Ambrosia beetles linked to spread of ROD
The first study to implicate ambrosia beetles in Rapid ʻŌhiʻa Death (ROD) has been published by the College of Tropical Agriculture and Human Resources (CTAHR) at the University of Hawai‘i.
The researchers identified Xyleborus ferrugineus, a non-native ambrosia beetle, as one culprit in the spread of Ceratocystis lukuohia during the spring season in the Puna area of Hawai’i Island. C. lukuohia is a tree-colonizing fungus that leads to widespread ROD in ʻōhiʻa lehua trees.
The researchers found the beetle frass contained 62% C. lukuohia DNA and that 17% of the frass had viable fungus spores with the potential to spread to healthy ʻōhiʻa trees. Frass is the sawdust and woody droppings produced by ambrosia beetles and other wood-boring insects when they bore into and colonize trees.
“Other players or species are creating potentially infectious frass as well. Once we have a better handle on what species we are dealing with, we can develop better management strategies," says researcher Kylle Roy, formerly of CTAHR’s Dept. of Plant and Environmental Sciences.
"This study shows the wood dust that ambrosia beetles create, when they attack and burrow inside ROD-killed trees, can contain living C. lukuohia fungal spores and is likely a contributing source of fungal inoculum on Hawai‘i Island,” adds researcher Marc Hughes, also at CTAHR. “Further research is needed to better understand to what extent boring dust plays in the larger context of ROD-induced mortality on the island.”
The researchers, working out of CTAHR’s Komohana Research and Extension Center on Hawai‘i Island, placed 200 frass traps onto ʻōhiʻa trees at four locations on the east side of the island. Each site had verified ROD mortality in the immediate vicinity, and the individual trees chosen for the study also showed symptoms of disease, including wilt, dieback, defoliation and active ambrosia beetle infestation.
The researchers confirmed the presence of C. lukuohia in the collected frass using a molecular test to detect its DNA. They also tested if this frass contained living spores of the C. lukuohia fungus by using a carrot-baiting method. Ambrosia beetle species responsible for producing the frass were identified as genus Xyleborus, with the majority being the non-native X. ferrugineus.
ROD is a recently emerged phenomenon that is decimating ʻōhiʻa lehua trees on Hawaiʻi Island. It is caused by two different fungi, C. lukuohia and C. huliohia. ʻŌhiʻa is the dominant native tree species of the Hawaiian archipelago and is culturally and ecologically important. The disease initially emerged in 2010 in Puna, Hawai‘i Island, and has spread across much of the island. The less virulent C. hulioha has recently been discovered on Kaua‘i.
Roy K, Ewing CP, Hughes MA, Keith L, Bennett GM. Presence and viability of Ceratocystis lukuohia in ambrosia beetle frass from Rapid ʻŌhiʻa Death-affected Metrosideros polymorpha trees on Hawaiʻi Island. For Path. 2018;e12476. https://doi.org/10.1111/efp.12476