Fungal symbionts in three exotic ambrosia beetles, Xylosandrus amputatus, Xyleborinus andrewesi, and Dryoxylon onoharaense (Coleoptera: Curculionidae: Scolytinae: Xyleborini) in Florida

The Biology, Impact, and Management of Xyleborus Beetles: A Comprehensive Review

Xyleborus beetles, a diverse group of ambrosia beetles, present challenges to forestry and agriculture due to their damaging burrowing behavior and symbiotic relationships with fungi. This review synthesizes current knowledge on the biology, ecology, and management of Xyleborus. We explore the beetles' life cycle, reproductive strategies, habitat preferences, and feeding habits, emphasizing their ecological and economic impacts. Control and management strategies, including preventive measures, chemical and biological control, and integrated pest management (IPM), are critically evaluated. Recent advances in molecular genetics and behavioral studies offer insights into genetic diversity, population structure, and host selection mechanisms. Despite progress, managing Xyleborus effectively remains challenging. This review identifies future research needs and highlights innovative control methods, such as biopesticides and pheromone-based trapping systems.

Flexibility in the ambrosia symbiosis of Xyleborus bispinatus

Introduction

Ambrosia beetles maintain strict associations with specific lineages of fungi. However, anthropogenic introductions of ambrosia beetles into new ecosystems can result in the lateral transfer of their symbionts to other ambrosia beetles. The ability of a Florida endemic ambrosia beetle, Xyleborus bispinatus, to feed and establish persistent associations with two of its known symbionts (Raffaelea subfusca and Raffaelea arxii) and two other fungi (Harringtonia lauricola and Fusarium sp. nov.), which are primary symbionts of invasive ambrosia beetles, was investigated.

Methods

The stability of these mutualisms and their effect on the beetle's fitness were monitored over five consecutive generations. Surface-disinfested pupae with non-developed mycangia were reared separately on one of the four fungal symbionts. Non-treated beetles (i.e., lab colony) with previously colonized mycangia were used as a control group.

Results

Xyleborus bispinatus could exchange its fungal symbionts, survive, and reproduce on different fungal diets, including known fungal associates and phylogenetically distant fungi, which are plant pathogens and primary symbionts of other invasive ambrosia beetles. These changes in fungal diets resulted in persistent mutualisms, and some symbionts even increased the beetle's reproduction. Females that developed on Fusarium sp. nov. had a significantly greater number of female offspring than non-treated beetles. Females that fed solely on Harringtonia or Raffaelea symbionts produced fewer female offspring.

Discussion

Even though some ambrosia beetles like X. bispinatus can partner with different ambrosia fungi, their symbiosis under natural conditions is modulated by their mycangium and possibly other environmental factors. However, exposure to symbionts of invasive beetles can result in stable partnerships with these fungi and affect the population dynamics of ambrosia beetles and their symbionts.

New species and new records of exotic Scolytinae (Coleoptera, Curculionidae) in Europe

Background

Bark and ambrosia beetles (Coleoptera, Scolytinae) are amongst the most important wood-boring insects introduced to Europe. During field investigations conducted between 2019 and 2021 in different countries and regions of Europe, many exotic species have been recorded providing new and relevant data.

New information

Dryoxylononoharaense (Murayama, 1933) is recorded in Europe for the first time. Xyleborinusattenuatus (Blandford, 1894) is a species new to Italy, while Xylosandrusgermanus (Blandford, 1894), Hypothenemuseruditus (Westwood, 1836) and Amasa sp. near A.truncata are new country records for Portugal. Cnestusmutilatus (Blandford, 1894), Phloeotribusliminaris (Harris, 1852) were collected in Italy and Amasa sp. near A.truncata was collected in France after the first discovery, confirming their establishment and their dispersal into new areas.

A monograph of the Xyleborini (Coleoptera, Curculionidae, Scolytinae) of the Indochinese Peninsula (except Malaysia) and China

