied in late summer after the honey is off the hives. Formic acid is reasonably effective at reaching the mites hidden away with larvae, under cappings (the cappings are porous and breathe), and has the added benefit of killing tracheal mites.
In midwinter, when there is little or no brood and the mites are all exposed, you can use oxalic acid vapour to kill Varroa. This can replace the early spring Apivar treatment if your mite levels are low after the late summer treatment.
You can also lay a narrow bead of food grade mineral oil along the tops of the top frames in the hive on a regular basis. This oil gets onto the bees, who groom themselves more as a consequence, and groom the mites off, one hopes to then fall through the screened bottom board! And the oil makes it hard for mites to hang on to the bees. It also smothers tracheal mites and makes it difficult for them to reach bee spiracles and then migrate into their tracheae. It is an easy, cheap and harm-free method of stacking the deck in favour of your bees…BUT it is only weakly effective. It helps, but not enough on its own. Use other treatment methods as well, and don’t get it on your queen as it will suppress her exudation of queen mandibular pheromone and perhaps trigger supercedure.
Some beekeepers opt to use grease patties, composed of icing sugar and vegetable shortening. The bees, in carrying this foreign substance out of the hive (bees have little tolerance for anything in the hive beside wax, nectar, pollen and themselves), get the shortening on themselves. They groom it off, knocking off Varroa in the process, and the shortening oils, like the mineral oil, make it very hard for the mites to cling onto bees. Heh heh.
Dusting icing sugar also causes increased grooming and mite fall, but needs to be done frequently (weekly) and causes problems with attracting ants and robbers.
Many beekeepers use drone trapping to reduce Varroa numbers in their hives. Varroa preferentially infest drone brood as it allows them to reproduce over a longer timeline, which equals more hatched fertile females. You allow drone brood to be laid in a larger than usual amount, by using drone brood frames, below, or by inserting shallow frames into deep supers (the bees draw free drone comb on the bottom of the frame, and the beekeeper cuts out the drone brood and with it any mites in with the drone brood once the brood is capped). Either way, you remove the drone brood and destroy it once capped, and the Varroa along with them. This is non-toxic and effective, but is a huge drain on colony resources. You must balance the result against the effects. Over time, drone trapping will also drive the Varroa population toward infesting worker brood.
Integrated Pest Management and “treatment free beekeeping” (it is anything but!) have received a lot of attention in recent years, as we all desperately search for non-toxic ways to protect honey bees from mites. I wonder what value many of the methods have given that we must treat for Varroa on a regular basis anyway, at least in our bee-dense area.
One bright light on the horizon is the work of Seattle based Eltopia. They are, in concert with the research laboratory of Dr. Marla Spivak, developing a drone brood frame that senses when the brood is capped over, then heats just enough to either sterilize or kill the mites in with the brood. No more reproduction = no more mites in your hive!
I have tried the all-natural, non chemical, soft approach and have concluded that I do not possess the requisite time, expertise, equipment or location for that to be successful. I wish it was, I truly do. But a hive crashing from Varroasis is not a pretty sight. In addition, a hive crashing from Varroa releases waves of bees who drift to any nearby hives, searching for a home.
I suspect we will find that Varroa infestation triggers behavioural changes in honey bees, pushing them to drift far, far more than is normal. This parasite triggered host effect is common in other parasites, and bestows on the parasite increased reproduction benefits. Alas.
I have taken the position that bees are no different than other domesticated animals, and that it is immoral to withhold treatment for them when they are sick or infested with parasites. So I treat as outlined above, and medicate if I find Nosema or the foulbroods, as per Ministry guidelines. But I long for the day we have either completely Varroa resistant bees, a non-toxic and completely effective remedy, or when we have extinguished the Varroa mite.
Personally, I think genetically modified Varroa or a Varroa-specific GMO virus are going to be our answer. One reason beekeepers are resistant to this sort of research is that Monsanto, the evil and much loathed multinational, is leading the way with research and research dollars…no doubt looking for a drench formula that they can then sell to beekeepers all over the world. I too have reservations about Monsanto, Bayer and their ways. But governments, particularly in Canada, are shutting down the flow of research dollars, particularly free, open ended, non-commercially targeted research. I deplore this. And am aware that the bee research being done now is funded by the Monsantos of the world.
So I will hold my nose and hope Monsanto, or someone like them, funds the research that leads to a Varroa solution. Because we owe this to the bees.
