Unfortunately, the Varroa Treatment was *Too* Effective
A strong hive was scheduled for Varroa management and we applied formic acid treatment. A rainy week followed and the bees could not leave the hive, and four days after that, the colony collapsed. A postmortem revealed many small hive beetle larvae, and the distinct smell of a slimeout. It was later determined that the colony absconded due to the excessive formic acid fumes. How did I know it was absconding? Because there were a relatively small number of dead bees on the bottom board. What happened to the hive materials? The frames were frozen and later reused (formic acid does not remain in the wax), whereas the hive bodies were rinsed and stowed away.
This post is mainly about the small hive beetles, their eggs and larvae. It covers how Varroa could (but according to The Science™, does not) interact with beetles to worsen bee morbidity, and ends with some not-so-relevant reports on African honey bees and their superior resistance to these two pests.
Precursor: How Slimeouts Occur
In a temperate climate (e.g. my part of Australia) there are small numbers of Aethina tumida (small hive beetle or “beetle” from hereon) either hidden in the hive or on their way to a beehive. The beetles are attracted to the odours produced by brood, pollen, ripening honey and the wax that holds them1. Beetles, like the wax moths, are more active at night, which is when they prefer to enter hives. Unlike moths, some beetles will enter hives during the day when the guard bees are distracted (as in during inspections). How the beetle times this, I do not know.
Once inside, the beetle moves into a dark recess: under frames, in the box corners, beneath the lid or even inside a faraway cell, all for the purpose of avoiding eviction from the bees. At this point, AI will have you think that bees will corral intruding beetles into “propolis prisons”, and that the beetles will “deceive workers into feeding them” by mimicking the antennae movements of hungry bees (reported by Ellis, 2002). This is true of African bees and is less pronounced in European bees (Ellis et al., 2003; reviewed in Ellis & Hepburn, 2006).
The authors argue that encapsulation is not as effective in European bees, possibly because it is not as developed as that of African bees (who have evolved alongside the beetles). Small adult beetle numbers can be kept under control, however they falter when the number increases, whereas African bees can withstand larger infestations, and this covered below.
Beetles Lay Eggs in Places Bees Cannot Reach
A member of my beekeeping club once said “small numbers of beetles are always present in a hive at low numbers- they’re inactive, waiting for an opportunity to lay eggs that are less likely to be removed by the bees” he later remarked that this “opportunity” comes in the form of colony stress, whether it’s due to disease, wet weather, chemicals or something else. Once the conditions are right, the beetles lay clusters of egg in crevices, empty comb, cracks in the frame and so on, with a single female capable of laying several hundred eggs over her lifespan (Meikle & Patt, 2011).
Supposing the conditions remain optimal for the beetle (i.e. temperature around ~32°C, relative humidity above >50%; Meikle & Patt, 2011), the eggs will hatch after 1-3 days and the beetle larvae emerge to feed on eggs, brood, honey and pollen. As they feed, they excrete the yeast Kodamaea ohmeri which consumes the sugars in honey, leading to fermentation and “that rotten smell” associated with a slimeout.
When larval feeding happens en masse, the colony collapses and the bees abscond, leaving the larva to feed freely (Amos et al., 2018). The odours emitted from fermenting hive products form a positive feedback loop, as Torto & collleagues (2007) found that more adult beetles will travel to bee-less hives when a slimeout is underway.
For visual learners, here’s a video of last season’s slimeout:
Varroa as a Cause of Slimeouts: General Colony Weakening
Theoretically, Varroa makes slimeouts more likely by weakening the colony population. There are not many formal studies that investigate whether there is synergy between Varroa & beetles and the general consensus is “it’s two big stressors combined”. That’s good enough for me, even if Delaplane & colleagues showed that large beetle numbers reduce Varroa populations (covered below).
As the mites feed on pupae and adult fat bodies, they spread viruses (deformed wing virus being the big one) and shorten the worker lifespan. Pupae that hatch into adults come out weaker and less effective at their normal duties and as this multiplies, the colony becomes understaffed, unable to manage their brood and hive stores. If this were due to pesticide poisoning, they’d be called “sublethal effects” as they reduce the quality of life rather than prematurely ending said life. I reckon “sublethal effects of Varroa” also works- the affected bee can still work, albeit at a lower capacity.
The sick bee has reduced grooming capability, guarding efficiency, hygienic removal, and perhaps metabolic capacity. As the infection spreads to hundreds of bees, the strain becomes evident in hive stores and altered brood patterns, as the colony struggles to manage the queen’s eggs and the intensive nature of comb building and honey dehydrating.
Theoretically, the adult beetles sense a reduction in the bee population and surmise that they have room to lay eggs and for the larvae to develop, and so begin laying said eggs.
