Living on the island of Tutuila in American Samoa, hints of Ooencyrtus pitosina’s existence to western science first came in 1884 when naval officer and entomologist Gervase Frederick Mathew first saw that the eggs of the Samoan swallowtail butterfly were being attacked by a minute insect.
While details of the behaviour were published in a scientific journal, however, the wasp was never formally described. A team of researchers has now done this in the hope of encouraging more research into this little-known wasp.
Andrew Polaszek, an entomologist at the Natural History Museum who led the research, says, ‘This might be a record for the length of time between identifying a species, and it later being described. It goes to show that describing a species is not as simple as just pointing at something you don’t recognise and giving it a name.’
‘When Ooencyrtus pitosina was seen in the 1880s, there would have been great difficulty in identifying whether or not it was a new species. Its host was only formally described about 20 years earlier, so a lack of understanding of the butterfly would have hindered finding out about the wasp.’
The Samoan islands are spread across more than 3,000 square kilometres of the Pacific Ocean. Their isolation means that they are home to a variety of animals and plants found nowhere else, such as the Samoan swallowtail, Papilio godeffroyi.
While it was once common to the entirety of the larger Samoan islands, it’s now only found on Tutuila, having become extinct on the archipelago’s other islands at some point in the late 1970s or early 1980s. Today, it covers just 5% of the range it used to inhabit.
The causes of its decline aren’t clear, but clearance of its forest habitat, as well as the impact of tropical storms, are thought to have pushed the lime berry (Micromelum minutum), the tree the Samoan swallowtail needs to lay its eggs on, into decline.
These plants are equally as important to O. pitosina, which lays its own eggs inside those of the Samoan swallowtail. Its larvae hatch and eat the butterfly eggs from the inside, eventually giving rise to adult wasps.
While it may seem like the wasps are also contributing to the butterfly’s decline, Andrew argues that they’re performing a vital role.
‘As the wasp is killing the butterfly, it initially seems like the wasp is the villain of the story,’ Andrew says. ‘Without the wasp, however, it’s possible that the lime berry could be wiped out on the island by being consumed by high numbers of the swallowtail’s caterpillars.’
‘In fact, it’s helping to keep the ecosystem in balance by keeping the butterfly population in check. If the swallowtail larvae overwhelmed the trees, then they would lose their source of food and push themselves closer to extinction.’
As a result of their close relationship, the fate of the three different species on Tutuila is locked together. With the Samoan swallowtail now classed as Endangered, it’s assumed that its loss would probably also lead to the extinction of O. pitosina.
Mark Schmaedick, a co-author of the research, adds, ‘While it seems likely that Mathew’s 1880s report of the egg parasitoid on P. godeffroyi in the Samoan islands and on the closely related Papilio schmeltzi in Fiji are both O. pitosina, there’s no way to check on that at the moment.’
‘It’s difficult to say whether it does parasitise more than just the Samoan swallowtail and whether it might occur outside the swallowtail’s range. We haven’t yet found any evidence that it does, but it also appears that the wasp might not be readily detected by typical general collecting methods.’
Were the wasp to become extinct, it would represent the loss of more than just one species. DNA sequencing suggests it isn’t that closely related to any other known wasp species. While it’s likely that closer relatives simply haven’t been found yet, O. pitosina is currently the only evidence of this unique branch of the tree of life.
Fortunately, just as the threats to the Samoan swallowtail affect the wasp, it also gets a boost from anything that benefits it. Proposals to reintroduce the butterfly from Tutuila into islands in the neighbouring nation of Samoa will need to take the wasp into account, which could help stabilise its population.
To ensure the success of the reintroduction, there will need to be a better understanding of the Samoan swallowtail and O. pitosina than there is now. The scientists hope to continue their research next year, aiming to avoid there being a sting in the tail for these two inextricably intertwined species.
The paper Ooencyrtus pitosina (Hymenoptera: Encyrtidae) – a natural enemy of Samoan swallowtail butterfly Papilio godeffroyi (Lepidoptera: Papilionidae) Is published in the journal Plos One.