The most alarm-worthy situations you will find in your colonies are the foulbroods, two catastrophic bacterial diseases that infest honey bees: European Foulbrood (EFB) and the more serious American Foulbrood (AFB) They are transmitted by bee drift of infected bees into new hives, and by the transfer of infected comb, wax and honey between hives. EFB and AFB are also thought to be vectored by Varroa mites.
This transfer is the reason we have a hive tool for each hive, and why beekeepers do not bring their tools into an apiary they are visiting and helping in. And why we have a wash station set up in the beeyard: if inspecting multiple hives, wash your hands between each hive inspection. It is cheap insurance against accidental disease transfer.
Both the foulbroods can be treated by administering an antibiotic treatment regime, and by putting the medicated bees onto clean, bare equipment. But AFB generates highly resistant spores, which can survive on old equipment or in old honey for literally decades. Typically, AFB infected bees are destroyed along with their equipment, usually via fire.
In the Lower Mainland of BC, we now have access to the Iotron facility, where using high energy electron beams, materials can be irradiated and thereby sterilized. You can bag and process your infected bee equipment at this facility, and that will eradicate the AFB spores. So our AFB protocol, followed precisely, now allows for the saving of infected bees and equipment.
In jurisdictions lacking access to a sterilization facility, burning bees and equipment is still the only way to stop the spread of AFB.
Note that AFB and EFB are rife in British Columbia, possibly due to the fact that many bee operations continue to medicate prophylactically to suppress the symptoms of AFB and EFB in their colonies. Essentially, they treat with antibiotics all the time. Once those colonies are sold or placed in fields and orchards for pollination, the antibiotic levels drop in the colony, and the disease re-blooms. Infected bees drift readily and infect other colonies. First signs of infection are dead, off-coloured, or odd looking larvae, and/or ragged holes in the capped brood, under which you find dead and/or rotted larvae. As the infection spreads, there can be a characteristic sour, rotted meat smell.
Detecting “off” brood is a key skill in beekeeping. If you have the least suspicion of foulbrood in the hive, call for help IMMEDIATELY. There are many colonies nearby, not just our own, who could both infect our apiary, or be infected by ours. There is no shame in finding foulbrood in your colony: the best beekeepers find it in their bees. But there is complete shame and blame for not recognizing and/or dealing with your foulbrood. A hive battling foulbrood may suddenly look uncharacteristically quiet, and will be dwindling for no good reason (ie nectar dearth).
If a hive seems too quiet, investigate! There is usually an reason and you need to find it! And a hive that fails to take a syrup feed is also usually in trouble.
You can send off-looking/rotted brood into the Ministry lab for diagnosis, a process that can take weeks (meanwhile, treat your bees….the longer the colony remains untreated, the more the disease will spread…to your hives and the hives of others). The affected larvae (or what remains of them) is scraped out of the cell with a toothpick, and the material collected is placed in a labelled ziplok bag.
Note that while you extract the larval goo, if it strings out in a long, elastic rope (this is called, inventively, “roping”), you likely have AFB on your hands. EFB rotted larvae do not generally rope out like this.
You can also keep a couple of $15 AFB test kits in your kit bag. These monoclonal antibody tests work instantly to diagnose AFB in your colony. They are used very much like a simple pregnancy test kit, and are available from beekeeping supply houses, including online sources. EFB test kits are also available…alas the test strip does not test for both, you must buy separate kits. However, if larvae are dying pre-capping, you can suspect EFB. If they are dying post-capping, you can suspect AFB. That is not a hard and fast rule, but applies in most cases.
Finally, you can do the inexpensive Holst Milk Test, in which larval contents are mixed with powdered milk and water. AFB affected larval matter will cause the resulting suspension to clear within 20 minutes. Non-AFB contents cause the suspension to stay cloudy.
Note that in British Columbia, you can choose to simply treat either foulbrood with Oxytet, turn the colony onto clean, bare equipment, feed medicated syrup while they build new comb, and bag all the old, contaminated equipment and send to Iotron for sterilization. No real need to verify if you have AFB or EFB as that protocol will deal with them both.
Unchecked, the foulbroods will kill a hive quickly.
Foulbroods are generally only a problem in a hive under stress. It may be that it was cold, wet, hungry, or mite-ridden, or headed by a poor queen. This makes nutritional support and site location priorities as you treat an infected hive.
