As the weather cools down throughout North America, let’s head to somewhere hot, really hot, to the Mauna Loa and Kilauea volcanos on the big island of Hawai’i, where lava erupts from vents at some 2000 degrees Fahrenheit and creates some of the youngest land on Earth. The Hawaiian Islands are an archipelago, a chain of land masses formed as the earth’s crust passes over a volcanic hot spot beneath the Pacific Ocean. The northernmost islands in the string, Kaua’i and Ni’ihau, were born around 5 million years ago but the youngster of the bunch is Hawai’i, which arose from the seafloor only some half million years ago. It continues to grow with ongoing eruptions and lava flows.
A hike across a lava flow on the Kīlauea Iki trail provides fascinating clues to the formation of the rich floral and faunal diversity of the Hawaiian Islands. One of the first colonists to a virgin larva field is the sacred ʻŌhiʻa tree, Metrosideros polymorpha. This keystone species comprises some 80% of the native forests on the big island and provides habitat and food for many of Hawai’i’s endangered birds and insects. Scattered across the moonscape of a 1959 larva flow are young ʻŌhiʻa trees whose roots miraculously pierce the ropy lava to collect water and nutrients, which support a leafy canopy and brilliant ʻŌhiʻa blossoms. While examining these remarkable trees, we observed several with Hershey-kiss-shaped galls on the lower surface of many leaves. Galls are abnormal growths on plants created by several species of insects and mites that secrete potent chemicals into the plant’s undifferentiated tissues. These chemicals derail the normal developmental processes of the plant and create food and refuge for the insect or mite at the expense of the unwitting plant host. We met other species on gall making insects in previous episodes on oaks, tupelo, spruce, and elm.
Spectacular eruptions of Mauna Loa and Kilauea volcanos on Hawai’i transport lava from deep within the earth to the surface to form new land. One of the first plants to colonize virgin larva flows is a keystone species, the sacred ʻŌhiʻa tree. Strange Hershey-kiss-shaped leaf galls are home to one of some three dozen psyllid species that evolved on the Hawaiian Islands. Tiny sap-sucking nymphs with wing buds will soon mature and pop the top of their gall to emerge as winged adults ready to mate and colonize new ʻŌhiʻa trees. Adult psyllids utilizing ʻŌhiʻa are similar in appearance to the adult boxwood psyllid depicted here next to its shed nymphal skin.
Galls on ʻŌhiʻa are formed by a clan of sucking insects called psyllids, members of a large order of insects known as the Hemiptera – stink bugs, assassin bugs, bed bugs and their pernicious relatives like aphids, whiteflies, adelgids, scale insects, and many others. The tiny gall-makers on ʻŌhiʻa are members of a genus of psyllids named Pariaconus, a taxon with 36 named species on the Hawaiian Islands associated with Metrosideros polymorpha. Many of these psyllids are free-living and don’t form galls, while others form unique and interesting galls on stems and leaves. Why so many species of Pariaconus on but one species of plant? Well, it turns out that ʻŌhiʻa has a remarkable ability to colonize and thrive in many different soil types, zones of varying rainfall, and at many different altitudes found on the Hawaiian Islands. From the time the ancient ancestors of modern day Pariaconus arrived on these islands, which are the most isolated islands on earth, they were able to evolve and diversify in the myriad habitats occupied by Metrosideros polymorpha. These varied and unique niches spawned the great diversity of psyllids seen on the islands today. This form of diversification or radiation of species on islands was made famous by Darwin and his observations of the Galápagos finches in “On the Origin of Species by Means of Natural Selection”.
But all is not well for the iconic and irreplaceable ʻŌhiʻa tree and the insects and other creatures that call it home. While it seems to tolerate its psyllid guests reasonably well, it is threatened by two species of lethal fungi, Ceratocystis lukuohia and Ceratocystis huliohia, causal agents of Rapid ʻŌhiʻa Death (ROD). First identified in 2014 on the big island, by 2019 it had infected more than 175,000 acres of trees. Through activities like moving infected wood, wounding trees with contaminated tools, and walking on or moving infected soil, humans are believed to help spread these fungi. Feral animals moving beneath infected trees and small bark beetles may also spread the disease. Efforts are underway to stop the spread of ROD and save this iconic tree so intertwined with Hawaiian culture and so important to the ecology and biodiversity of these unique and irreplaceable forests.
We thank Dan Gruner for stimulating discussions about the ecology of Hawaiian flora and fauna and Paula Shrewsbury for images used in this episode. The great article “Making the most of your host: the Metrosideros-feeding psyllids (Hemiptera, Psylloidea) of the Hawaiian Islands” by Diana M. Percy was consulted for this episode.
To learn more about ROD and its management, please visit the following websites: