Phoretic and internal transport of Raffaelea lauricola by different species of ambrosia beetle associated with avocado trees

Authors: CRUZ, LUISA - University Of Florida; MENOCAL, OCTAVIO - University Of Florida; Kendra, Paul; CARRILLO, DANIEL - University Of Florida

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/30/2021
Publication Date: 5/10/2021
Citation: Cruz, L. F., O. Menocal, P. E. Kendra, and D. Carrillo. 2021. Phoretic and internal transport of Raffaelea lauricola by different species of ambrosia beetle associated with avocado trees. Symbiosis. https://doi.org/10.1007/s13199-021-00776-2.
DOI: https://doi.org/10.1007/s13199-021-00776-2.

Interpretive Summary: Ambrosia beetles are wood boring insects that feed on symbiotic fungi. These beetles are known to carry fungal spores in specialized storage organs (mycetangia), introduce the spores into host trees, and subsequently farm fungal gardens on which they feed. However, it is possible that spores can also be transported phoretically, carried passively by adhering to the insect’s body (exoskeleton). An ARS scientist (Miami, FL), in collaboration with the University of Florida, initiated a study to determine if the laurel wilt fungus (Raffaelea lauricola) could be transported phoretically by ambrosia beetles in avocado groves. In all five beetle species studied, R. lauricola was recovered from the exoskeleton, confirming phoretic transport. These results improve our understanding of laurel wilt epidemiology, indicating that (1) multiple species can vector the fungal pathogen and (2) transmission can occur by phoresy in addition to via mycetangia, thereby accelerating the spread of laurel wilt in Florida avocado groves.

Technical Abstract: Fungus farming ambrosia beetles carry their nutritional mutualistic fungi in specialized structures called mycetangia. Fungal propagules could also be dispersed phoretically on the beetle’s exoskeleton. In this study we determined the phoretic presence and abundance of Raffaelea lauricola, the causal agent of the laurel wilt disease in avocado, on five ambrosia beetle species. Beetles were collected by three methods: in flight, excavated from logs, and from logs placed in emergence chambers. Beetles were assayed for the presence of internal (gut and mycetangium) and external (attached to the exoskeleton) R. lauricola. The pathogen was isolated from the exoskeleton of all beetle species. The collection method significantly influenced the frequency of pathogen recovery, and the abundance of both internal and phoretic R. lauricola was species-specific. The abundance of internal pathogen propagules was greater than the phoretic propagules. Besides R. lauricola other fungal species including mutualistic, opportunistic, saprophytic, and entomopathogenic fungi, were isolated from the beetle’s exoskeleton. However, phoretic R. lauricola was more prevalent and abundant than any other phoretic fungi across the beetle species. Our results suggest that phoresy is a common mechanism of transportation of wood inhabiting fungi and that phoretic R. lauricola may potentially contribute to LW infection in avocado.