The slippery life of the small hive beetle

Alfred Hitchcock would have loved the small hive beetle, Aethina tumida. If he had only known, he might have scrapped his movie The Birds in favor of The Beetles. Small hive beetles lead a horror story life, fraught with mystery, intrigue, and destruction. The beetles, aided by their partner in crime—a symbiotic yeast—produce slime, mimic pheromones, and ferment honey.

If all that is not odd enough, an adult hive beetle can coax a honey bee worker into feeding it. Even the larvae are creepy. Reminiscent of fictional lemmings that leap from cliffs, beetle larvae of a certain age travel in packs and dive from the hive entrance to the ground below. Weirdness knows no bounds.

Beetles imported from Africa

In its homeland of sub-Saharan Africa, the small hive beetle is only a minor pest. Having evolved together with the local honey bees, Apis mellifera scutellata and A. m. capensis,¹ the small hive beetles are kept in check and do not normally destroy colonies of honey bees.²

However, once it crossed oceans, the small hive beetle found a vulnerable host without a ready defense. The European sub-species of honey bees are easy prey, partly because they abscond less frequently than the African bees. In addition, they leave more resources behind for the beetles to consume when they do abscond.³

Widespread Damage

Since the small hive beetle was first spotted in North America in 1996, it has spread rapidly, defying predictions about its inability to survive in colder climates. It was first reported in both Canada and Australia in 2002 and has since scattered throughout other regions as well.

Damage to colonies infected by small hive beetles can be severe, potentially causing colony demise. In the early years after its introduction, the small hive beetle seemed to be a problem limited to weak or small colonies, or to unguarded honey houses and stored equipment. But more recently, beekeepers have reported that even large and robust colonies can be destroyed by the beetles. Elevated beetle populations, especially those that approach 1,000 adults per hive,⁴ are especially problematic.

An adult small hive beetle may enter a hive along with a nucleus colony, a package, or a split. Or, left to its own devices, a beetle may discover a colony by detecting honey bee pheromones or other attractive hive odors.⁵

Imagine Life in their Shoes

To better understand the life cycle of these creatures, suppose for a moment you are one. Let’s say you, Bonnie the Beetle, emerged from the soil a week ago.  Before doing anything else, you mate with the handsome dudes in your area. After a week or so,⁶ you go for a nice long flight to look for a place to live and raise your young. There’s no point in staying too close to home because lovely, habitable hives are everywhere.

You find a suitable domicile by sniffing the air. Honey bee alarm pheromone smells inviting, as do the odors of pollen, ripening honey, and bee brood. Your ability to fly about six miles (10 km) makes this an easy task.¹ Once you find a hive you like, you simply fly in. What a relief to be out of the light!

Laying your Eggs

Where you lay your eggs depends on how welcoming your hosts decide to be. If the host bees are friendly, you may lay your eggs atop stored pollen or directly on bee brood. To reach the brood, you chew a tiny hole through the wax cappings or make a small slit in the side of a brood cell. With your long and flexible ovipositor, you can reach deep into protected places. You drop your clutch of eggs—a dozen or two—in the tight space and then move on to another. With a little luck, you may lay 1000 eggs, or maybe double that, in your lifetime.⁴

If the host bees are not so benevolent, you may decide to leave your eggs in cracks and slits along the edge of the brood nest. Unprotected eggs may be eaten by worker bees, so site selection is important. You want to avoid ornery bees, so you seek tiny out-of-the-way crevices where they can’t quite reach you. If in doubt, you try the secluded spaces between the top bars, in feeders, in the corners of the bottom board, or up on top of the inner cover. It’s a world of choices.

Avoiding the Jailers

Still, caution is necessary. Sometimes an army of cranky nurse bees will corral you and your friends into a corner, denying you food and preventing access to the brood nest. With no get-out-of-jail-free cards, you are incarcerated behind a line of guards that watches you 24/7.

At such times, you must be devious. Play nice. If you stroke the bees’ mandibles with your antennae, they will eventually feed you. When she receives just the right touch, a nurse bee will extend her proboscis into your waiting mouth and, in a process called trophallaxis, share regurgitated nectar from her honey stomach.⁴ How sweet it is. And remember, patience pays. You can play this waiting game for 180 days or more.²

Larval feces is loaded with yeast spores

Meanwhile, while you’re marking time behind bars, your eggs begin to hatch. Within 2-4 days, hungry larvae emerge and promptly begin a search for food.² The menu includes pollen, nectar, honey, and brood, and the larvae will burrow through combs and cappings to get it. And like creatures everywhere, the more they eat, the more they defecate. But this larval feces is special, loaded with the spores of a yeast called Kodamaea ohmeri.⁷ The yeast spores love the moist environment of the hive, and soon germinate and mature into a slimy mat that coats the combs with a glistening and slippery mantle.

The yeast can bloom quickly. A pristine frame of honey can transform into a slimy mess in as little as 30 hours. The shimmering, gelatinous sheet weakens the wax cappings that cover the honeycomb, causing the honey to leak from the cells and then ferment. In severe cases, the ruined honey will drip down through the hive and run out the entrance.

The Wandering Stage

After about 13 days of gorging in the hive,⁸ your offspring are ready to rock and roam. The larvae gather together near the hive entrance and wait for the witching hour. Just after dusk, under the cover of darkness, the larvae wriggle from the hive and drop to the ground. This group’s exit from the colony has been described as a “mass exodus” or a “larval regiment.” As soon as they hit terra firma, these wandering larvae begin to look for a place to burrow into the soil. Remarkably, most find a spot within three feet (90 cm) of the hive entrance.

