Juan Carlos Cambronero-Heinrichs

Wood-boring ambrosia beetles and their offspring obligately depend on cultivated fungi that they maintain within host trees. Stressed trees produce at least two potent antimicrobials, ethanol and acetic acid, which typically suppress the growth of most fungi. Ethanol facilitates the growth of ambrosia beetles’ fungal mutualists and thus aids in host colonization; however, acetic acid’s effects are unknown. Here, we evaluated the effects of acetic acid on tunneling and offspring production by two exotic ambrosia beetle species, Xylosandrus germanus and Anisandrus maiche . Cut wood stems (i.e., bolts) were infused with water alone, 5% ethanol, or mixtures of 5% ethanol with acetic acid at various dilutions. Infestations by X. germanus and A. maiche resulted in more frass, larvae, pupae, and adults in the combined 5% ethanol-acetic acid treatment, than in the 5% ethanol alone or water control treatments. X-ray micro-computed tomography revealed that bolts infused with this 5% ethanol-acetic acid mixture had larger insect gallery volumes than those with only 5% ethanol, which in turn were larger than the water control. A positive relationship was found between frass ejected by each beetle and gallery volume across all experiments. This study demonstrates that acetic acid and ethanol synergistically increase offspring production while facilitating gallery expansion by the larvae. Understanding how ambrosia beetles utilize stress-induced compounds within host trees to benefit their fungiculture could lead to novel management strategies for these invasive insects.

Microbial-fungal symbioses are fundamental drivers of ecological networks and agricultural systems, orchestrating critical processes such as nutrient cycling, maintenance of soil and plant health, and enhancement of host resilience across diverse environments (Martin & Van Der Heijden 2024;Williams et al. 2023). These interactions, spanning a continuum from mutualism to commensalism and antagonism, involve fungi engaging with taxonomically diverse partners (Martin & Tan 2025). Under the pressures of global environmental change, land degradation, and intensifying agricultural demands, understanding the mechanistic basis and ecological consequences of these interactions has become increasingly urgent and a clear research priority This Research Topic aims to consolidate pioneering research focused on the mechanistic basis and diversity of microbial-fungal partnerships, addressing key questions related to the molecular and ecological mechanisms that regulate microbiome-host interactions. The 12 publications included in this Research Topic have expanded our understanding of the role of these across diverse contexts, including medicinal plants, fruit crops, hyphosphere-associated bacteria, and rhizosphere and endophytic interactions. The present Research Topic reinforces the complexity of microbial-fungal symbioses across a wide range of ecological and applied contexts, from plant-associated microbiomes to soil and insect systems. The published studies demonstrate that inter-kingdom interactions are central to regulating diverse functions, including plant fitness, metabolic dynamics, ecosystem resilience, and responses to environmental and anthropogenic pressures.The findings presented here provide new conceptual and methodological insights for understanding how microbial consortia can be explored to improve agricultural sustainability and ecosystem restoration. Future research should further elucidate the molecular and ecological basis regulating these interactions, with a focus on translating these findings into scalable biotechnological applications.

Ambrosia beetles (Curculionidae: Scolytinae and Platypodinae) are wood-boring insects studied as examples of fungus-insect symbiosis and for their success as invasive species. While most research on their microbiota has focused on fungal associates, their bacterial communities remain largely understudied. In this review, we synthesize current knowledge on the bacterial microbiota of ambrosia beetles, identify critical gaps in the field, and provide recommendations for future research. To date, eight metabarcoding studies have explored bacterial communities in ambrosia beetles, analyzing a total of 13 species, mostly within the tribe Xyleborini (Scolytinae). These studies have examined the presence of bacteria in ambrosia beetle mycetangia, organs specialized for transporting fungal symbionts, as well as bacterial diversity in fungal gardens and whole beetles, across different life stages, and under varying environmental conditions. In general, bacterial communities appear to be highly specific to the beetle species, and differ between the beetles and their fungal gardens. Most studies employed 16S rRNA gene metabarcoding, and the optimal primer combination for characterizing bacterial communities in environmental samples is 515F/806RB (V4). Various methods for collecting beetles have been used, such as ethanol-baited traps, direct collection from galleries, logs kept in emergence cages, and rearing, but which of them to select when planning a study depends on the specific aim. A significant knowledge gap remains regarding the functional roles of dominant bacterial taxa, as metabarcoding studies often assume that these roles are similar to those played in other beetle species, such as bark beetles. More studies should be conducted to test hypotheses regarding the various factors influencing microbial composition and function, and advanced molecular techniques, including (meta-) genome and transcriptome sequencing, which have been employed in only a limited number of studies, could offer great potential to help bridging this knowledge gap.

Ambrosia beetles (Curculionidae: Scolytinae and Platypodinae) are fungus-farming woodborers that can cause damage to the trees they colonize. Some of these beetles target stressed plants that emit ethanol, and management strategies have proposed using ethanol-injected trees as trap trees to monitor or divert dispersing adult females away from valuable crops. In this study, we used container-grown trees from 8 species to compare the effect of ethanol injection versus flooding on ambrosia beetle host selection and colonization success. Our aims were to understand whether ethanol injection is a suitable technique for different ambrosia beetle species and whether its effectiveness varies depending on the tree species used. In addition, we quantified the amount of ethanol in tree tissues to understand whether ethanol concentration could reflect observed differences among treatments and tree species. Our findings demonstrated that ethanol-injected trees were significantly more selected by both Xyleborinus saxesenii and Xylosandrus spp. and that significantly more adult beetles of both taxa emerged from ethanol-injected than flood-stressed trees. In addition, we showed that ethanol injection can trigger attacks by X. saxesenii and Xylosandrus spp. on a variety of deciduous tree species, nullifying the effects of the species-specific characteristics observed on flood-stressed trees, which can only partially be attributed to the amount of ethanol within the plant. This supported the idea that practitioners can potentially select any species of deciduous trees in management programs for ambrosia beetles based on ethanol-injected trees.

