When Honey Bees Eat Larvae

Pupal Cannibalism and Nutrient Recycling to Maintain Hive Health

Considering that honey bees are descended from wasps, it’s no surprise they can consume animal tissue. Within the colony, cannibalism is a response to environmental stressors where there is a shortage of proteins (pollen). Apis mellifera workers consume younger larvae (<3 days of age) for proteins, generally sparing the older larvae (>3 days of age) who have had more resources spent on them. The consumed proteins are recycled to fuel royal jelly secretions for the queen and brood. Diseased larvae are also consumed to reduce the risk of transmission inside the hive.

How do pollen shortages trigger cannibalisation, and are hygienic bees more likely engage in the act? No, the consumed proteins do not end up in honey (no protein ends up in honey…), so the product remains “vegetarian”, as we define it. Unfortunately, not much research is done on honey bee cannibalism, and I cover what I can.

Larval Cannibalisation is Triggered by Pollen Shortages

Consider nectar dearths, those periods in the late summer and autumn where there is generally a shortage of flowering plants. To obtain nutrients, some colonies turn to robbing behaviour. With proper management, beekeepers can feed hives sugar syrup to maintain carbohydrate intake. What about pollen shortages?

Schmickl & Crailsheim triggered cannibalism by artificial pollen shortages. After five days in this experiment, larvae less than three days of age were cannibalised, resulting in both a reduction in the colony’s protein demands and an increase in protein supply (Schmickl & Crailsheim, 2001). Older larvae were spared, and their cells were capped earlier to preserve the colony’s resource investments. After all, the older the larvae, the bigger the gain of protein per cannibalised larva, but also the higher the loss as older larvae have already received more nursing than younger ones.

Reduced Brood Rearing During Pollen Shortages

In times of pollen shortages, only 32% of one-day-old larvae reached the capping stage, compared to the 80% in the control group (Schmickl & Crailsheim, 2001). Once those larvae reached three days of age, cannibalisation was rarely observed (~2%), perhaps this is the colony’s marker of “too much invested to lose” versus an expendable protein source?

Larval Cannibalisation is Part of the Colony’s Response to Environmental Change

Eating brood is an extreme response to sudden environmental shifts. Such rapid changes in environmental conditions and related shortages of nutrients induce a cascade of behavioural reactions in the honeybee colony:

1. First, the colony increases foraging intensity.

2. If ineffective, the nurse bees start producing proteins from their bodily (fat) reserves and adjust nursing intensity.

3. If ineffective, cannibalism of larvae comes next.

I suggest that cannibalism occurs in pollen dearths rather than nectar ones since all hives are guaranteed to contain carbohydrates in the form of honey – after all, honey is stored food for the colony. The chances of finding rich pollen stores is slim. Pollen shortages occur in the absence of flowering plants, and pure pollen cannot be stored indefinitely like honey can.

Thinking logically, if one colony does not have enough pollen, then the same is true of their neighbours. There is no point in risking forager lives by engaging in pollen robbing, and so the colony looks inwards for protein sources. It’s okay to disagree, honey bees are no logicians.

Hygienic Behaviour Increases the Rate of Larval Cannibalisation Due to a Lower Detection Threshold

Is cannibalism a part of hygienic behaviour? if you reduce the number of infected individuals in a colony, then you reduce the risk of disease spreading. However, reducing the infected population by eating them is sure to spread the disease further, right?

This is what Posada-Florez and colleagues found when analysing adult bees that cannibalised larvae infected with Deformed Wing Virus (DWV; Posada-Florez et al., 2021). In their study, Varroa-Sensitive hygienic bees consumed larvae infected with DWV. In one instance, cannibalisation of a single infected pupa led to infection of 2,00 worker bees, as the virus spread to other workers through oral trophallaxis.

The hygienic bees had a higher sensitivity to detecting the DWV-infected pupa compared to control, developing and spreading a low-level infection. However, this infection remained asymptomatic from the orally-acquired DWV. The authors suggest that this could be due to the sensitivity of hygienic bees, who are able to detect infection at earlier stages and thus cannibalise larvae at low(er) infection levels. Of course, high oral doses resulted in DWV symptoms (Posada-Florez et al., 2021).

Is Feeding Pollen Substitutes to Stop Larval Cannibalisation Necessary?

Short answer, yes, it meets their protein needs. However, true substitutes for pollen do not exist; these pollen provisions are of a lower quality than the variety they find in the wild. Artificial pollen feeds tend to be based on soybean, milk, linseed oil and/or yeast, and their long term use is not recommended for optimal bee health¹.

If brood cannibalisation is capable of maintaining colony protein levels, then what if hives were fed a patty made of brood? This is what Pavlovic and colleagues investigated, concluding that bees can thrive on pollen substitutes made from yellow mealworm larvae (Tenebrio molitor – a beetle species; Pavlovic et al., 2022).

