Population divergence of aggregation pheromone responses in Ips subelongatus in northeastern China

Performance of two Ips bark beetles and their associated pathogenic fungi on hosts reflects a species-specific association in the beetle-fungus complex

When Ips bark beetles invade and colonize the host plants, their associated pathogenic fungal partners are carried into the phloem of the host trees. Host trees are lethally attacked by the beetle-fungus complex and the collective damage severely limits forestry production worldwide. It is of great importance to verify whether bark beetles and their associated fungi show concordant performance in terms of biology, physiology, and biochemistry on host trees. In this study, the two Ips bark beetles Ips typographus and Ips subelongatus (Coleoptera: Curculionidae, Scolytinae), their respective associated pathogenic fungi Endoconidiophora polonica and Endoconidiophora fujiensis, and their respective host plants Picea jezoensis and Larix olgensis were selected as test material. Cross-inoculation experiments were conducted indoors and outdoors to investigate the differences in reproduction and development of two beetles and infectivity of two fungi on two plants, as well as the differences in physiological responses of two plants to two fungal infections. The results showed that I. typographus and E. polonica had excellent host performance on P. jezoensis; however, neither successfully colonized and infected L. olgensis. In contrast, I. subelongatus and E. fujiensis showed strong host suitability on L. olgensis and some degree of suitability on P. jezoensis, although the host suitability of P. jezoensis for E. polonica was significantly higher than that for E. fujiensis. In addition, we found that the absolute amount of ergosterol accumulated on the lesion was positively correlated with lesion area. The ergosterol amount and lesion area were both strongly correlated with the release of host monoterpenes, but had no obvious correlation with the concentration of fungi-induced phenols on the lesion area and the side-chain oxidation of lignin in the xylem of the infected sites. Based on these results, we confirmed that "I. typographus-E. polonica" and "I. subelongatus-E. fujiensis" complexes both showed the most suitable consistent performances on their own traditional hosts, establishing a stable species-specific association relationship in these two beetle-fungus complexes, with the "I. subelongatus-E. fujiensis" complex showing broader host suitability. From the perspective of physiological responses of plants to fungal infections, monoterpenes are an important indicator of host suitability.

Insights into the Divergence of Chinese Ips Bark Beetles during Evolutionary Adaptation

Simple Summary

Bark beetle species of the genus Ips are among the major pests of Chinese conifer forests. Based on mitochondrial genome and SNP, we investigated the phylogenetic relationships and evolutionary trends of 19 populations of six Ips species that had serious outbreaks in recent years. Our results demonstrated the relationships between Ips evolution and host plants, pheromones, and altitudinal differences, and provided new insights into the mechanism of adaptive evolution of Ips bark beetles.

Abstract

Many bark beetles of the genus Ips are economically important insect pests that cause severe damage to conifer forests worldwide. In this study, sequencing the mitochondrial genome and restriction site-associated DNA of Ips bark beetles helps us understand their phylogenetic relationships, biogeographic history, and evolution of ecological traits (e.g., pheromones and host plants). Our results show that the same topology in phylogenetic trees constructed in different ways (ML/MP/BI) and with different data (mtDNA/SNP) helps us to clarify the phylogenetic relationships between Chinese Ips bark beetle populations and Euramerican species and their higher order clades; Ips bark beetles are polyphyletic. The structure of the mitochondrial genome of Ips bark beetles is similar and conserved to some extent, especially in the sibling species Ips typographus and Ips nitidus. Genetic differences among Ips species are mainly related to their geographic distribution and different hosts. The evolutionary pattern of aggregation pheromones of Ips species reflects their adaptations to the environment and differences among hosts in their evolutionary process. The evolution of Ips species is closely related to the uplift of the Qinghai-Tibet Plateau and host switching. Our study addresses the evolutionary trend and phylogenetic relationships of Ips bark beetles in China, and also provides a new perspective on the evolution of bark beetles and their relationships with host plants and pheromones.

Ophiostomatoid fungi including a new species associated with Asian larch bark beetle Ips subelongatus, in Heilongjiang (Northeast China)

Ips subelongatus (Coleoptera, Scolytinae) is an important bark beetle species that infests Larix spp. in Asia. Individuals of this beetle are vectors of ophiostomatoid fungi, on their exoskeletons, that are transmitted to infested trees. In this study, the symbiotic assemblage of ophiostomatoid fungi associated with I. subelongatus in Northeast China was studied. Fungal isolates were identified based on their morphological characters and sequences of ITS, beta-tubulin, elongation factor 1-alpha and calmodulin gene regions. In total, 48 isolates were collected and identified, residing in six taxa. These included a novel species, described here as Ophiostoma gmelinii sp. nov.

