Climate change will lead to higher frequencies of natural disasters, including flooding events. Consequently, more severe and widespread attacks by invasive ambrosia beetles associated with stressed ...trees are predicted to occur. In this scenario, understanding which tree species are expected to be more susceptible, as well as which ambrosia beetle species could be expected to have the biggest impacts, represent a research priority. To this aim, potted-trees of eight tree species (four in the order Fagales and four in the order Rosales) were first exposed to flood-stress in a common garden experiment; then emerging insects were counted and logs obtained from infested trees were analyzed through X-ray tomography to evaluate host selection patterns (entry holes), colonization success (percentage of branched galleries and offspring production), and potential damage to the tree (volume of the log occupied by galleries) by native and exotic ambrosia beetle species. Four ambrosia beetle species attacked flood-stressed trees: the native Anisandrus dispar and Xyleborinus saxesenii, and the exotic Xylosandrus crassiusculus and Xylosandrus germanus. Flood-stressed Rosales species were consistently more attacked than Fagales species, with different ambrosia beetle species showing distinct preferences for certain tree species. Rosales were also generally more suitable for ambrosia beetle reproduction than Fagales, even though differences were also evident among tree species within the two orders. The exotic X. crassiusculus was the most successful species, showing higher percentage of branched galleries and higher offspring production. Our study showed a differential susceptibility of common tree species to native and exotic ambrosia beetles in a flood-stress scenario. In addition, it highlighted that X-ray tomography provides a novel method for characterizing the host selection and colonization of trees by ambrosia beetles and can be considered a reliable methodology to improve our comprehension of these mechanisms.
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•Ambrosia beetles are associated to broadleaved trees stressed by flooding.•X-ray tomography can consistently quantify colonization of xylem by ambrosia beetles.•Rosales are more attacked and colonized than Fagales in a common garden experiment.•Xylosandrus crassiusculus showed the highest attack rate and colonization success.
Abstract
Ambrosia beetles are fungal-growing insects excavating galleries deep inside the wood. Their success as invaders increased scientific interest towards them. However, most studies on their ...microbiota targeted their fungal associates whereas the role of bacterial associates is understudied. To explore the role of abundant microbial associates, we isolated bacteria from active galleries of two widespread ambrosia beetles, Xylosandrus crassiusculus and X. germanus. These isolates were classified within the Erwiniaceae family and through a phylogenetic analysis including isolates from other insects we showed that they clustered with isolates obtained from ambrosia and bark beetles, including Erwinia typographi. The whole genome analysis of the isolate from active galleries of X. crassiusculus suggested that this bacterium plays both a nutritional role, by providing essential amino acids and enzymes for the hydrolysis of plant biomass, and a defensive role, by producing antibiotics. This defensive role was also tested in vitro against fungi, including mutualists, common associates, and parasites. The bacteria inhibited the growth of some of the common associates and parasites but did not affect mutualists. Our study supported the hypothesis of a mutualist role of Erwiniaceae bacteria in ambrosia beetles and highlighed the importance of bacteria in maintaining the symbiosis of their host with nutritional fungi.
What is the functional role of an Erwiniaceae bacterium that is part of the core microbiota of ambrosia beetles?
Abstract
Exotic ambrosia beetles (Coleoptera: Curculionidae: Scolytinae), such as Xylosandrus crassiusculus (Motschulsky), Xylosandrus germanus (Blandford), and Xylosandrus compactus (Eichoff) are ...serious pests in southeastern ornamental nurseries. Preventative pyrethroid trunk sprays effectively reduce boring damage. However, it is unclear how pyrethroids such as permethrin prevent attack. Thus, the objective was to determine how permethrin-treated bolts interact with invading ambrosia beetles. In 2022, a study with 2 independent trials was conducted in a nursery on red maple (Acer rubrum L.), bolts during March and April, respectively. The treatments were (i) nonbaited, nontreated bolt, (ii) ethanol baited bolt, (iii) nonbaited bolt + glue painted on bolt, (iv) ethanol baited bolt + glue, (v) ethanol baited bolt + glue + permethrin, (vi) ethanol baited bolt + glue + permethrin + verbenone, and (vii) ethanol baited bolt + glue + verbenone. Ambrosia beetles trapped on glue, beetles which fell into the pail with soap solution under the bolts, and entry holes on bolts were quantified. Permethrin prevented beetle attacks but did not reduce the number of ambrosia beetles landing on the treated bolts. Verbenone reduced ambrosia beetles from landing on the bolts but did not prevent boring into bolts. The numbers of ambrosia beetles in soapy water were not significantly different among treatments. Ambrosia beetles are landing on permethrin-treated bolts but not boring into the bolts, implying that fresh permethrin residues may not be necessary for ambrosia beetle management.
