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When the caterpillars of a beautiful butterfly were introduced to the tiny island of Sottunga in the Land archipelago, the scientists hoped to study how the emerging butterflies would spread across the landscape.
However, the researchers failed to realize that their introduction of the Glanville pied butterfly (Melitaea cinxia) led to the emergence of three more species on the Baltic Sea island, which emerged from the butterfly like Russian pupae.
Some of the caterpillars contained an ichneumon wasp, Hyposoteric horticolathat bursts from the caterpillar before it can pupate and become a butterfly.
Some of these tiny wasps had another, even smaller, rarer parasite, a “hyperparasitoid” wasp known as Mesochore see Stigmatikus. It kills the ichneumon wasp around the same time as the wasp kills the caterpillar and hatches from the caterpillar’s carcass 10 days later.
There was also a bacterium carried by the female H. horticola Wasps and passed on to their offspring. Through an unknown mechanism Wolbachia pipientis Increases the parasitic wasp’s susceptibility to being taken over by the tiny parasitic wasp M. stigmaticusthat can only live on H. horticola Wasp.
Perhaps most surprising, given that small island populations are notoriously critically endangered, it is perhaps most surprising that all four species survive on the 27 square kilometer island even 30 years after they were originally introduced.
Research into the genetics of the parasitic wasp and its bacterium has shown that this survival is all the more remarkable given that the Glanville pied butterfly has experienced multiple population breakdowns on Sottunga.
“The Glanville piebald butterfly population has had astounding drops at times over the past 30 years and we expected the genetic diversity to be very low in the years following these drops,” said Dr. Anne Duplouy from the University of Helsinki and lead author of the Learn.
“But this butterfly seems to be recovering from isolated population crashes somehow, and the genetic diversity in Ã…land is still impressively high, despite all the bottlenecks the butterfly has been through.”
The parasites may have survived on the island due to their superior flight skills. Unlike many butterflies, the Glanville pied butterfly is a poor distribution type, and individuals living naturally on neighboring islands cannot fly more than 7 km (4.3 miles) over open water to Sottunga to replenish this population.
But the little parasitic wasp H. horticola appears to have been able to fly, or at least be lifted by strong winds, to move between the islands of the Ã…land archipelago, an autonomous region of Finland where Swedish is the official language.
Since H. horticola was accidentally introduced into Sottunga, the wasp has been discovered on other islands in the north where it has not yet been detected. These individuals share genotypes similar to Sottunga, suggesting that they came from the wasps that were accidentally brought to this island.
Duplouy said the study, published in Molecular Ecology, could serve as a warning to projects trying to reintroduce or restore rare species, and shows how easily other organisms – or pathogens – besides the target species can be accidentally released.
“Endangered species reintroduction comes from the heart, a good place, but we still have a lot to learn about the species we are reintroducing and the habitat we plan to reintroduce them into before we do,” she said.
The bacterium Wolbachia pipientis, is found in insects worldwide (at least 40% of all species), but if accidentally introduced into uninfected populations, it could reduce the breeding success of species conservationists want to help.
While the Glanville pied butterflies and associated parasitoids have survived against all odds on Sottunga, population crashes – normally caused by drought on the island – also highlighted the dangers of the climate crisis, according to Duplouy.
More droughts could lead to more frequent crashes and a loss of genetic diversity from which the butterfly and its dependent species may no longer be able to recover.
Researchers count the Glanville populations every September when their sociable caterpillars are clearly visible in communal “netsâ€.
“The butterflies are still there, but at the moment the conditions are very tough because of the drought,” said Duplouy. “We worry every year that it won’t be there, but at the moment we still have it.”
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