Despite the benefits derived from the use of pharmaceuticals, these compounds are currently considered contaminants of emerging concern because of their presence and persistence in the environment. This study aimed to determine the toxicity of 27 pharmaceuticals and the interaction effects of binary mixtures of selected compounds towards two model organisms: the microcrustacean Daphnia magna and the bacterium Aliivibrio fischeri (Microtox test). Six compounds, namely polymyxin B, polymyxin E, fluoxetine, diphenhydramine, clenbuterol and ketoprofen exhibited moderate toxicity towards D. magna. Additionally, three compounds (cefotaxime, polymyxin B, polymyxin E) also showed a moderate toxic effect on A. fischeri. The comparison of such results with model estimations showed inaccuracy in the predicted data, highlighting the relevance of experimental ecotoxicological assays. The assayed mixtures contained four selected drugs of high-hazard according to their reported concentrations in wastewater and surface water (diphenhydramine, trimethoprim, ketoprofen, and fluoxetine); data revealed interactions only in the fluoxetine-containing mixtures for D. magna, while all mixtures showed interactions (mostly synergistic) for Microtox. Chronic effects on the reproduction of D. magna were observed after exposure to fluoxetine and diphenhydramine, although higher sensitivity was determined for the latter, while the mixture of these compounds (which showed acute synergy in both models) also affected the reproduction patterns. Nonetheless, all the effects described at the acute or chronic level (for individual compounds or mixtures) were determined at concentrations higher than commonly reported at environmental levels. This work provides valuable ecotoxicological information for the risk assessment of pharmaceuticals and their mixtures in the environment.

Chagas disease, caused by the protozoan Trypanosoma cruzi, is a zoonosis primarily found in rural areas of Latin America. It is considered a neglected tropical disease, and Triatoma dimidiata is the main vector of the parasite in Central America. Despite efforts, Chagas disease continues to be a public health concern, and vector control remains a primary tool to reduce transmission. In this study, we tested the hypothesis that highly abundant bacteria in the gut of T. dimidiata inhibit the growth of T. cruzi. To achieve this, bacterial diversity in the gut of T. dimidiata specimens from Costa Rica was characterized by metabarcoding of the 16S rRNA, microbial isolation was performed, and the effect of freeze-dried supernatants of the isolates on T. cruzi was investigated. Metabarcoding showed that the most abundant genera in the gut were Corynebacterium, Tsukamurella, Brevibacterium, and Staphylococcus. Barcoding and sequences comparison confirmed that 8 of the 30 most abundant amplicon sequence variants (ASVs) were isolated, and 2 of them showed an inhibitory effect on the growth of T. cruzi epimastigotes. These bacteria correspond to isolates of Tsukamurella and Brevibacterium, which were respectively the second and sixth most abundant ASVs in the gut of T. dimidiata. Notably, only the isolate of Brevibacterium showed a significant difference in growth inhibition against epimastigotes of both T. cruzi strains tested. These findings suggest that the gut microbiota of T. dimidiata may play an active role in modulating parasite development.

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.

I am not that special. Others dream of becoming scientists and taking the academic path to pursue a doctoral degree. Now, I am a Ph.D. student in beautiful Veneto, Italy, in an entomology group at the University of Padova. I am part of a Crop Science Ph.D. school, where my project focuses on the microbiology of ambrosia beetles. Obtaining the position was not easy. I first attempted to enter graduate school in 2017, and it was not until 2020 that I moved to study in Europe. Experiencing rejection was disappointing, but the application process became more familiar with each attempt. To help reassure others going through this process, I will share my reflections on my setbacks in obtaining a Ph.D. position.

Río Celeste (“Sky-Blue River”) is a river located in the Tenorio National Park (Costa Rica) that has become an important hotspot for eco-tourism due to its striking sky-blue color. A previous study suggested that this color is not caused by dissolved chemical species, but by precipitation of lightscattering aluminosilicate particles at the mixing point of two colorless streams, the acidic Quebrada Agria and the neutral Río Buenavista. We now present microbiological information on Río Celeste and its two tributaries, as well as a more detailed characterization of the particles that occur at the mixing point. Our results overturn the previous belief that the light scattering particles are formed by the aggregation of smaller particles coming from Río Buenavista, and rather point to chemical formation of hydroxyaluminosilicate colloids with Quebrada Agria as the main contributor to the phenomenon, with Río Buenavista acting as a secondary source of silica to the reaction. We also show how the sky-blue color of Río Celeste arises from the tight interaction between chemical and biological processes. Sulfur-oxidizing bacteria generate an acidic environment in Quebrada Agria, which in turn cause dissolution and mobilization of aluminum and other metals, while in Río Buenavista the growth of diatoms transforms dissolved silicon into colloidal biogenic forms. The local interaction between these two well-known biological activities gives rise to the unique Río Celeste phenomenon, in what constitutes a textbook example of emergent behavior in environmental microbiology.