Altruistic suicide against Varroa

The parasitic mite is decimating beehives. Biologist Paul Page has shown that an Asian forager has found a countermeasure through a system of “altruistic suicide.” A model that its European cousin could follow?

It runs, it runs, the varroa. It even flies. This mite, from the same family as ticks, actually moves on adult worker bees, clinging to their necks or hiding in the folds of their abdomens. It then pierces a hole in the cuticle, draws some haemolymph to obtain nutrients, and in doing so transmits diseases: “Of the some twenty viruses identified in bees, most are associated with the presence of Varroa destructor,” explains biologist Paul Page.

Once inside the hive, varroa infiltrates the cells of larvae that are fed royal jelly by young workers and subsequently sealed with a wax cap. The mite takes advantage of the roughly twenty hours preceding cell capping to jump from a worker, enter a cell, and hide beneath the larva. Once the cell is sealed, “it pierces the larval skin, begins feeding on its haemolymph, and transmits diseases that weaken it even before it becomes an adult.”

The intruder in the cell is always a gravid female, which lays her offspring there: “The first egg is usually a male, while the three or four that follow develop into female varroa mites. The male then inseminates his sisters, which can leave the cell when the worker emerges 10 to 11 days later.” This means that within three weeks the varroa population in a hive can double or even triple—“an increase that becomes exponential over the season if nothing is done to treat the colony against this parasite.” Varroa is thus one of several causes contributing to the bee mortality observed in recent years.

Last year, Paul Page published in the journal “Scientific Reports” the results of a three-year study conducted in China and Thailand on the parasite’s original host, the Asian honey bee (Apis cerana). It should be noted that the European honey bee (Apis mellifera) has been infested by Varroa destructor for only about a century. Humans are responsible, having moved European bee colonies in pursuit of an increasingly global exploitation of the species. Via Russia, beekeepers brought European honey bees to the borders of the Asian honey bee’s range—China, Mongolia, Korea, Japan: “And as often happens when two genetically close species come into contact, varroa was able to pass from an Apis cerana worker to an Apis mellifera worker and rapidly adapt to this new host.”

The immune system under threat

In addition to transmitting pathogenic viruses, the presence of varroa also weakens the colony’s immune system. “Bees are debilitated by the parasite, more frequently exhibit deformed wings, and can no longer fly, thereby reducing the amount of food brought back to the hive; the decline is gradual, but a colony heavily infested with varroa cannot survive more than a year without the help of a knowledgeable beekeeper,” explains Paul Page.

When varroa arrived in Europe in the 1980s, beekeepers initially turned to chemical treatments using synthetic acaricides. “At first this worked well, but the parasite quickly developed physiological resistance to these products.” Today, more natural products based on formic and oxalic acids, as well as plant-derived compounds, are used. These are fairly effective, as a hive can be cleared of “up to 95% of its varroa.” However, this is still insufficient, since the remaining 5% will reproduce and rebuild their population. “Today in Switzerland, it is impossible to find a colony without varroa. The parasite moves from one colony to another by transferring from one worker to another on a flower or when a worker mistakenly enters a neighbouring hive. The more colonies there are in an apiary, the greater the likelihood that diseases will spread among them.”

A promising study

The study conducted by Paul Page and his colleagues at Agroscope and the University of Bern nonetheless opens up new perspectives. Its aim was to understand why the Asian honey bee is more resistant to varroa than the European honey bee: “In the Asian honey bee, the parasite develops only on drones, which are rarer and present only at certain times of the season, whereas in the European honey bee it is found on both drones and workers—throughout the entire beekeeping season—thus generating a much greater potential for invasion and population growth.”

The Asian honey bee, the original host of Varroa destructor, has co-evolved with its parasite and has therefore developed “defensive behaviours to limit its harmful effects.” It is also less domesticated and more aggressive than its European cousin, which has become docile through selective breeding for apiculture and may therefore be less able “to defend itself against invaders, parasites, or pathogens.” Paul Page nevertheless identified another decisive factor: a phenomenon of “altruistic suicide” in varroa-infested larvae of Asian honey bees.

A defence through hygiene

Under comparable experimental conditions of varroa infestation, researchers observed upon opening the cells that in Asian honey bees—unlike in European honey bees—“in a large proportion of infested cells, the workers were already decomposing or had prematurely halted their development into adults, and were therefore unable to emerge from their cells on their own. This alone is sufficient to disrupt the parasite’s reproductive cycle or compromise its survival.”

In addition, Asian honey bees have developed a hygienic behaviour that is only weakly expressed in European honey bees: “They are able to detect an infested cell before the end of development, then remove the diseased larva or pupa and carry it out of the colony, thereby simultaneously eliminating the parasite.”

Because this “altruistic suicide” and this hygienic behaviour are “behavioural traits, they must have a genetic basis. Through selective breeding, it should therefore be possible to obtain European honey bee colonies in which these traits are reinforced.” Although this idea is highly promising, its drawback is that it can take a great deal of time: “Queen selection is not straightforward, as it is based on the traits of an entire colony. Moreover, a queen mates on average with about twelve different males, which generates behavioural variation among her offspring.” The final hour of varroa has therefore not yet arrived.

Source: https://www.migrosmagazine.ch/abeilles-contre-varroa-mortel-combat