Beetles Prefer to Lay Eggs in Capped Cells; Larvae Eat Pupae
Observations by Elzen and colleagues found that when beetles are offered honey, pollen and pupae, the adult beetles preferred to lay eggs on the pupae (Elzen et al., 2000). They noted that the pupae were reduced to “amorphous shells [of exoskeleton])”, suggesting that the larvae consumed the entire pupa except for its hard parts2.
From a metabolic point of view, this makes sense. Pollen is hard to digest on account of the grain’s shell and structure; the nurse bee caste diverts a portion of their energies into powering the enzymes that break these hardened grains. Once digested, the pollen grain yields lipids and protein, at a high costly. Honey is simple carbohydrates and no other macronutrients- you can’t grow on carbs alone. Ghosh & colleagues (2016) analysed worker bees from a per-100-gram perspective, finding that:
- 5-6 day old larvae contained 35g protein, 14g fat and 46.1 g carbohydrates
- 2-5 day old pupae were 45.9g protein, 16.0g fat and 34.3g carbohydrates
- At the imago or “final morph prior to emerging from the cell” stage, the fat content decreased to 14.5g and carbohydrate to 6.9g. Protein increased to 51g.
- This is nutritional value is also attributed to adult bees
- It’s wrong to conclude that beetle larvae would benefit most from eating almost-adult bees for the increased protein, as it does not account for the “hard bits” that would have formed by this stage (exoskeleton, legs etc…).
- At the imago or “final morph prior to emerging from the cell” stage, the fat content decreased to 14.5g and carbohydrate to 6.9g. Protein increased to 51g.
The authors had analysed the bees from an “insects as human food” perspective, and beetles determined that brood is a better source of nutrition long before we had nutritional contents. From these results one can see why beetles prefer laying eggs on pupae: high protein content and a balanced carbohydrate/fat profile ideal for development. If beetles could lay eggs on larvae directly they would, however the bees will remove them as part of their hygienic duties. Piercing the cell and laying eggs on the pupa-to-be is much less risky tactic.
Beetles Prefer Laying Eggs from an Adjacent, Empty Cell
The adult beetle struggles to pierce the capped brood cell because she would be harassed by the workers. However if that brood cell had an empty cell adjacent to it, the beetle can scurry into that gap, pierce the side wall and lay eggs unobstructed (Ellis & Delaplane, 2008). The egg can develop in relative safety and feast on the pupa when it emerges, assuming the bees don’t remove it first.
The Presence of Varroa did not Impact Beetle Behaviour
Elis & Delaplane found that beetles would lay in a capped cell regardless of its Varroa status, perhaps because they cannot detect the presence of the mite, or the beetle does not care for the squatters (Ellis & Delaplane, 2008).
Their study observed an indirect interaction – the more Varroa in a hive, the more beetle larvae were spotted. Not because of synergy, rather because the high mite population meant a weaker colony, which translated to reduced hygienic behaviour in terms of beetle egg/larvae removal.
More direct research from Delaplane & colleagues found the presence of mites & beetles in one hive “did not interact such that damage by one was affected by changing levels of the other” (Delaplane et al., 2010). In more plain terms, when excess beetle numbers caused damage, the mites did not have a reproductive advantage due to the “weakened colony”. If anything, the authors report that when beetles surged in number, mite populations decreased, and they suggested beetle presence (which are easier to detect and manage than mites) triggered hygienic behaviours, and that this general increase in hygienic behaviour (whether due to increased grooming or cell uncapping), reduced mite populations.
For the sake of completeness, when Varroa levels increased, the population of the tracheal mite Acarapis woodi increased, suggesting some synergy between those two species (Delaplane et al., 2010).
Do I Manage SHB or Varroa First?
Ideally, manage both simultaneously; it’s not hard. Varroa requires chemical management, and alas, these chemicals do not affect the beetles. Integrated pest management techniques work better for Varroa; I have not heard of IPM for beetles because it is relatively easier to control them. Some methods I use for controlling beetles:
- Oil traps using the “silver bullet” – add a cheap cooking oil into the trays and place 3-4 trays between each frame per box. The beetles will (theoretically) seek refuge in these trays as they are a dark spot the bees cannot access, and drown in the oil
- Drawback: bees will seal the openings. If you have a few hives, scraping the wax off at each inspection is no issue… Multiple hives? Best off using another approach: Apithor
- Apithor traps use a beetle bait laced with the insecticide fipronil. Adult beetles are attracted to the scent, where they ingest the bait and succumb to fipronil poisoning. The device is placed on the bottom board or on the top bars and lasts for several months.
