Butterflies released in Finland contained parasitic wasps – with more wasps inside | Butterflies
When the caterpillars of a beautiful butterfly were introduced to the small island of Sottunga in the Åland archipelago, scientists hoped to study how the emerging butterflies would disperse across the landscape.
But the researchers did not realize that their introduction of the Glanville fritillary (Melitaea cinxia) led to the emergence of three other species on the island in the Baltic Sea, which arose from the butterfly like Russian dolls.
Some caterpillars contained a parasitic wasp, Horticultural hyposoter, which springs from the caterpillar before it can pupate and become a butterfly.
Living inside some of these little wasps was another even smaller and rarer parasite, a “hyperparasitoid” wasp known as Mesochore cf. stigma. It kills the parasitic wasp at about the same time as the wasp kills the caterpillar and emerges 10 days later from the carcass of the caterpillar.
There was also a bacteria carried by the female. H. horticola wasps and transmitted to its offspring. By an unknown mechanism, Wolbachia pipientis increases the susceptibility of the parasitic wasp to be taken over by the tiny parasitic wasp Mr. stigmatic, who can only live H. horticola wasp.
Perhaps most surprisingly, given that small island populations are notoriously vulnerable to extinction, all four species still survive on the 27 km² island 30 years after initial introduction.
A study the genetics of the parasitic wasp and its bacteria have shown this survival all the more remarkable given that the Glanville fritillary has experienced several population crashes on Sottunga.
“The Glanville fritillary population has experienced incredible crashes at times over the past 30 years and we would expect there to be very low genetic diversity in the years following these crashes,” said Doctor Anne Duplouy from the University of Helsinki and lead author of the study.
“But this butterfly somehow seems to be recovering from isolated population accidents, and the genetic diversity in Åland is still impressive, despite all the bottlenecks the butterfly has gone through.”
Parasites may have survived on the island due to their superior flight skills. Unlike many butterflies, the Glanville’s Fritillary is a poor disperser, and individuals living naturally on neighboring islands cannot fly more than 4.3 miles (7 km) in open water to Sottunga to complete this population.
But the little parasitic wasp H. horticola seems to have been able to fly or at least be lifted by strong winds to move between the islands on the island archipelago, an autonomous region of Finland where Swedish is the official language.
Since H. horticola was accidentally introduced to Sottunga, the wasp was discovered on other islands to the north, where it had not been previously reported. These individuals present genotypes similar to those of Sottunga, suggesting that they come from wasps accidentally introduced to this island.
Duplouy said the study, which is published in Molecular Ecology, could serve as a warning to projects seeking to reintroduce or restore rare species, showing how easily other organisms – or pathogens – can be inadvertently released alongside target species.
“The reintroduction of endangered species comes from the heart, a good place, but we have a lot to learn about the species we are reintroducing and the habitat where we want to reintroduce them before we do so,” she said.
The bacteria, Wolbachia pipientis, is found in insects around the world (in at least 40% of all species), but if accidentally introduced into uninfected populations, it could reduce the reproductive success of the species that environmentalists are trying to help .
While Glanville’s fritillaries and its associated parasitoids have survived through thick and thin on Sottunga, Duplouy said population accidents – which are usually caused by drought on the island – have also shown the dangers of the climate crisis.
More droughts could cause more regular accidents and a loss of genetic diversity from which the butterfly and its dependent species may be unable to bounce back.
Researchers count the populations of Glanville each year in September, when its gregarious caterpillars are very visible in the communal “webs”.
“The butterflies are still there but at the moment the conditions are really difficult because of the drought,” said Duplouy. “Every year we’re really worried he’s not around, but right now we still have him.”