The genus Ambrosiella accommodates species of Ceratocystidaceae (Microascales) that are obligate, mutualistic symbionts of ambrosia beetles, but the genus appears to be polyphyletic and more diverse than previously recognized. In addition to Ambrosiella xylebori, Ambrosiella hartigii, Ambrosiella beaveri, and Ambrosiella roeperi, three new species of Ambrosiella are described from the ambrosia beetle tribe Xyleborini: Ambrosiella nakashimae sp. nov. from Xylosandrus amputatus, Ambrosiella batrae sp. nov. from Anisandrus sayi, and Ambrosiella grosmanniae sp. nov. from Xylosandrus germanus. The genus Meredithiella gen. nov. is created for symbionts of the tribe Corthylini, based on Meredithiella norrisii sp. nov. from Corthylus punctatissimus. The genus Phialophoropsis is resurrected to accommodate associates of the Xyloterini, including Phialophoropsis trypodendri from Trypodendron scabricollis and Phialophoropsis ferruginea comb. nov. from Trypodendron lineatum. Each of the ten named species was distinguished by ITS rDNA barcoding and morphology, and the ITS rDNA sequences of four other putative species were obtained with Ceratocystidaceae-specific primers and template DNA extracted from beetles or galleries. These results support the hypothesis that each ambrosia beetle species with large, complex mycangia carries its own fungal symbiont. Conidiophore morphology and phylogenetic analyses using 18S (SSU) rDNA and TEF1α DNA sequences suggest that these three fungal genera within the Ceratocystidaceae independently adapted to symbiosis with the three respective beetle tribes. In turn, the beetle genera with large, complex mycangia appear to have evolved from other genera in their respective tribes that have smaller, less selective mycangia and are associated with Raffaelea spp. (Ophiostomatales).

Recent molecular studies have found that the ambrosia beetle Euwallacea fornicatus Eichhoff (Coleoptera: Curculionidae: Scolytinae) is a complex of cryptic species, each carrying a different species of symbiotic fungus, in the genus Fusarium, which they farm within galleries inside woody hosts. Several of these beetle species have become invasive pests around the world for attacking and infecting healthy trees with their phytopathogenic fungal symbionts. Diet and rearing protocols were developed for two members of the E. fornicatus species complex, polyphagous shot hole borer (PSHB) and tea shot hole borer (TSHB), using sawdust from host trees, allowing collection of data on beetle biology, phenology and sex ratios. Adults developed within 22 days at 24 °C. Single PSHB or TSHB foundresses averaged 32.4 and 24.7 adult female offspring, respectively, and up to 57 and 68 female adults within 6–7 weeks. A strong predictor of the number of offspring in a colony was the number of entry holes. Average sex ratios (% male) for PSHB and TSHB, respectively, were 7.4% and 7.2%. Being haplodiploid, virgin PSHB foundresses were able to produce and mate with male offspring, then subsequently produce female offspring, confirming that they have arrhenotokous reproduction. A cold tolerance study found significant mortality rates among PSHB colonies exposed to −5° or −1 °C but not colonies exposed to 0°, 1° or 5 °C. Given Hamilton's local mate competition (LMC) theory, a number of LMC predictions were violated. PSHB sex ratios were not affected by the number of foundresses; approximately 14% of broods did not contain males; males did not usually eclose before females but eclosed around the same time (22–23 days); and PSHB males were found walking outside of their natal galleries on the trunk of a heavily infested tree in the field. Alternatives to LMC are considered, such as early forms of sociality (maternal care, cooperative brood care), local resource enhancement and kin selection.