- One trap per hive is sufficient, however multiple traps may be used in severe infestations. It also runs into the issue of bees sealing the entrances, however they are easier to clean than the silver bullets
- Vegetable oil on the bottomboard tray. Any beetle or larvae that falls through the hive’s mesh bottom drowns in the oil, rather than continuing its journey to the soil or back inside the hive
- Frequent cleaning & oil replacement required, otherwise it will develop mould that scores high on the “ew” factor
- You can also use chux wipes or similar washcloth-type material – the beetle will get stuck on the fibers and starve to death
- Suck up adult beetles with a modified car vacuum… Dad’s idea!
With mites, your long-term approach is a combination of drone brood trapping & uncapping, using hygienic bees and of course, miticides. Many skip the first two because of the labour intensiveness and opt for the miticide approach, fair enough – be sure to use it properly and rotate products to avoid the mites’ developing resistance. “but it won’t happen to me!” I hear you say. Sure champ, we also said that about bacteria.
If Your Hive Dies, Freeze, Clean and Reuse the Frames
If the colony collapses and you know it is from Varroa and/or beetles (rather than say, American foulbrood), then the frames can be frozen for few days to kill remaining beetle eggs/larvae & mites, then placed inside another colony. Any damage or scraps will be cleaned by the bees and the frame will me modified to suit the colony’s needs. For example, if you added a frame of drone brood and the colony does not need to rear brood, they will reform it into smaller cells and extra wax either discarded or reused elsewhere.
If the colony collapsed while you were using chemical treatments and you wish to reuse the frames for honey production, or would prefer to start over chemical-free, then discard the wax and apply a new foundation. There is no need to do this if the chemical treatments were an organic acid like formic or oxalic acids, which do not dissolve into the wax.
I mention the above because I hear comments at my beekeeping club of “when I add frames to my hives I always find a lot of wax debris on the bottom board”, and so I’ve addressing this frequently asked question above; it’s okay, let them bee.
African Honey Bees have Adapted to Small Hive Beetles
As mentioned above, European bees can form “beetle prisons” too, except their tactics are not as refined and therefore “effective” as the African bees, which will I segue into now as a conclusion to this article:
The 2003 study by Ellis & colleagues showed the Cape honey bee, Apis mellifera capensis, exhibiting hygienic behaviour in response to the presence of small hive beetle eggs. As both species are native to the region, they have developed a sort of evolutionary arms race against one another:
- Beetles confined to empty honey combs have learnt to pierce the adjacent cell and lay eggs.
- The pierced cell triggers a cleaning response in the infested brood combs, resulting in removal of said eggs.
Additionally, the Cape bee constructs prisons using propolis to control small hive beetles (Neumann et al., 2001). The guard bees hold a constant number of beetles inside this prison cell, increasing the number of propolis prisons when beetle density increases, rather than the numbers of beetles per existing cell.
The authors suggest that this defensive behaviour evolved as a way to manage beetle infestation and limit their proliferation within the honey bee colony. However this imposes a selection pressure in which the beetles most resistant to starvation survive. Some 20-years-later, Papach and colleagues (2020) showed that beetle populations within their native African geographies had a higher tolerance to starvation (females survived up to two months without food) than beetles in their invasive North American range. This adaptation against imprisonment is the next instalment of the evolutionary arms race between host & pest – maybe in further generations down the line we will see bees counter the starvation resistance with… Harassment to make beetles burn more energy?
Africanised Honey Bees are less Impacted by Varroa
Extracted from the article on AHBs
The hybrid Africianised honey bee with European & African genetics is less impacted by Varroa– These colonies were untreated, and yet had a lower mite count compared to other South American colonies who used European-lineage bees (Tibata et al., 2021).
Further, the wild Africanised bees had lower Varroa mite infestation levels compared to European honey bees within the same region. They had no apparent symptoms of the honey bee diseases associated with Varroa infestations – Deformed Wing Virus, Black Queen Cell Virus & Israeli Acute Paralysis Virus. Although these pathogens are found at low levels in any hive, the presence of Varroa enhances their transmission and severity (Martin, 2001, Sumpter & Martin 2004).
The authors reported a low Varroa infestation level in wild, untreated Africanised colonies (a mite load of 4.5%, determined via alcohol washing). This count was lower than other colonies in Latin America, such as mite load of 6% in Mexico (Tapia-Gonzalez et al., 2007). In North America, the mite counts for stationary beekeeping operations were reported at 6%, slightly higher than the Mexican counts (Traynor et al., 2016).
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Foonotes
- One of the reasons why you’re told to clean up burr comb and not to leave it lying around, as it’ll attract moths and beetles. If it’s a large amount you risk attracting the larger creatures in your area. ↩︎
- This is unlike Varroa, who feeds only on the bee’s fat bodies (Ramsey et al., 2019). Mites are also smaller than beetle larvae (who eventually grow larger than the pupae). ↩︎