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The Classroom

The Classroom – February 2024

- February 1, 2024 - Jamie Ellis - (excerpt)

The Classroom - ABJ - Jamie Ellis
Oxalic acid chemistry

 I would like to know if the oxalic acid treatment involves a liquid phase before vaporization or if oxalic acid sublimes, going directly from solid to vapor. I am not a chemist, and there is a lot of confusion on this topic in the available information.

Marie-Hélène Majeau
Québec, November


My team and I get this question a lot. Many folks are interested in using oxalic acid (OA) as a Varroa control. One of the common ways to administer OA to colonies is to heat the crystals, causing the OA to become a gas that permeates the hive. The question, though, is does that OA sublimate or vaporize? What is the difference anyway?

Vaporization is the process of a liquid becoming a gas. Imagine putting an ice cube into a pot that is heated on a stove. If the pan is hot enough, the ice cube (a solid) will melt (turn into a liquid) and then turn into a gas (water vapor that is released). If you do this long enough, the entire cube of ice will turn into water and then into a gas. This process is called vaporization. Sublimation, on the other hand, is when a solid becomes a gas without passing through a liquid phase.

I chatted with my UF colleague (Dr. Cameron Jack) about this as he is our resident expert on OA. In fact, I have heard him answer this question many times. Dr. Jack, also being interested in this question, reached out to a chemist who shared that pure OA sublimates when heated (it goes from a solid directly to the vapor phase without passing through the liquid phase). The catch is that the OA registered for use in honey bee colonies is not pure OA. Instead, it is oxalic acid dihydrate. Heating this product causes it to break down into OA and water. Thus, you can split hairs about what happens next. The OA dihydrate does not sublimate. It goes through a brief liquid phase that includes water before becoming a vapor in our hives. Thus, the OA dihydrate we use vaporizes in our colonies. However, some argue that since it breaks down into OA and water, only the water vaporizes; the OA itself sublimates. So, I guess a little of both is happening, depending on what side of the fence you stand.

Queen mating flights

 When the virgin queen flies off to mate and travels to a drone congregation area that is outside the distance her hive drones normally frequent, does she “turn on” her queen pheromone when she arrives at her destination? After mating with enough drones and she begins her flight back to her hive, does she then turn off the pheromone? Do males from the drone congregation area follow her back to her hive? Does she just outrun her pursuers? Alternatively, is there a “newly mated” scent that in some way alerts the males that she has been fertilized and does not need any further attention? If the queen scent is always on, why do the drones in her hive not all fight to mate with her before she even leaves for the mating flight and why do they not show great interest when she comes back?

Bob Mooney
New York, November


The mating behavior of queens and drones is triggered only when flying and when both are at drone congregation areas (DCAs), or at least when both are flying to DCAs. This is a good thing, because worker bees can detect eggs that are fertilized by the queen’s brothers and will abort those eggs. Furthermore, ample research has shown that queens that mate with multiple, genetically diverse males will produce healthier colonies than those that mate with only a few males. Thus, honey bees are hardwired to mate while flying to avoid inbreeding and this leads to healthier/more productive colonies.

That said, there is currently no indication that queens can turn on/off their pheromone production as they leave/return to the nest. In theory, drones from other hives could wait outside a single nest and try to catch virgin queens leaving/returning to the nest. However, this would pay small dividends. It benefits queens and drones to go to DCAs rather than just mate right outside the hive. From the drone’s perspective, he is more likely to encounter a virgin queen at a location to which queens travel purposefully. Colonies do not produce virgin queens consistently throughout the year. Most drones waiting outside a hive to mate would never live long enough to encounter a virgin queen. On the other hand, if nature found a way to get all virgin queens and drones from an area to go to the same place, both sexes would have greater likelihood of encountering the other sex.

Queens are cranking out the pheromones as they leave the nest, during their time at a DCA, and as they return to the nest. Thus, they do pick up errant drones on the way to the DCA and on the way home from the DCA. Little is known about queen sexual encounters going to/from the DCA. It is certainly conceivable that breeding can occur when flying to/from a DCA. Most research of which I am aware about queen mating behavior is conducted at a DCA. We know that is where most of the mating events occur. Even still, I admit it is within the realm of possibility that queens will occasionally mate before they get to/after they leave a DCA.