But worry not. Wandering larvae are resilient and strong. If soil conditions under the hive are not perfect, the larvae will take off in search of something better. Such vagabonds have been known to travel 220 yards (200 m) in search of better soil. Their primary concern is soil moisture, especially summer moisture.⁹ Relentless in their pursuit of ideal conditions, larvae have been known to wander for up to 48 days and still manage to pupate successfully.¹⁰

Pupa to Adult Beetle

Once they find a suitable place, the larvae dig down 4-8 inches (10-20 cm) where they prepare a smooth-walled cradle for themselves in which to pupate. Pupation is usually complete in three to four weeks, although it may take anywhere from 8 to 78 days, depending on conditions.⁹ The adult then emerges from the soil, just like you did. When all goes according to plan, one year can yield six generations of beetles.

Okay, enough of being Bonnie the Beetle. You can hang up your wings now and come back to real life.

A Close Relationship

Although the small hive beetle life cycle is strange, the yeast-beetle relationship is crazier still. It turns out that when the yeast, K. ohmeri, grows on top of pollen, it produces an odor that mimics honey bee alarm pheromone.⁵ Since small hive beetles are attracted to alarm pheromone in a big way, the growing yeast attracts more and more beetles. Like a siren call, the yeast odor announces that small hive beetles have successfully colonized this hive. “Come join us!” the odor seems to say. And join they do. A hive that had few beetles may suddenly attract hundreds.

Contrary to early accounts, small hive beetles prefer to lay their eggs directly in brood comb. Beetle larvae that emerge within a brood cell have a ready supply of protein and other nutrients in the form of a honey bee pupa.  Multiple beetle larvae may spill from a single brood cell and begin burrowing through combs and honey as they search for more food. Beekeepers have reported cutting open brood cells to find a dozen or more beetle larvae actively consuming a dead pupa.

Signs of trouble in the hive

Sometimes the exoskeletons of bee pupae can be seen on the landing board or on the ground beneath a hive, their interiors completely sucked dry by the beetles. Such skeletons, often accompanied by leaking honey, could be a beekeeper’s first sign of trouble.

Oddly enough, honey bees do not pursue small hive beetle larvae. While a phalanx of guards imprisons the adult beetles, the beetle larvae freely roam through the brood nest and honeycombs, eating and defecating as they go.

Adults can overwinter in the hive as long as the bees feed them, and heat from the cluster keeps them warm.¹¹ Egg laying is suspended during the winter months but can resume as the weather warms in spring.

How Yeast Spores get into the Larvae

It is still unknown how the yeast spores get into the larvae. It is possible that the spores survive within the larva during metamorphosis into an adult beetle, even though the digestive tract gets completely reorganized in the process. Alternatively, the larva may shed yeast spores in its feces as it digs into the soil prior to pupation, only to become re-infected as an emerging adult. Others speculate that the spores may reside in pollen, which in turn is carried into the hive by foraging bees and then eaten by the larvae.¹²

Hayes et al.¹³ grew slime in the laboratory by placing small hive beetles, water, brood comb, and honeycomb in plastic bags. They found “the best slime (with the highest yeast production) was obtained when a combination of honeycomb and brood comb was used.” Honeycomb alone did not produce beetles or slime, brood comb alone produced beetles but minimal slime, but in combination, brood comb and honeycomb produced numerous larvae within a viscous mantle of slime.

They also found that the slime became more attractive to small hive beetles as it aged, possibly due to an increasing amount of ethanol. Ethanol is attractive to many insects, helping them locate rotting fruit and similar foods. In addition, other compounds such as isobutanol, isopentanol, and isopentyl acetate were produced by the slime.⁷ Isopentyl acetate is the well-known main component of honey bee alarm pheromone.

Slime may come suddenly

Many beekeepers have reported seeing little or no slime in a hive and then, several days later, seeing masses of it covering many frames. The slime seems to materialize out of nowhere, far exceeding the expansion of the beetles themselves. This sudden appearance occurs because the yeast can multiply much more quickly than the beetles.

Severely infested honey bee colonies are unable to remove the slime.  As the slime spreads the bees tend to move up into the supers, trying to get away from the mess. Egg laying drops or may cease altogether. At some point, the colony may abscond or collapse.¹

Pairs of Pests

Many parallels exist between the varroa mite-virus association and the small hive beetle-yeast association. In each case, it is probably the unseen partner that causes the most damage. It is possible that honey bees could live with varroa if it weren’t for the viruses. Likewise, perhaps the bees could live with small hive beetles if it weren’t for the yeast.

And just as mites are getting harder to control, small hive beetles are getting harder to control as well. Simply maintaining strong and healthy colonies may no longer be enough. Since controlling an insect on an insect is a tricky business, we may well have to begin looking at the yeast as the potential weak spot in the beetle life cycle.

Looking back on it, perhaps Alfred Hitchcock could have written his movie about The Yeasts. Now that’s creepy.

Rusty
Honey Bee Suite

References

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10. Cuthbertson GS, Wakefield ME, Powell ME, Marris G, Anderson H et al. 2013. The small hive beetle Aethina tumida: A review of its biology and control measures. Current Zoology 59 (5): 644-653.
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12. Conklin TA. 2012. Investigations of small hive beetle-yeast associations (Doctoral dissertation). Retrieved from https://etda.libraries.psu.edu/catalog/15226.
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