The virus particles enter the cocoon worm through the bee sting along with the bee’s own eggs. The viruses then empty their contents into the cocoon’s cells and deliver genes that differ from those of normal viruses. These genes suppress the cocoon’s immune system and control its growth, making it a safe breeding ground for baby bees.

The insect world is full of parasitic wasp species that spend their infancy eating other living insects. For reasons that scientists do not fully understand, bees have repeatedly tamed disease-causing viruses and turned them into biological weapons. Currently, many examples of this have been described and new research has pointed to more examples. Nowadays, researchers are discovering the mechanism of this process by studying viruses in different stages of domestication.

A prominent example of a virus domesticated by honey bees includes a group called bracoviruses, which are thought to be derived from a virus that infected the honey bee or its host, the cocoon worm, about a hundred million years ago. That ancient virus inserted its DNA into the honey bee genome. Since then, this DNA has become part of the bee’s genome and passed on to subsequent generations.

Over time, bees diversified into new species and viruses diversified along with them. Bracoviruses are found in about 50,000 honeyless bee species, including M. demalitor. Other domesticated viruses originated from different wild viruses that entered the bee genome at different times.

As the bee-virus combination evolves, the virus genome spreads throughout the bee’s DNA. Some genes are lost, but a certain set of them remain that are essential for making the initial infectious virus particles. These pieces are located in different places in the bee genome, but they can still communicate with each other. “They’re still making products that work together to make virus particles,” says University of Georgia entomologist Michael Strand. “But instead of having a complete viral genome like the wild virus, the domesticated virus particles act as a delivery vehicle for the bee’s weapons.”

The weapons that bees use are very different. Some of them are proteins, while others are genes on small pieces of DNA. Most of them bear little resemblance to what is seen in bees or viruses, so it is not clear where they originated. They are also constantly changing and are in an evolutionary arms race with cocoon defenses or other measures.

In many cases, researchers have yet to discover what the genes and proteins do inside the bee host or prove that they function as weapons. But they have discovered some details. For example, M. demolitor bees use bracoviruses to deliver a gene called glc1.8 to the immune cells of moth larvae, which causes the infected immune cells to produce mucus that prevents them from attaching to the bees’ eggs. Other genes in M. demolitor bee bracoviruses cause immune cells to destroy themselves, while others prevent larvae from suffocating the parasite in melanin sheaths.

Taming viruses can be dangerous. However, the wild relatives of domesticated viruses can be lethal, causing cells to produce virus particles and then burst them to release their contents. Some of them cause the body of insects to dissolve and turn them into sticky juice. In fact, even under domesticated conditions, sometimes specialized cells inside bees’ ovaries must burst to release viral particles. The bee must find a way to control that virus so that it does not infect and destroy the bee itself.

How did bees evolve to be able to domesticate bees? More importantly, they have neutralized viruses. Virus particles cannot reproduce because they do not contain the genes that are critical for making new virus particles. They remain in the bee genome.

Bees also control where and when domesticated virus particles are produced. The reason for this is probably to reduce the risk of recalcitrance of the virus. Bracovirus particles are made only in one part of the female reproductive system and only for a limited time, and the key genes of the virus have been completely destroyed so that domesticated viruses cannot replicate their DNA. This loss is seen even in recently domesticated viruses, suggesting that this step is critical.

Any gene that is not useful for the bee gradually accumulates mutations. In bracoviruses, so much time has passed that unused genes are detectable. In recently domesticated viruses, residues are still detectable.

It is not unusual to have a genome full of dead viruses. Viruses always jump into animal genomes. Even our DNA is full of remnants of viruses. However, only parasitic bees are known to have the ability to maintain a complete set of genes that work together to make viral particles.

Like other virus-harvesting bees, the D. longicaudata bee injects virus particles into its host, which in this case is a fruit fly larva. Kaufman and Burke, along with Taylor Harrell, have shown that without poxviruses, most bee larvae die. But unlike fully domesticated viruses, this poxvirus replicates outside the bee and produces new virus particles in larval cells. The bee takes advantage of the poxvirus but does not completely control it.

Kelsey Kaufman, an entomologist at the University of Tennessee, says this poor control could be an indication of the type of virus the bees are starting with. Most domesticated viruses originate from a type of virus called nodiviruses, which can integrate into the bee genome more easily than poxviruses. But it is also possible that the bees have not had enough time yet. In fact, the association between bees and poxviruses is so new that it appears to exist in only one species of bee.

The virus is only found in certain tissues and only reproduces when the eggs are developing, which could mean that the bee has already developed defenses. Viruses also seem to be losing their ability to transmit without the help of bees. Kaufman has tried feeding flies many viruses and they didn’t seem to get infected that way.

Kaufman calls the poxvirus system exciting because little is known about how the domestication process of viruses begins. We cannot go back and see how this process begins. However, the new system can provide us with an insight into the beginning of this process. Although no one knows for sure why parasitic bees continue to domesticate viruses, researchers speculate that it has something to do with their lifestyle.

Internal parasites live inside their hosts, which are dangerous environments that they actively try to kill. From the bee’s point of view, viruses are like packages full of tools to solve this most catastrophic problem.

A study published in 2023 supports this idea. This research examined the genome of more than 120 species of honey bees, ants, and honey bees. The researchers examined these genomes for signs of the types of viruses that are susceptible to domestication. They inferred the presence of domesticated viruses by identifying viral genes that were functionally conserved over evolutionary time. Such conservation is expected if the genes help bees to survive or reproduce.

As expected, non-parasitic insects showed little evidence of having these domesticated viruses. The same was true for parasites that grow outside their hosts’ bodies where the host’s immune system cannot attack them. But in parasites that grow inside other insects, domesticated viruses seem to be much more common.

Julien Varaldi, a biologist at Claude Bernard Lyon 1 University in France and one of the authors of the study, says that there is a special connection between viruses and these internal parasites. This shows that viruses play an important role in the evolution of this lifestyle. With hundreds of thousands of bee species and countless strains of viruses, it’s highly likely that the two creatures could mate. In this situation, there are many opportunities for development.