In addition, I put a question mark against any queen whose hive falls ill with a foulbrood. It may be that her genetic heritage produces workers susceptible to the foulbroods. I generally replace the queen of a foulbrood-stricken hive once the hive is treated, cured, and back on its feet. Just in case.
There are other common conditions that affect honey bees, principally sacbrood and chalkbrood. Both are stress related, so the best treatment and prevention is good husbandry technique…providing good nutritional support and clean, warm, dry housing.
Note that there is anecdotal evidence that formic acid treatment is associated with low chalkbrood levels. It is always good to do a formic acid treatment at least once a year, not just for Varroa but for tracheal mites…and if it drops your chalkbrood spore count, all the better!
The Nosema Twins
There is a final class of serious diseases of honey bees: the microsporidian caused Nosema apis and Nosema ceranae. Both the Nosemas affect and damage gut function in bees, and bees are little more than flying digestive systems, a digestive system that functions as flight engine, baby bottle, and storehouse.
There is a great page on Nosema, here.
The microsporidians are fungi-related unicellular organisms. Unfortunately for the bees, who exchange gut contents frequently in the course of normal hive activities (feeding larvae, feeding drones and queen, transferring nectars, making bee bread, moving stores), the Nosemas freely transfer in the act of exchanging gut contents. Infection with Nosema apis produces the classic tell tale hive-wide diarrhea. Bee feces, waxy smears of ochre to burnt umber colour, are evacuated in the hive, and ofen are sprayed across the face of the hive exterior as bees exit the hive, desperate to defecate. Normally bees never, ever defecate in the hive, or even close to it.
In contrast, Nosema ceranae may never leave tell tale streaks in or out of the hive.
What they both will cause is an inexplicable dwindling of the hive population.
A colony that should be expanding simply seems to stall out or even dwindle. Dwindling can be fiendishly difficult to see. The give-away in a multi-hive apiary is the hive that seems quieter than the others, particularly when the expectation is that it will be equal to its peers.
Observe the hives midafternoon on a warm, sunny day. Which look quiet? Do an inspection on them ASAP and see what is up in there!
For our purposes, Nosema is expected when you see fecal streaking as in the photos above (note that dysentery can be transitory and nothing to worry about…maybe the bees got into old syrup…but when dwindling or inactivity at at the hive entrance seem apparent, ring that inner beekeeping alarm bell!), or when you feel the hive is not expanding as it should.
In the early spring, through the nectar flow, hives should almost physically boom. There will be lots and lots of bees coming and going, pollen coming in, and big orientation flights on warm, sunny late afternoons. You can be in the apiary in the early afternoon, puttering about at various chores, all the hives looking routine, when suddenly one hive wakes up and a big cloud of bees hovers in a constant figure 8 flight pattern at the entrance of the hive, with bees crawling on the face above the entrance. It looks a lot like a swarm event, or robbing, but the orderly, circling flights with bees all looking at the hive, and an sense of intensity but calm are hallmark signs of orientation flights.
In a hive dwindling from Nosema…no orientation flights, no clouds of bees. Alas, Nosema can permanently impair the laying rate of the hive queen.
Any hive that has weathered a Nosema episode needs to have their queen monitored for fertility and lay rate. It is probably best to replace her, once the hive is in recovery. As with the foulbroods, replace the queen, just in case…
Nosema is treated by the application of Fumadil-B, fumagillin. I only treat when I am reasonably certain Nosema is in play. Many beekeepers dose bees in spring and fall, or during/after transport, as Nosema can also be a stress based disease (cold, wet weather, malnutrition, transport stress).
Dosing in the absence of symptoms drives resistance, so I prefer to wait until I see symptoms, which admittedly can be subtle. As a general rule of thumb, I would dose any hive that is dwindling, if I cannot find a good reason for that dwindling (symptoms of foulbrood, which also causes dwindling, are usually quite obvious).
A course of Fumadil-B may help unexplained dwindling and, IMHO, won’t hurt.
Whenever you are puzzled by a hive that fails to thrive, or has worrying symptoms, reach out for help. Experienced beekeepers are happy to come have a peek and see what’s up, you can find them at your local bee club or through an online beekeeping forum. Taking photos of your problem when you post to a forum can be a big help in diagnosing your problems.
List of useful links for learning about bees and beekeeping, from Serenata Flowers.