A last point about queen pheromones: The pheromonal bouquet of queens changes considerably in her early life. For example, it will be different upon her emergence from when she takes her mating flight. It takes ~1-2 weeks for the newly emerged queen to take her mating flight. She undergoes further sexual maturation during this time. Correspondingly, her pheromone output and composition also change during this time. Upon her sexual maturation, she becomes attractive to drones, but only when flying. This is what keeps drones in the hive from being attracted to her. For some reason, they also need to be flying to want to mate with a queen. This is a biological issue that is poorly understood.

A last point about DCAs: They are not static locations where groups of drones are always flying looking for queens. Instead, they are quite loose in structure until virgin queens visit the area. Thus, queens can pick up a few drone suitors while flying to the DCAs and returning home from them. However, the bulk of the drones are flying loosely in a DCA and coalesce in purpose when a queen moves through the area. This triggers mating behavior in the queen. The queen determines when she no longer wants to mate, and she leaves the DCA. I am sure a few straggler drones follow her out of the DCA, but she shows no sexual interest and loses them en route back to the hive. Of course, she may mate with drones before formally arriving at a DCA and after leaving one, but most mating events occur at the DCA.

Do worker honey bees move eggs?

 Do worker bees move eggs? For example, will bees move eggs from worker cells to queen cells, or from damaged cells to adjacent ones? It seems like most people pooh-pooh the idea, but I have heard the occasional “expert” state very authoritatively that they can and do so. Has anyone ever recorded evidence of bees moving eggs?

Eugene Makovec (Editor, ABJ)
Missouri, July


These are great questions that I, too, have heard people ask many times. I once heard Dr. Wyatt Mangum (columnist in this journal) discuss this during a lecture. Correspondingly, I thought I would reach out to him and ask him to answer this question, given his thorough insight on the topic. I include his response below, thanking and acknowledging him for his contribution:

Beekeepers have long debated whether or not bees can move eggs into queen cups. Imagining a worker bee removing an egg or young larva from its cell floor, carrying it through a crowded brood nest, somehow locating an empty queen cup and placing the tiny bit of bee life there, all without injuring it, seems quite a remarkable behavioral feat. In contrast, we have seen repeatedly the fundamental observation in the study of social insects: A big brain is not always required to accomplish a seemingly complicated task.

One way to decide the contention concerning moving brood is to look at the evidence from carefully conducted observational studies. Although their main goals were not to document bees moving eggs or young larvae, their designs (the setups) provided situations to make those determinations possible.

The first two studies involve Mark Winston, well known for his many contributions to apiculture. In the 1970s, he was in South America studying the events following queen removal from Africanized Honey Bee (AHB) colonies. He made daily detailed observations on nine colonies as they reared emergency queen cells. (An emergency queen cell begins as an enlarged worker cell with the cell walls elongated downward as the larva grows.) Of the viable queen cells, Winston explained, “48% (29 of 62 [actually 47%]) were empty when first elongated (one or two day after dequeening) but contained eggs or young larvae on subsequent days which developed into viable queens.” 

The brood appearing in the previously empty, newly built, emergency queen cells had to have been moved there by the bees. The documented queen production from the 29 queen cells eliminated the eggs originating from workers (Winston, 1979). The high frequency of occurrence, estimated as 47%, is startling, but this colony was an AHB population. What about bees with a more European origin in North America (EHB)?

In British Columbia, Canada, Punnett and Winston (1983) conducted a similar study to the one with AHBs, detailing the events after dequeening eight colonies. By daily direct observation they determined 4.2% (5 of 118) of viable queen cells were empty when first elongated. Moreover, the bees had moved eggs or young larvae into these newly constructed queen cups. After eliminating the worker bees as an origin of young brood, the frequency of brood moving in EHBs dropped compared to AHBs (4.2% vs. 47%), roughly a factor of ten.

Turning back to the 1950s, we find another scientist encountering situations where bees moved young brood to queen cups (which in the older literature were sometimes called queen cell cups). From the United Kingdom, Colin G. Butler wrote his famous “The World of the Honey Bee,” the first edition published in 1954. In the visually starved bee biology books of the 1950s, his book proudly proclaimed two-color photographs and 87 black-and white photographs –– taken by the author.