Using Insect Larvae as a More Effective Pollen Substitute

In the aforementioned study, the colonies fed with pollen substitutes made from yellow mealworm larvae and sugar ended up consuming significantly less food than colonies fed with yeast or pollen patties. Despite eating less, they maintained a healthy body weight comparable to the other groups (fed on yeast, pollen or sugar patties), suggesting that T. molitor flour is highly nutritious, and is more bioavailable for bees.

Dried yellow mealworm larvae contains around 33% fat and 51% protein (Zhao et al., 2016). Having a higher nutrient concentration than pollen (which varies between 1.5% to 48.4% protein and 1.2% to 24.6% lipid; Vaudo et al., 2020), mealworm patties can mixed with sugar to achieve the desired ratios of protein, fats & carbs lipids and carbohydrates. For reference, the contents of live bee brood are 76.8% moisture, 4.7% lipid, 9.4% protein, 0.8% ash and 8.0% carbohydrate (Finke, 2005). Remove the moisture, and the nutritional profile is inferior to mealworm larvae.

Hocherl & colleagues showed that bees will consume more of a less-nutritious source (corn pollen) compared to a superior diet from multifloral sources (Höcherl et al., 2012). However, if bees were offered a choice of consuming foraged pollen versus mealworm patties, the pollen will be selected over the T. molitor. You can’t fight nature, even if her replacement is “more nutritious and easier to obtain”… That previous sentence was not about bees. Nonetheless, bees consumed less mealworm patties compared to other products, suggesting that it is indeed a more nutritious supplemental feed as less amounts were required to achieve the same health effects.

If you will not “eat ze bug”, then your bees might.

Social Immunity

This deserves its own article

Social immunity is an entomological concept whereby social insects adopt a defensive strategy that goes beyond the individual, consisting of behavioural, physiological, and environmental modifications to protect the colony from diseases and parasites. Examples of social immunity in bees include:

• Behavioural shifts like consuming infected larvae (and other hygienic behaviours), increasing hive temperatures in times of Chalkbrood infection, or allocating the task of removing dead bees to specific individuals.
• Environmental modifications, like the use of propolis coatings on hive walls, increasing hive temperatures to combat disease and swarming as a means of migrating to a better location.
• Diet-driven physiological changes. Royal jelly can transfer pathogenic bacterial fragments to brood, stimulating their immune system (Harwood et al., 2021). Further, bees can choose to favour certain plants based on their nutritional needs (Leponiemi et al., 2023)

Other mechanisms include hygienic behaviour, temperature regulation, the use of antimicrobial substances, and manipulation of the internal hive environment to reduce pathogen loads. The most common form of social immunity in bees is through the secretion of glucose oxidase². In the study by Jones and colleagues, they noted that the number of individuals inside the hive drives immunity (“group density”; Jones et al., 2018). Bees living in low density colonies (cages) had increased glucose oxidase activity, which suggests a compensatory mechanism for lacking adequate worker numbers.

In contrast, the authors suggest that when glucose oxidase levels decrease in high density colonies, there may be increased immune activity at the individual bee level, a dataset that was unanalysed by the authors. Social immunity enables individuals to allocate more resources to growth and development, rather than spend it defending themselves from disease (Ardia et al., 2012). Insects that were constantly infected or had chronic immune system activation ended up being smaller and had a higher metabolic rate, requiring more energy to maintain a more-active immune system.

Wrapping up: Larval Cannibalism in Immunity and Nutrition

From a nutrition point of view, larval cannibalism is a last resort to ensuring the colony maintains its health – the costs to brood and the next generation of bees is large, as that colony will take time to regain strength. It is rare for the beekeeper to encounter sudden pollen shortages in the field, however, should they happen, there is always supplemental feeding.

Larval cannibalism is one adaption used by bees to maintain social immunity; whether to slow disease spread, or remove unworthy larvae from the hive. The fact that cannibalistic bees get infected themselves, but do not show symptoms or die, suggests that much higher doses are required to infect an adult. I am curious to know how infections progress post-ingestion- could reducing the numbers of infected larvae while (potentially) increasing the number of infected adults result in a dilution of the pathogen, thus helping the colony overcome it?

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Footnotes

1. Once natural forage is available, the bees will ignore the patties. At this stage, beekeepers should remove any remaining supplemental feed, to prevent small hive beetle larvae and other pests from consuming them. ↩︎
2. Glucose oxidase converts glucose molecules into gluconic acid and hydrogen peroxide. The former gives honey some of its acidic properties, the latter is a general antiseptic that inhibits microbial growth. ↩︎