Spore Acquisition and Survival of Ambrosia Beetles Associated with the Laurel Wilt Pathogen in Avocados after Exposure to Entomopathogenic Fungi

Abstract

Laurel wilt is a disease threatening the avocado industry in Florida. The causative agent of the disease is a fungus vectored by ambrosia beetles that bore into the trees. Until recently, management strategies for the vectors of the laurel wilt fungus relied solely on chemical control and sanitation practices. Beneficial entomopathogenic fungi (EPF) are the most common and prevalent natural enemies of pathogen vectors. Laboratory experiments demonstrated that commercial strains of EPF can increase the mortality of the primary vector, Xyleborus glabratus, and potential alternative vectors, Xylosandrus crassiusculus, Xyleborus volvulus and Xyleborus bispinatus (Coleoptera: Curculionidae: Scolytinae). Our study provides baseline data for three formulated commercially-available entomopathogenic fungi used as potential biocontrol agents against X. crassiusculus, X. volvulus and X. bispinatus. The specific objectives were to determine: (1) the mean number of viable spores acquired per beetle species adult after being exposed to formulated fungal products containing different strains of EPF (Isaria fumosorosea, Metarhizium brunneum and Beauveria bassiana); and (2) the median and mean survival times using paper disk bioassays. Prior to being used in experiments, all fungal suspensions were adjusted to 2.4 × 106 viable spores/mL. The number of spores acquired by X. crassiusculus was significantly higher after exposure to B. bassiana, compared to the other fungal treatments. For X. volvulus, the numbers of spores acquired per beetle were significantly different amongst the different fungal treatments, and the sequence of spore acquisition rates on X. volvulus from highest to lowest was I. fumosorosea > M. brunneum > B. bassiana. After X. bispinatus beetles were exposed to the different suspensions, the rates of acquisition of spores per beetle amongst the different fungal treatments were similar. Survival estimates (data pooled across two tests) indicated an impact for each entomopathogenic fungus per beetle species after exposure to a filter paper disk treated at the same fungal suspension concentration. Kaplan–Meier analysis (censored at day 7) revealed that each beetle species survived significantly shorter in bioassays containing disks treated with EPF compared to water only. This study demonstrated that ambrosia beetles associated with the laurel wilt pathogen in avocados are susceptible to infection by EPF under laboratory conditions. However, the EPF needs to be tested under field conditions to confirm their efficacy against the beetles.

Recent advances toward the sustainable management of invasive Xylosandrus ambrosia beetles

“…Moreover, another entomopathogenic fungus, Isaria fumosorosea Wize (strain TR-78-3) (Ascomycota: Hypocreales), significantly reduced survival of X. compactus in laboratory trials both after direct application to the beetles and indirect treatment of hazelnut branches (Kushiyev et al 2018). Avery et al (2018) confirmed the susceptibility of X. crassiusculus to B. bassiana, I. fumosorosea and M. brunneum commercial strains, but spore acquisition by treated beetles was significantly higher after exposure to B. bassiana compared to the other two fungi. Mukasa et al (2019) observed more than 70% mortality of X. compactus in laboratory and field tests with an atoxigenic L-strain of Aspergillus flavus Link (Eurotiales: Trichocomaceae) isolated from Robusta coffee (Coffea canephora Pierre ex A. Froehner) farms in Uganda.…”

Microbial Biocontrol Strategies for Ambrosia Beetles and Their Associated Phytopathogenic Fungi

“…The cryptic habit of the beetles warrants the repeated application of contact insecticides to avoid infestation and of systemic insecticides with long residual efficacy to kill those beetles already inside the trees (Paine et al, 2011;Castrillo et al, 2016;Eatough Jones and Paine, 2018). Early detection and sanitation measures are also implemented to dispose of infested wood and limit the spread of wood borers (Eatough Jones and Paine, 2015;Avery et al, 2018). A wide array of systemic fungicides has been tested to reduce the spread of ambrosia fungi, propiconazole being one of the most promising active ingredients so far (Mayfield et al, 2008;Freeman et al, 2012).…”

Characterization of the Exo-Metabolome of the Emergent Phytopathogen Fusarium kuroshium sp. nov., a Causal Agent of Fusarium Dieback

“…However, to achieve this, more information about the mechanisms of pathogenicity of this species is required. In this sense, it is assumed that the fungus is vectored by the insect, thus gaining free access to the host’s xylem [ 13 ], and FD is the consequence of fungal mass accumulation on stressed trees, among other possibilities [ 4 ]. Since Fusarium comprises many species that cause human and plant diseases worldwide [ 14 ], an important question would be to determine its contribution to pathogenesis.…”