Identification of the Achilles heels of the laurel wilt pathogen and its beetle vector

Abstract

Ambrosia beetles harbor fungal symbionts that serve as food sources for larvae and adults. These beetles lay their eggs along tunnels in xylem sapwood, which is the substrate for fungal growth. Symbiotic fungi of the genus Raffaelea found in invasive and indigenous ambrosia beetles include the highly virulent plant pathogen Raffaelea lauricola affecting members of the Lauraceae family. R. lauricola is responsible for the deaths of > 500 million trees since 2005. Infection by as few as 100 spores can kill a healthy tree within months. Our data show that R. lauricola is cold-adapted with optimal growth between 16 and 26 °C, with little to no growth at temperatures ≥ 30 °C. The fungus is halophilic and shows a dramatic decrease in growth at pH ≥ 6.8. Fungicide resistance profiling revealed sensitivity of R. lauricola to prochloraz, dichlorofluanid, most conazoles, dithiocarbamates, and zineb (zinc fungicide), whereas the related species Raffaelea arxii showed more limited fungicide sensitivity. Entomopathogenic fungi potentially useful for beetle control were generally highly resistant to most fungicides tested. Coupling pH decreased the concentration for 95% inhibition of fungal growth (IC) of the most potent R. lauricola fungicides by 3-4-fold. Use of avocado bark plug insect bioassays revealed that commercially available Beauveria bassiana can be used as a biological control agent capable of effectively killing the beetle vectors. These data provide simple and practical recommendations to specifically target R. lauricola while having minimal effects on other symbiotic and entomopathogenic fungi, the latter of which can be used to manage the beetle vectors.

Acquisition of fungi from the environment modifies ambrosia beetle mycobiome during invasion

“…Insects, like many other organisms, live in association with bacterial and fungal symbionts (Douglas, 2015; Gurung, Wertheim & Falcao Salles, 2019), which can have a positive (i.e., mutualistic), negative (i.e., parasitic) or neutral (i.e., commensalistic) impact on their host’s fitness. These symbionts can also facilitate (Lu et al, 2010; Himler et al, 2011; Adams et al, 2011; Vilcinskas et al, 2013) or limit (Zhou et al, 2018; Umeda & Paine, 2019) the invasion process of their insect host. When invading a new environment, insects and their microorganisms experience biotic and abiotic forces that can lead to the loss of part of the microbiome (Lester et al, 2017).…”

“…Nonetheless, the spread of several species has been limited by climatic conditions (e.g., humidity, temperature). Exotic ambrosia beetles are indeed able to survive only in areas suitable for the growth of their fungal symbionts (Marini et al, 2011; Rassati et al, 2016a; Rassati et al, 2016b; Zhou et al, 2018; Umeda & Paine, 2019). This scenario could however change if an exotic beetle is able to acquire native fungi from the invaded environment.…”

Host density dependence and environmental factors affecting laurel wilt invasion

“…We used a recursive partitioning algorithm to predict host and environmental factors that were involved in laurel wilt disease incidence levels, and found that increased maximum temperatures, high relative humidities, and high wind speeds along with small stem diameters limited disease incidence within a plot. A recent paper found that found that temperatures above 26°C inhibited colony growth in R. lauricola (Zhou et al 2018). Another recent study found that the vector X. glabratus is sensitive to high temperatures and increased relative humidity, with limited flight activity (Brar et al 2012).…”

Peer Review #2 of "Acquisition of fungi from the environment modifies ambrosia beetle mycobiome during invasion (v0.1)"

“…Nonetheless, the spread of several species has been limited by climatic conditions (e.g., humidity, temperature). Exotic ambrosia beetles are indeed able to survive only in areas suitable for the growth of their fungal symbionts (Marini et al, 2011;Rassati et al, 2016b,a;Zhou et al, 2018;Umeda & Paine, 2019). This scenario could however change if an exotic beetle is able to acquire native fungi from the invaded environment.…”

“…Insects, like many other organisms, live in association with bacterial and fungal symbionts (Douglas, 2015;Gurung, Wertheim & Falcao Salles, 2019), which can have a positive (i.e., mutualistic), negative (i.e., parasitic) or neutral (i.e., commensalistic) impact on their host's fitness. These symbionts can also facilitate (Lu et al, 2010;Himler et al, 2011;Adams et al, 2011;Vilcinskas et al, 2013) or limit (Zhou et al, 2018;Umeda & Paine, 2019) the invasion process of their insect host. When invading a new environment, insects and their microorganisms experience biotic and abiotic forces that can lead to the loss of part of the microbiome (Lester et al, 2017).…”