Xylosandrus crassiusculus, a fungus‐farming wood borer native to Southeastern Asia, is the most rapidly spreading invasive ambrosia species worldwide. Previous studies focusing on its genetic ...structure suggested the existence of cryptic genetic variation in this species. Yet, these studies used different genetic markers, focused on different geographical areas and did not include Europe. Our first goal was to determine the worldwide genetic structure of this species based on both mitochondrial and genomic markers. Our second goal was to study X. crassiusculus' invasion history on a global level and identify the origins of the invasion in Europe. We used a COI and RAD sequencing design to characterize 188 and 206 specimens worldwide, building the most comprehensive genetic data set for any ambrosia beetle to date. The results were largely consistent between markers. Two differentiated genetic clusters were invasive, albeit in different regions of the world. The markers were inconsistent only for a few specimens found exclusively in Japan. Mainland USA could have acted as a source for further expansion to Canada and Argentina through stepping stone expansion and bridgehead events. We showed that Europe was only colonized by Cluster II through a complex invasion history including several arrivals from multiple origins in the native area, and possibly including bridgehead from the United States. Our results also suggested that Spain was colonized directly from Italy through intracontinental dispersion. It is unclear whether the mutually exclusive allopatric distribution of the two clusters is due to neutral effects or due to different ecological requirements.
Ethanol emitted by stressed trees is an olfactory cue used by ambrosia beetles (Coleoptera; Curculionidae; Scolytinae) to locate susceptible hosts to colonize. In addition, ethanol enhances the ...growth of ambrosia beetle fungal symbionts, improving colonization. Whether host selection and colonization are affected also by the amount of ethanol produced by stressed trees and by tree species is unclear. To investigate these mechanisms, we induced attacks by ambrosia beetles in bolts of eight tree species by coring and filling them with either 5% or 90% ethanol solutions in water. For each ethanol concentration, bolts of the eight different tree species were replicated six times in a randomized complete block design. Entry holes were used as a proxy for host selection whereas gallery development stage was used as a proxy for colonization. Ethanol concentration differentially affected host selection of the three ambrosia beetles that were active during this study. Anisandrus dispar Fabricius preferentially attacked bolts with 90% ethanol concentration, Xylosandrus crassiusculus (Motschulsky) preferentially attacked bolts with 5% ethanol concentration, and Xyleborinus saxesenii (Ratzeburg) attacked bolts irrespective of ethanol concentration. Colonization of X. crassiusculus reflected the same pattern observed for entry holes. The effect of host tree species on host selection was most prominent for X. saxesenii, while X. crassiusculus established a higher number of developed galleries in Ostrya carpinifolia Scopoli bolts than on five of the other tested tree species. Our results suggest that ethanol concentration and host tree species may influence ecological niche partitioning among ambrosia beetle species.
