Entomological Russian Dolls

Thirty years ago, the Glanville fritillary (Melitaea cinxia) was introduced to the tiny (27 square km) island of Sottunga in Finland's Aland archipelago. The intention was to see how this butterfly distributes itself. The insect is not a very good flier, so it is restricted to that island. Although Glanville fritillary numbers have undergone repeated 'crashes', the butterfly recovers and still survives in this location (https://www.theguardian.com/environment/2021/sep/14/butterflies-finland-parasitic-wasps). What was not expected, however, was that some Glanville fritillary caterpillers contained a parasitic wasp Hyposoter horticola. The wasp kills the caterpiller, before it can pupate. Even more unexpected, was the presence of a tinier wasp, Mesochorus cf stigmaticus that parasitised and killed H. horticola. Finally, there was a bacterium, Wolbachia pipientis, carried by some female H. horticola, predisposing them to infection by the smaller wasp. Weirdly all 4 species have survived on Sottunga. Some of the wasps, have even reached neighbouring islands. This study provides a clear warning to people undertaking introduction/re-introduction exercises. You clearly have to be very careful not introduce 'passengers', along with the intended species. These unintended introductions could have very detrimental effects on ecosystems.