The Southeast Asian xyleborine ambrosia beetle fauna is reviewed for the first time. Thirty-four genera and 315 species are reviewed, illustrated, and keyed to genera and species. Sixty-three new species are described: Amasa cycloxyster sp. nov., Amasa galeoderma sp. nov., Amasa gibbosa sp. nov., Amasa lini sp. nov., Amasa tropidacron sp. nov., Amasa youlii sp. nov., Ambrosiophilus caliginestris sp. nov., Ambrosiophilus indicus sp. nov., Ambrosiophilus lannaensis sp. nov., Ambrosiophilus papilliferus sp. nov., Ambrosiophilus wantaneeae sp. nov., Anisandrus achaete sp. nov., Anisandrus auco sp. nov., Anisandrus auratipilus sp. nov., Anisandrus congruens sp. nov., Anisandrus cryphaloides sp. nov., Anisandrus feronia sp. nov., Anisandrus hera sp. nov., Anisandrus paragogus sp. nov., Anisandrus sinivali sp. nov., Anisandrus venustus sp. nov., Anisandrus xuannu sp. nov., Arixyleborus crassior sp. nov., Arixyleborus phiaoacensis sp. nov., Arixyleborus setosus sp. nov., Arixyleborus silvanus sp. nov., Arixyleborus sittichayai sp. nov., Arixyleborus titanus sp. nov., Coptodryas amydra sp. nov., Coptodryas carinata sp. nov., Coptodryas inornata sp. nov., Cyclorhipidion amasoides sp. nov., Cyclorhipidion amputatum sp. nov., Cyclorhipidion denticauda sp. nov., Cyclorhipidion muticum sp. nov., Cyclorhipidion obesulum sp. nov., Cyclorhipidion petrosum sp. nov., Cyclorhipidion truncaudinum sp. nov., Cyclorhipidion xeniolum sp. nov., Euwallacea geminus sp. nov., Euwallacea neptis sp. nov., Euwallacea subalpinus sp. nov., Euwallacea testudinatus sp. nov., Heteroborips fastigatus sp. nov., Heteroborips indicus sp. nov., Microperus latesalebrinus sp. nov., Microperus minax sp. nov., Microperus sagmatus sp. nov., Streptocranus petilus sp. nov., Truncaudum bullatum sp. nov., Xyleborinus cuneatus sp. nov., Xyleborinus disgregus sp. nov., Xyleborinus echinopterus sp. nov., Xyleborinus ephialtodes sp. nov., Xyleborinus huifenyinae sp. nov., Xyleborinus jianghuansuni sp. nov., Xyleborinus thaiphami sp. nov., Xyleborinus tritus sp. nov., Xyleborus opacus sp. nov., Xyleborus sunisae sp. nov., Xyleborus yunnanensis sp. nov., Xylosandrus bellinsulanus sp. nov., Xylosandrus spinifer sp. nov.. Thirteen new combinations are given: Ambrosiophilus consimilis (Eggers) comb. nov., Anisandrus carinensis (Eggers) comb. nov., Anisandrus cristatus (Hagedorn) comb. nov., Anisandrus klapperichi (Schedl) comb. nov., Anisandrus percristatus (Eggers) comb. nov., Arixyleborus resecans (Eggers) comb. nov., Cyclorhipidion armiger (Schedl) comb. nov., Debus quadrispinus (Motschulsky) comb. nov., Heteroborips tristis (Eggers) comb. nov., Leptoxyleborus machili (Niisima) comb. nov., Microperus cruralis (Schedl) comb. nov., Planiculus shiva (Maiti & Saha) comb. nov., Xylosandrus formosae (Wood) comb. nov. Twenty-four new synonyms are proposed: Ambrosiophilus osumiensis (Murayama, 1934) (= Xyleborus nodulosus Eggers, 1941 syn. nov.); Ambrosiophilus subnepotulus (Eggers, 1930) (= Xyleborus cristatuloides Schedl, 1971 syn. nov.); Ambrosiophilus sulcatus (Eggers, 1930) (= Xyleborus sinensis Eggers, 1941 syn. nov.; = Xyleborus sulcatulus Eggers, 1939 syn. nov.); Anisandrus hirtus (Hagedorn, 1904) (= Xyleborus hirtipes Schedl, 1969 syn. nov.); Cnestus protensus (Eggers, 1930) (= Cnestus rostratus Schedl, 1977 syn. nov.); Cyclorhipidion bodoanum (Reitter, 1913) (= Xyleborus misatoensis Nobuchi, 1981 syn. nov.); Cyclorhipidion distinguendum (Eggers, 1930) (= Xyleborus fukiensis Eggers, 1941 syn. nov.; = Xyleborus ganshoensis Murayama, 1952 syn. nov.); Cyclorhipidion inarmatum (Eggers, 1923) (= Xyleborus vagans Schedl, 1977 syn. nov.); Debus quadrispinus (Motschulsky, 1863) (= Xyleborus fallax Eichhoff, 1878 syn. nov.); Euwallacea gravelyi (Wichmann, 1914) (= Xyleborus barbatomorphus Schedl, 1951 syn. nov.); Euwallacea perbrevis (Schedl, 1951) (= Xyleborus molestulus Wood, 1975 syn. nov.; Euwallacea semirudis (Blandford, 1896) (= Xyleborus neohybridus Schedl, 1942 syn. nov.); Euwallacea sibsagaricus (Eggers, 1930) (= Xyleborus tonkinensis Schedl, 1934 syn. nov.); Euwallacea velatus (Sampson, 1913) (= Xyleborus rudis Eggers, 1930 syn. nov.); Microperus kadoyamaensis (Murayama, 1934) (= Xyleborus pubipennis Schedl, 1974 syn. nov.; =Xyleborus denseseriatus Eggers, 1941 syn. nov.); Stictodex dimidiatus (Eggers, 1927) (=Xyleborus dorsosulcatus Beeson, 1930 syn. nov.); Webbia trigintispinata Sampson, 1922 (= Webbia mucronatus Eggers, 1927 syn. nov.); Xyleborinus artestriatus (Eichhoff, 1878) (= Xyelborus angustior [sic] Eggers, 1925 syn. nov.; = Xyleborus undatus Schedl, 1974 syn. nov.); Xyleborinus exiguus (Walker, 1859) (= Xyleborus diversus Schedl, 1954 syn. nov.); Xyleborus muticus Blandford, 1894 (= Xyleborus conditus Schedl, 1971 syn. nov.; = Xyleborus lignographus Schedl, 1953 syn. nov.). Seven species are removed from synonymy and reinstated as valid species: Anisandrus cristatus (Hagedorn, 1908), Cyclorhipidion tenuigraphum (Schedl, 1953), Diuncus ciliatoformis (Schedl, 1953), Euwallacea gravelyi (Wichmann, 1914), Euwallacea semirudis (Blandford, 1896), Microperus fulvulus (Schedl, 1942), Xyleborinus subspinosus (Eggers, 1930).

Four mycangium types and four genera of ambrosia fungi suggest a complex history of fungus farming in the ambrosia beetle tribe Xyloterini

Ambrosia beetles farm fungal cultivars (ambrosia fungi) and carry propagules of the fungal mutualists in storage organs called mycangia, which occur in various body parts and vary greatly in size and complexity. The evolution of ambrosia fungi is closely tied to the evolution and development of the mycangia that carry them. The understudied ambrosia beetle tribe Xyloterini included lineages with uncharacterized ambrosia fungi and mycangia, which presented an opportunity to test whether developments of different mycangium types in a single ambrosia beetle lineage correspond with concomitant diversity in their fungal mutualists. We collected representatives of all three Xyloterini genera (Trypodendron, Indocryphalus, and Xyloterinus politus) and characterized their ambrosia fungi in pure culture and by DNA sequencing. The prothoracic mycangia of seven Trypodendron species all yielded Phialophoropsis (Microascales) ambrosia fungi, including three new species, although these relationships were not all species specific. Indocryphalus mycangia are characterized for the first time in the Asian I. pubipennis. They comprise triangular prothoracic cavities substantially smaller than those of Trypodendron and unexpectedly carry an undescribed species of Toshionella (Microascales), which are otherwise ambrosia fungi of Asian Scolytoplatypus (Scolytoplatypodini). Xyloterinus politus has two different mycangia, each with a different ambrosia fungus: Raffaelea cf. canadensis RNC5 (Ophiostomatales) in oral mycangia of both sexes and Kaarikia abrahamsonii (Sordariomycetes, genus incertae sedis with affinity for Distoseptisporaceae), a new genus and species unrelated to other known ambrosia fungi, in shallow prothoracic mycangia of females. In addition to their highly adapted mycangial mutualists, Trypodendron and X. politus harbor a surprising diversity of facultative symbionts in their galleries, including Raffaelea. A diversity of ambrosia fungi and mycangia suggest multiple ancestral cultivar captures or switches in the history of tribe Xyloterini, each associated with unique adaptations in mycangium anatomy. This further supports the theory that developments of novel mycangium types are critical events in the evolution of ambrosia beetles and their coadapted fungal mutualists.

Plasticity of mycangia in Xylosandrus ambrosia beetles

Insects that depend on microbial mutualists evolved a variety of organs to transport the microsymbionts while dispersing. The ontogeny and variability of such organs is rarely studied, and the microsymbiont's effects on the animal tissue development remain unknown in most cases. Ambrosia beetles (Coleoptera: Curculionidae: Scolytinae or Platypodinae) and their mutualistic fungi are an ideal system to study the animal-fungus interactions. While the interspecific diversity of their fungus transport organ-mycangia-is well-known, their developmental plasticity has been poorly described. To determine the ontogeny of the mycangium and the influence of the symbiotic fungus on the tissue development, we dissected by hand or scanned with micro-CT the mycangia in various developmental stages in five Xylosandrus ambrosia beetle species that possess a large, mesonotal mycangium: Xylosandrus amputatus, Xylosandrus compactus, Xylosandrus crassiusculus, Xylosandrus discolor, and Xylosandrus germanus. We processed 181 beetle samples from the United States and China. All five species displayed three stages of the mycangium development: (1) young teneral adults had an empty, deflated and cryptic mycangium without fungal mass; (2) in fully mature adults during dispersal, the pro-mesonotal membrane was inflated, and most individuals developed a mycangium mostly filled with the symbiont, though size and symmetry varied; and (3) after successful establishment of their new galleries, most females discharged the bulk of the fungal inoculum and deflated the mycangium. Experimental aposymbiotic individuals demonstrated that the pronotal membrane invaginated independently of the presence of the fungus, but the fungus was required for inflation. Mycangia are more dynamic than previously thought, and their morphological changes correspond to the phases of the symbiosis. Importantly, studies of the fungal symbionts or plant pathogen transmission in ambrosia beetles need to consider which developmental stage to sample. We provide illustrations of the different stages, including microphotography of dissections and micro-CT scans.

A selective fungal transport organ (mycangium) maintains coarse phylogenetic congruence between fungus-farming ambrosia beetles and their symbionts

Thousands of species of ambrosia beetles excavate tunnels in wood to farm fungi. They maintain associations with particular lineages of fungi, but the phylogenetic extent and mechanisms of fidelity are unknown. We test the hypothesis that selectivity of their mycangium enforces fidelity at coarse phylogenetic scales, while permitting promiscuity among closely related fungal mutualists. We confirm a single evolutionary origin of the Xylosandrus complex-a group of several xyleborine genera that farm fungi in the genus Ambrosiella. Multi-level co-phylogenetic analysis revealed frequent symbiont switching within major Ambrosiella clades, but not between clades. The loss of the mycangium in Diuncus, a genus of evolutionary cheaters, was commensurate with the loss of fidelity to fungal clades, supporting the hypothesis that the mycangium reinforces fidelity. Finally, in vivo experiments tracked symbiotic compatibility throughout the symbiotic life cycle of Xylosandrus compactus and demonstrated that closely related Ambrosiella symbionts are interchangeable, but the probability of fungal uptake in the mycangium was significantly lower in more phylogenetically distant species of symbionts. Symbiont loads in experimental subjects were similar to wild-caught beetles. We conclude that partner choice in ambrosia beetles is achieved in the mycangium, and co-phylogenetic inferences can be used to predict the likelihood of specific symbiont switches.

Partnerships Between Ambrosia Beetles and Fungi: Lineage-Specific Promiscuity Among Vectors of the Laurel Wilt Pathogen, Raffaelea lauricola

Nutritional mutualisms that ambrosia beetles have with fungi are poorly understood. Although these interactions were initially thought to be specific associations with a primary symbiont, there is increasing evidence that some of these fungi are associated with, and move among, multiple beetle partners. We examined culturable fungi recovered from mycangia of ambrosia beetles associated with trees of Persea humilis (silk bay, one site) and P. americana (avocado, six commercial orchards) that were affected by laurel wilt, an invasive disease caused by a symbiont, Raffaelea lauricola, of an Asian ambrosia beetle, Xyleborus glabratus. Fungi were isolated from 20 adult females of X. glabratus from silk bay and 70 each of Xyleborus affinis, Xyleborus bispinatus, Xyleborus volvulus, Xyleborinus saxesenii, and Xylosandrus crassiusculus from avocado. With partial sequences of ribosomal (LSU and SSU) and nuclear (β-tubulin) genes, one to several operational taxonomic units (OTUs) of fungi were identified in assayed individuals. Distinct populations of fungi were recovered from each of the examined beetle species. Raffaelea lauricola was present in all beetles except X. saxesenii and X. crassiusculus, and Raffaelea spp. predominated in Xyleborus spp. Raffaelea arxii, R. subalba, and R. subfusca were present in more than a single species of Xyleborus, and R. arxii was the most abundant symbiont in both X. affinis and X. volvulus. Raffaelea aguacate was detected for the first time in an ambrosia beetle (X. bispinatus). Yeasts (Ascomycota, Saccharomycotina) were found consistently in the mycangia of the examined beetles, and distinct, putatively co-adapted populations of these fungi were associated with each beetle species. Greater understandings are needed for how mycangia in ambrosia beetles interact with fungi, including yeasts which play currently underresearched roles in these insects.

Human Antimicrobial Peptide Isolated From Triatoma infestans Haemolymph, Trypanosoma cruzi-Transmitting Vector

The importance of antimicrobial peptides (AMPs) in relation to the survival of invertebrates is well known. The source and the mode of action on the insects' immune system of these molecules have been described from different perspectives. Insects produce their own AMPs as well as obtain these molecules from various sources, for example by absorption through the intestinal tract, as previously described for Boophilus microplus. Blood-sucking barber bug Triatoma infestans attracts social, economic and medical interest owing to its role in the transmission of Chagas disease. Despite new studies, descriptions of AMPs from this insect have remained elusive. Thus, the aims of this work were to characterize the antimicrobial potential of human fibrinopeptide A (FbPA) obtained from the T. infestans haemolymph and identify its natural source. Therefore, FbPA was isolated from the T. infestans haemolymph through liquid chromatography and identified by mass spectrometry. This peptide exhibited antimicrobial activity against Micrococcus luteus. Native FbPA from human blood and the synthetic FbPA also exhibited antimicrobial activity. The synthetic FbPA was conjugated with fluorescein isothiocyanate and offered to the insects. The haemolymph collected after 72 h exhibited fluorescence at the same wavelength as fluorescein isothiocyanate. Our experiments show that beyond intrinsic AMP production, T. infestans is able to co-opt molecules via internalization and may use them as AMPs for protection.

Detecting Symbioses in Complex Communities: the Fungal Symbionts of Bark and Ambrosia Beetles Within Asian Pines

Separating symbioses from incidental associations is a major obstacle in symbiosis research. In this survey of fungi associated with Asian bark and ambrosia beetles, we used quantitative culture and DNA barcode identification to characterize fungal communities associated with co-infesting beetle species in pines (Pinus) of China and Vietnam. To quantitatively discern likely symbioses from coincidental associations, we used multivariate analysis and multilevel pattern analysis (a type of indicator species analysis). Nearly half of the variation in fungal community composition in beetle galleries and on beetle bodies was explained by beetle species. We inferred a spectrum of ecological strategies among beetle-associated fungi: from generalist multispecies associates to highly specialized single-host symbionts that were consistently dominant within the mycangia of their hosts. Statistically significant fungal associates of ambrosia beetles were typically only found with one beetle species. In contrast, bark beetle-associated fungi were often associated with multiple beetle species. Ambrosia beetles and their galleries were frequently colonized by low-prevalence ambrosia fungi, suggesting that facultative ambrosial associations are commonplace, and ecological mechanisms such as specialization and competition may be important in these dynamic associations. The approach used here could effectively delimit symbiotic interactions in any system where symbioses are obscured by frequent incidental associations. It has multiple advantages including (1) powerful statistical tests for non-random associations among potential symbionts, (2) simultaneous evaluation of multiple co-occurring host and symbiont associations, and (3) identifying symbionts that are significantly associated with multiple host species.

Fungal Associates of the Xylosandrus compactus (Coleoptera: Curculionidae, Scolytinae) Are Spatially Segregated on the Insect Body

Studies of symbioses have traditionally focused on explaining one-to-one interactions between organisms. In reality, symbioses are often much more dynamic. They can involve many interacting members, and change depending on context. In studies of the ambrosia symbiosis-the mutualism between wood borer beetles and fungi-two variables have introduced uncertainty when explaining interactions: imprecise symbiont identification, and disregard for anatomical complexity of the insects. The black twig borer, Xylosandrus compactus Eichhoff, is a globally invasive ambrosia beetle that infests >200 plant species. Despite many studies on this beetle, reports of its primary symbionts are conflicting. We sampled adult X. compactus and infested plant material in central Florida to characterize the fungal symbiont community using dilution series, beetle partitioning, and DNA-based identification. X. compactus was consistently associated with two fungal taxa, Fusarium spp. and Ambrosiella xylebori Multivariate analyses revealed that A. xylebori was strongly associated with the beetle mycangium while Fusarium spp. were associated with the abdomen and external surfaces. The Fusarium spp. carried by X. compactus are not members of the Ambrosia Fusarium Clade, and are probably not mutualists. Fungal community composition of the mycangium was less variable than external body surfaces, thus providing a more consistent fungal inoculum. This is the first report of spatial partitioning as a mechanism for maintenance of a multimember ambrosia fungus community. Our results provide an explanation for discrepancies among previous reports, and suggest that conflicting results are not due to differences in symbiont communities, but due to inconsistent and incomplete sampling.