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

Abstract

Fungus farming ambrosia beetles carry their nutritional mutualistic fungi in specialized structures called mycetangia. Fungal propagules are also dispersed phoretically on the beetle’s exoskeleton. 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: Xyleborus bispinatus, Xyleborus volvulus, Xyleborus affinis, Xyleborinus saxesenii and Xylosandrus crassiusculus. Beetles were captured while in flight, excavated from logs, and from logs placed in emergence chambers. Beetles collected by the three methods were assayed for the presence of internal (gut and mycetangium) and external (attached to the exoskeleton) colony forming units (CFUs) of R. lauricola. The pathogen was recovered 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. Internal CFUs recovery was greater than phoretic recovery. Besides R. lauricola, other cycloheximide tolerant fungi, including mutualistic and entomopathogenic fungi, were isolated from the beetle’s exoskeleton. However, phoretic CFUs of R. lauricola were more prevalent and abundant than any other mutualistic phoretic fungi across the beetle species. Our results suggest that phoresy is a common mechanism of transportation of wood-inhabiting fungi and that phoretic transmission of R. lauricola may potentially contribute to the infection in avocado.