Something is killing Guam's nanasu plants, which are often seen in ornamental hedges on the island.
Working with the University of Guam, experts at the University of Florida have identified the tiny bark beetle which is attacking the nanåsu plants, which are also known as beach naupaka or by its scientific name, Scaevola taccada. Nanåsu is a major component of the beach strand community on Pacific Islands where it is also used as an ornamental landscaping plant. Ironically, this same plant species is considered to be an invasive species in Florida and the Caribbean.
A few weeks ago Ilene Quitugua, a contractor who monitors the health of landscape plants for the Guam Visitor's Bureau and Denson Fairfield, of Landscape Management Systems, a company that maintains landscapes for GVB, noticed that nanåsu plants used in ornamental hedges throughout Tumon Bay were starting to die.
The University of Guam College of Natural and Applied Sciences Extension and Outreach was contacted and asked to investigate the cause of the problem and make recommendations for solving it. UOG extension agent Phoebe Wall and UOG entomologist Dr. Aubrey Moore collected plant samples. When dead and dying branches were cut open and examined under a microscope, Moore found many minute bark beetles feeding inside. Images were taken and sent to Dr. Jiri Hulcr at the University of Florida who is a world expert on bark beetles. Dr. Hulcr offered to help identify the beetles and asked for specimens to be sent to him. Getting the scientific name for a pest unlocks the global knowledge base for that species. This allows scientists to answer questions such as:
Is this an invasive species (a species from elsewhere which is likely to cause harm)?
What is the range of plants that the pest feeds on, and what kind of damage does it do?
What methods have been used to successfully control the pest?
Does the pest transmit plant pathogens?
Accurate identification of insect pests is very important and far more difficult than most people expect. Species determination is usually performed by highly skilled taxonomists who specialize in a relatively small group of species. Bark beetles are among the hardest insects to identify. They belong to a family of beetles called Curculionidae, which contains more than 86,000 species and many more yet to be described.
Specimens from Guam arrived at the University of Florida and were identified as Hemicryphalus argutus by Andrew Johnson, a graduate student working in Hulcr's lab. Johnson is the world's leading taxonomist in a huge group of tiny bark beetles known as Cryphalini, or the pygmy borers. Almost nothing is known about this species. The only previous record is from specimens collected on Saipan during 1944 and 1945.
Although this is the first time H. argutus has been observed on Guam, it is probably not an invasive species. Moore thinks it is likely that this is an endemic species, meaning that it evolved on Guam and other Mariana Islands. So why are we seeing what is becoming known as Scaevola dieback? One possible cause to be investigated is plant pathogens. Many species of bark beetles related to H. argutus infect plants with fungus on which they feed. It is the fungus that actually kills the plant, not the tiny beetles.
Or, we may have the wrong suspect. The plot thickens as the Scaevola dieback investigation proceeds.