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.
Highlights
Ambrosia beetles have symbiotic relationships with fungi that contribute to their ecological, biological and economic importance [1,2,3]
This study demonstrated that ambrosia beetles associated with the laurel wilt pathogen in avocados are susceptible to infection by entomopathogenic fungi (EPF) under laboratory conditions
This study demonstrated the efficacy of entomopathogenic fungi (EPF) against adult X. bispinatus and X. volvulus and assessed their actual dose delivered by immersion
Summary
Ambrosia beetles have symbiotic relationships with fungi that contribute to their ecological, biological and economic importance [1,2,3]. Ambrosia beetles associated with plant pathogenic fungi that cause diseases, such as Raffaelea lauricola T.C. Harr., Fraedrich & Aghayeva and Fusarium spp., are becoming relevant pests for the forestry and agricultural sector in many countries [4,5,6]. Ambrosia beetles vector the fungus R. lauricola, which can cause laurel wilt disease in redbay, Persea borbonia (L.), silk bay, P. humilis Nash, swamp bay, P. palustris (Raf.) Spreng., and avocado, Persea americana Mill. The ability of some ambrosia beetles to act as vectors of plant pathogenic fungi in trees has brought much attention for finding management strategies that could reduce the impact of these insects
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