Xylosandrus crassiusculus
is an invasive ambrosia beetle comprising two differentiated genetic lineages, named cluster 1 and cluster 2. These lineages invaded different parts of the world at ...different periods of time. We tested whether they exhibited different climatic niches using Schoener’s D and Hellinger’s I indices and modeled their current potential geographical ranges using the Maxent algorithm. The resulting models were projected according to future and recent past climate datasets for Europe and the Mediterranean region. The future projections were performed for the periods 2041–2070 and 2071–2100 using 3 SSPs and 5 GCMs. The genetic lineages exhibited different climate niches. Parts of Europe, the Americas, Sub-Saharan Africa, Asia, and Oceania were evaluated as suitable for cluster 1. Parts of Europe, South America, Central and South Africa, Asia, and Oceania were considered as suitable for cluster 2. Models projection under future climate scenarios indicated a decrease in climate suitability in Southern Europe and an increase in North Eastern Europe in 2071–2100. Most of Southern and Western Europe was evaluated as already suitable for both clusters in the early twentieth century. Our results show that large climatically suitable regions still remain uncolonized and that climate change will affect the geographical distribution of climatically suitable areas. Climate conditions in Europe were favorable in the twentieth century, suggesting that the recent colonization of Europe is rather due to an increase in propagule pressure via international trade than to recent environmental changes.
Ambrosia beetles (Xylosandrus spp.) are pests of woody ornamental nurseries and tree nut orchards. Growers use ethanol-infused bolts and ethanol-mediated bottle traps for monitoring ambrosia beetles ...in the spring.To refine these monitoring tools, we investigated the 1) sensitivity of the commercially available ethanol pouches placed on bolts compared with that of ethanol-infused bolts for ambrosia beetle attacks; and 2) the effect of aging of ethanol pouches on ambrosia beetle captures in bottle traps. Experiments were conducted in ornamental nurseries and pecan orchards in 2019 and 2020. For objective 1, the treatments included a bolt infused with locally purchased ethanol, an ethanol pouch attached to a bolt, and a control (bolt without ethanol). For objective 2, ethanol pouches were initially field-aged for 0, 1, 2, 3, and 4 wk and ambrosia beetle captures were documented weekly for 4 wk. Ethanol pouches older than 5 wk were replaced with fresh pouches at the pecan sites while ethanol pouches at nursery sites were aged up to 8 wk.The ethanol-pouch comparison study showed that the number of ambrosia beetle attacks was significantly greater on ethanol-infused bolt traps than with the ethanol pouches on bolts at the nursery and pecan sites in both years.The age of the ethanol pouches did not affect captures of Xylosandrus crassiusculus (Motschulsky) and X. germanus (Blandford) in bottle traps within 8 wk of deployment. Results provide practical implications into the sensitivity of the different ethanol lure types in association with bolt traps and the longevity of the ethanol pouches for grower use.
Exotic Xylosandrus spp. ambrosia beetles established in non-native habitats have been associated with sudden and extensive attacks on a diverse range of living trees, but factors driving their shift ...from dying/dead hosts to living and healthy ones are not well understood. We sought to characterize the role of host physiological condition on preference and colonization by two invaders, Xylosandrus germanus and Xylosandrus crassiusculus. When given free-choice under field conditions among flooded and non-flooded deciduous tree species of varying intolerance to flooding, beetles attacked flood-intolerant tree species over more tolerant species within 3 days of initiating flood stress. In particular, flood-intolerant flowering dogwood (Cornus florida) sustained more attacks than flood-tolerant species, including silver maple (Acer saccharinum) and swamp white oak (Quercus bicolor). Ethanol, a key host-derived attractant, was detected at higher concentrations 3 days after initiating flooding within stems of flood intolerant species compared to tolerant and non-flooded species. A positive correlation was also detected between ethanol concentrations in stem tissue and cumulative ambrosia beetle attacks. When adult X. germanus and X. crassiusculus were confined with no-choice to stems of flood-stressed and non-flooded C. florida, more ejected sawdust resulting from tunneling activity was associated with the flood-stressed trees. Furthermore, living foundresses, eggs, larvae, and pupae were only detected within galleries created in stems of flood-stressed trees. Despite a capability to attack diverse tree genera, X. germanus and X. crassiusculus efficiently distinguished among varying host qualities and preferentially targeted trees based on their intolerance of flood stress. Non-flooded trees were not preferred or successfully colonized. This study demonstrates the host-selection strategy exhibited by X. germanus and X. crassiusculus in non-native habitats involves detection of stress-induced ethanol emission and early colonization of living but weakened trees.
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