Diseases and Vectors

A vector is a living organism that transmits an infectious agent from one organism to another. Introduced organisms and diseases can work together to devastate indigenous populations that are defenseless against new pathogens.

The ‘ōhia tree is the most important tree in Hawai‘i. It’s one of the first trees to colonize new lava, and makes up over 80% of the native forests from sea level to high in the mountains. Many birds, snails, and plants depend on the the ‘ōhia for survival. It’s especially important for capturing rainfall and replenishing the underground aquifers.

Rapid ‘ōhia death (R.O.D.) is a fungal disease that is spreading through the native Hawaiian forests. A tree can go from healthy looking to dead in weeks. Two species of fungus Ceratocystis huliohia and Ceratocystis lukuohia cause different symptoms, but are both deadly to the ‘ōhia tree. The fungus infects the core of the ‘ōhia tree, and kills it by blocking the uptake of water.

The fungal spores are spread from one tree to another when an invasive boring ambrosia beetle (Xyleborus ferrugineus) chews into the infected wood of a dead tree. The beetle pushes out sawdust, called frass, containing beetle waste and fungus spores. The spores are carried by the wind, water, animals, or people to infect neighboring trees. This is why it’s important to always wash your shoes before and after going into the native forest.

The relationship between the mosquito and spotted dove (Spilopelia chinensis) is another example of two introduced species causing devastating effects. In Hawai‘i, avian malaria is caused by Plasmodium relictum a parasitic microorganism. It’s transmitted by the invasive southern house mosquito (Culex quinquefasciatus).

The spotted dove was brought to the Hawaiian Islands from Asia around 1860, and continued to be released in the Islands by State agencies until the 1960s. The spotted dove was both kept in captivity for food, and released into the wild as a game bird for hunting. The spotted dove is suspected to have introduced avian malaria to the Hawaiian Islands, and is still a reservoir for the disease.

Avian malaria is especially devastating to indigenous Hawaiian birds. Native birds are almost totally absent from lower elevations where mosquitos are. They have found relative refuge in the cold mountain forests, but climate change is expanding the areas where mosquitos can exist. Warmer weather allows mosquitos to live at higher elevations. Efforts to control mosquito populations are ongoing.