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We report that associations between mutualistic fungi and their economically and ecologically important woodwasp hosts are not always specific as was previously assumed. Woodwasps in the genus Sirex engage in obligate nutritional ectosymbioses with two species of Amylostereum, a homobasid\iomycete genus of white rot fungi. In the present study, the Amylostereum species and genotypes associated with three species of Sirex native to eastern North America and one relatively recent invasive Sirex from Europe were investigated by comparing intergenic spacer regions (IGS). Sirex spp. were sampled over 6 years from 23 sites in six US states, ranging from Maine in the northeast to Louisiana in the southeast, to obtain samples of Amylostereum from mycangia of adult females. Two of the native Sirex species (Sirex nigricornis and Sirex nitidus) were associated with either Amylostereum chailletii or Amylostereum areolatum, refuting the hypothesis of strict species-specific relationships. However, the invasive Sirex noctilio and the native Sirex cyaneus were each collected with only A. areolatum or A. chailletii, respectively, although S. noctilio was associated with two different IGS genotypes of A. areolatum and S. cyaneus occurs sympatrically with the other native Sirex. In Pinus, the preferred host tree of S. nigricornis and S. noctilio, these species co-occurred in 25.9 % of trees sampled, and horizontal transmission of fungal strains from S. noctilio to S. nigricornis was documented, although only in one tree. The extent that further spread and establishment of S. noctilio will alter the composition of symbionts carried by native Sirex is unknown but will depend in part on the degree of flexibility in these host–symbiont associations.

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We provide an overview of both traditional and innovative control tools for management of three Xylosandrus ambrosia beetles (Coleoptera: Curculionidae: Scolytinae), invasive species with a history of damage in forests, nurseries, orchards and urban areas. Xylosandrus compactus, X. crassiusculus and X. germanus are native to Asia, and currently established in several countries around the globe. Adult females bore galleries into the plant xylem inoculating mutualistic ambrosia fungi that serve as food source for the developing progeny. Tunneling activity results in chewed wood extrusion from entry holes, sap outflow, foliage wilting followed by canopy dieback, and branch and trunk necrosis. Maintaining plant health by reducing physiological stress is the first recommendation for long-term control. Baited traps, ethanol-treated bolts, trap logs and trap trees of selected species can be used to monitor Xylosandrus species. Conventional pest control methods are mostly ineffective against Xylosandrus beetles because of the pests’ broad host range and rapid spread. Due to challenges with conventional control, more innovative control approaches are being tested, such as the optimization of the push–pull strategy based on specific attractant and repellent combinations, or the use of insecticide-treated netting. Biological control based on the release of entomopathogenic and mycoparasitic fungi, as well as the use of antagonistic bacteria, has yielded promising results. However, these technologies still require validation in real field conditions. Overall, we suggest that management efforts should primarily focus on reducing plant stress and potentially be combined with a multi-faceted approach for controlling Xylosandrus damage.

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Ambrosia beetles Xylosandrus crassiusculus and X. germanus are among the most important exotic pests of orchards and nurseries in the US and are difficult to control using conventional insecticides because of their cryptic habits. The use of biological control agents may prove effective by targeting both beetles and fungal symbionts inside tree galleries: entomopathogenic fungi could be used to target beetle foundresses and their brood, or mycoparasitic fungi, e.g., Trichoderma harzianum, could be used to target their associated fungal symbionts. We used a combination of in vitro assays and beetle bioassays to examine competition between symbionts and biological control fungi and the impact of biological control fungi on beetle brood production. The in vitro assays showed T. harzianum outcompeted different strains of Ambrosiella roeperi and A. grosmanniae associated with X. crassiusculus and X. germanus, respectively, whether in primary or secondary resource capture assays. In contrast, entomopathogenic fungi Beauveria bassiana and Metarhizium brunneum blocked the spread of symbionts only in primary competition assays. Complementary beetle bioassays showed that beetle galleries in T. harzianum-treated beech stems had sparse symbiont growth, many with no or only a small number of eggs present. Brood numbers produced by foundresses in T. harzianum-treated stems were comparable to those in stems treated with either entomopathogen at the higher dosages, in which brood reduction was likely due to foundress mortality prior to laying eggs or after laying only a small number of eggs. These results show the potential of using biological control fungi in targeting ambrosia beetle populations either directly by killing foundresses and reducing brood production or indirectly by suppressing symbiont growth in their galleries.