In his classic queen supersedure studies, Butler (1957) investigated the inability of a queen to spread her queen substance, as the pheromone was then called, throughout the brood nest, thereby allowing some colonies to rear replacement queens. The movement of numerous queens in large colonies (two brood chambers) was restricted while allowing the bees full access to them (no partial contact with queen cages). A three-inch long fine wire restricted the queen’s movement. The wire looped around the queen’s “waist” (between the thorax and abdomen) and kept her away from edges of the comb, the typical locations of queen cups. 

Tethered queens could not lay eggs, although they did drop a few eggs during the study. With the queens restricted, out of their reach of the queen cups, they could not oviposit in them. Over the duration of tethering (6-33 days), Butler gave the colonies combs of young larvae, the source of emergency queen cells should the bees try to supersede the restricted queens. From 42 colonies with tethered queens, 17 reared replacement queens. Of these 17 colonies, five colonies had from 1-3 queen cups with eggs appearing in them. Since the queens could not oviposit in the cups, the bees would have had to pick up dropped eggs and place them in the queen cups (as Butler acknowledged).

Whenever I see a bee encountering eggs out-of-place (on the cell rims or on the glass of my observation hives), she eats them in a second or two. Seeing a bee carrying away an egg would be a stunning sight (and difficult to photograph). Butler also did not think the eggs were laid by workers because “none of the signs typical of the presence of ‘laying workers’ were found.” (Multiple eggs in brood cells are the well-known symptom. Other behaviors (beyond the scope of this brief response) occur among laying workers and nonlaying workers who may have active ovaries. I am still trying to understand this behavior because it might occur before multiple eggs accumulate in the brood cells.)

Assuming a queen would mark her dropped eggs with her queen-laid egg pheromone, they would be protected against worker-bee removal while in the queen cups. Worker bees remove each other’s eggs because they lack this queen pheromone. In general, this behavior is called worker policing. Therefore, the persisting and surviving eggs at hatching would have likely originated from a queen. Given the demands of the study, Butler did not follow the developmental fates of these eggs. Nevertheless, a worker-policing perspective, a behavior unknown to Butler in 1957, provides additional support for the eggs being queen-laid. Interestingly, Butler had observed queens laying eggs in queen cups in an observation hive (page 55 of the 1954 edition of his book).

Overall, these observational studies indicate that our EHBs have a set of behaviors conferring the ability to move eggs or young larvae to at least queen cups, although at low frequencies. If a similar study were conducted today on local bee populations of the mid-Atlantic (my location) and the frequency of this brood-moving behavior was somewhat higher, I would not be surprised, given the recently documented summer swarming behavior and colony usurpations (that indicate genetic changes in these local bees).

Butler, C. G. (1954). The world of the honeybee. Collins, St. James Place, London.

Butler, C. G. (1957). The process of queen supersedure in colonies of honeybees (Apis mellifera Linn.). Insectes Sociaux, 4(3), 211-223.

Punnett, E. N., & Winston, M. L. (1983). Events following queen removal in colonies of European-derived honey bee races (Apis mellifera). Insectes Sociaux, 30(4), 376-383.

Winston, M. L. (1979). Events following queen removal in colonies of Africanized honeybees in South America. Insectes Sociaux, 26(4), 373-381.

Small hive beetle problem

 I had a hive that had a lot of hive beetles when checking in late October. The number of bees was not enough to take care of ten frames in a deep. I dumped and shook out the bees and beetles on the ground after locating the queen and putting her in a 5-frame nucleus. I know, the beetles would probably climb back into the hive but hey, what the heck. I put Swiffer sheets and some Beetle Blasters with oil into the nuc. I checked the hive this week on a 60°F (15.5°C) day. The queen and bees were doing okay and had adequate stores of honey and still a lot of hive beetles.

Do you have any ideas on how to get beetles out and save the nuc? I noticed Rusty Burlew’s article in the December issue on controlling hive beetles. I am thinking of putting either a piece of red Plexiglas or make the entire top of red Plexiglas and seeing if that will run them out. I may shake bees and beetles out of the nuc onto one-eighth inch (3.2 mm) screen wire and reload the bees and move to another location and see if that works. What do you think?

Bob Limbird
Arkansas, December


Small hive beetles are annoying critters! I am glad you are taking action to control them. The good news is that