Fruit Flies Drink Wasps Under the Table

When it comes to drinking alcohol, parasitic wasps are total lightweights compared to the tiny fruit fly, which uses this advantage to protect itself from its mortal enemy.

Female parasitic wasps like to inject their eggs into fruit fly larvae along with a venom that suppresses their hosts’ cellular immune response. A wasp larva hatches inside the fruit fly larva and begins to eat its host from the inside out before emerging as an adult wasp. This obviously destroys the fruit fly progeny and is a major threat to the species.

To protect their larvae, fruit flies have figured out that depositing them in an alcohol-rich environment - overripe, fermenting fruit - will help protect them from the parasitic wasps' eggs. This isn't something they do regularly, but only when they notice wasps in the neighborhood, researchers at Emory University have discovered.

“The adult flies actually anticipate an infection risk to their children, and then they medicate them by depositing them in alcohol,” says Todd Schlenke, the evolutionary geneticist whose lab led the research.

Adult fruit flies detect the wasps by sight, and appear to have much better vision than previously realized, he adds. “Our data indicate that the flies can visually distinguish the relatively small morphological differences between male and female wasps, and between different species of wasps.”

Previously, Schlenke's team published a study showing how fruit fly larvae infected with wasps prefer to eat food high in alcohol. This behavior greatly improves the survival rate of the fruit flies because they have evolved high tolerance of the toxic effects of the alcohol, but the wasps have not.

“The fruit fly larvae raise their blood alcohol levels, so that the wasps living in their blood will suffer,” Schlenke says. “When you think of an immune system, you usually think of blood cells and immune proteins, but behavior can also be a big part of an organism’s immune defense.”

For the latest study, the researchers asked whether the fruit fly parents could sense when their children were at risk for infection, and whether they then sought out alcohol to prophylactically medicate them.

Adult female fruit flies were released in one mesh cage with parasitic wasps and another mesh cage with no wasps. Both cages had two petri dishes containing yeast, the nourishment for lab-raised fruit flies and their larvae. The yeast in one of the petri dishes was mixed with 6 percent alcohol, while the yeast in the other dish was alcohol free.  After 24 hours, the petri dishes were removed and the researchers counted the eggs that the fruit flies had laid.

The results were dramatic. In the mesh cage with parasitic wasps, 90 percent of the eggs laid were in the dish containing alcohol. In the cage with no wasps, only 40 percent of the eggs were in the alcohol dish.

“The fruit flies clearly change their reproductive behavior when the wasps are present,“ Schlenke says. “The alcohol is slightly toxic to the fruit flies as well, but the wasps are a bigger danger than the alcohol.”

The fly strains used in the experiments have been bred in the lab for decades. “The flies that we work with have not seen wasps in their lives before, and neither have their ancestors going back hundreds of generations,” Schlenke says. “And yet, the flies still recognize these wasps as a danger when they are put in a cage with them.”
Photo by Alex Wild
Photo by Alex Wild

Further experiments showed that the flies are extremely discerning about differences in the wasps. They preferred to lay their eggs in alcohol when female wasps were present, but not if only male wasps were in the cage.

The adult female fruit flies only react to the presence of female wasps that infect fruit-fly larvae, above, and not to male wasps, or to other species of wasps that do not infect their larvae.

Carol Clark, Emory University

Fruit Fly, Drosophila Melanogaster
Fruit Fly

Drosophila Melanogaster
Head of a Fruit Fly