Chemical signal interactions of the bark beetle with fungal symbionts, and host/non-host trees

The symbiosis between the bark beetle (Ips subelongatus) and its fungal symbiont (Endoconidiophora fujiensis) poses a serious threat to larch forests. However, the signaling pathways between these symbiotic partners and their host/non-host trees are not fully understood. Inoculation of the host larch (Larix principis-rupprechtii) with two strains of E. fujiensis induced a rapid and long-term release of monoterpenes. Although the fungi had a level of tolerance to these compounds, many monoterpenes inhibited fungal growth in culture. Moreover, monoterpenes with stronger inhibitory effects on fungal growth exhibited weaker synergistic effects on the attraction of I. subelongatus to aggregation pheromone. Surprisingly, individual isomers of aggregation pheromone components promoted fungal symbiont growth in a culture medium. Non-host volatiles (NHVs) were tested and shown to completely inhibit the growth of fungal symbionts in culture but had no effects on beetle responses to aggregation pheromone, with the exception of (Z)-3-hexen-1-ol. These results reveal convergence and mutualism patterns in the evolution of I. subelongatus and E. fujiensis with respect to host tree volatiles but not in response to NHVs. Ultimately, we put forward a hypothesis that host plants are ecological and evolutionary determinants of bark beetle-fungus symbioses in terms of their complex signaling interactions.

Ophiostomatoid fungi associated with Ips subelongatus, including eight new species from northeastern China

Ips subelongatus is a major pest that infects larch plantations over large areas of northern and northeastern China. Ips species are closely associated with ophiostomatoid fungi that are morphologically well-adapted for dispersal by beetles. These associations result in important threat for coniferous forests worldwide. The aim of this study was to characterize the ophiostomatoid communities associated with I. subelongatus infesting Larix species and sympatric Pinus sylvestris var. mongolica in northeastern China forests. Morphological and multilocus phylogenetic approaches (based on six markers: ITS, LSU, 60S, β-tubulin, EF-1α, and CAL gene regions) allowed identifying 14 species of four genera (Ceratocystiopsis, Endoconidiophora, Leptographium and Ophiostoma). Eight species are showed to be new to science. Most strains resided in two Ophiostoma species complexes, viz. the O. clavatum and the O. ips complexes, all together accounting for 76.8% of all isolates. Ophiostoma hongxingense sp. nov., O. peniculi sp. nov., and O. subelongati sp. nov. (O. clavatum complex) and O. pseudobicolor sp. nov. (O. ips complex) were the four dominant species. The ophiostomatoid communities associated with larch bark beetles, I. cembrae and I. subelongatus, in Europe and Asia, China and Japan, also were compared. These comparisons showed distinct, specific assemblage patterns.

Forecasting Spring Flight of Ips subelongatus (Coleoptera: Curculionidae: Scolytinae) in Japanese larch Larix kaempferi (Pinales: Pinaceae) Forests in the Republic of Korea

Ips subelongatus is a species of bark beetle experiencing population outbreaks in Korea. In this study, a predictive model and empirical prediction were used to forecast the spring flight of these beetles in Japanese larch forests. The number of beetles caught in pheromone traps was investigated in larch forests thinned in 2009, 2010, 2012, or 2013. Data from the sites thinned in 2009, 2010, or 2012 were used in the predictive model based on a degree-day model that was validated using data from the site thinned in 2013. The lower threshold temperature for flight (LTF) and a thermal sum for the spring flight of I. subelongatus were estimated. The empirical prediction that beetles initiate their flight when daily maximum temperatures reach 16 or 20°C was tested using daily maximum temperature and the beetles caught. The LTF was estimated as 5.97°C, with 42.95 degree-days required for initiation of spring flight. The median flight dates were estimated with a discrepancy from 1 to 3 d by the predictive model. Using the empirical prediction, differences between the day when daily maximum temperature reached 16 or 20°C and flight peak days ranged from 4 to 45 d. These results demonstrate that the predictive model is more suitable than the empirical prediction for predicting the spring flight of I. subelongatus. Overwintering I. subelongatus adults seem to need to gain a determined thermal sum before initiating spring flight rather than merely waiting for the daily maximum temperature to exceed a critical temperature.

Male-Produced Aggregation Pheromone of Coffee Bean Weevil, Araecerus fasciculatus

The coffee bean weevil (CBW), Araecerus fasciculatus (De Geer, 1775), is a cosmopolitan stored-product pest, especially in tropical and subtropical areas. Previous reports showed evidence for aggregation behavior of CBW and this study aimed to identify and evaluate the aggregation pheromone of this pest. Whole-body solvent extractions were analyzed by coupled gas chromatography-mass spectrometry and comparison of male and female extracts showed a male-specific compound which was identified as squalene, 2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene. This compound was shown to attract male and female CBW in Y-tube olfactometer and storage trap assays. The longevity of septum lures was also evaluated in a warehouse. Squalene could be useful in monitoring and controlling this pest.

Revisiting the Male-Produced Aggregation Pheromone of the Lesser Mealworm, Alphitobius diaperinus (Coleoptera, Tenebrionidae): Identification of a Six-Component Pheromone from a Brazilian Population

The lesser mealworm, Alphitobius diaperinus Panzer 1797 (Coleoptera: Tenebrionidae), is a cosmopolitan insect pest affecting poultry production. Due to its cryptic behavior, insecticide control is usually not efficient. Thus, sustainable and effective methods would have an enormous and positive impact in poultry production. The aim of this study was to confirm the identity of the male-produced aggregation pheromone for a Brazilian population of A. diaperinus and to evaluate its biological activity in behavioral assays. Six male-specific compounds were identified: (R)-limonene (1), (E)-ocimene (2), 2-nonanone (3), (S)-linalool (4), (R)-daucene (5), all described before in an American population, and a sixth component, (E,E)-α-farnesene (6), which is apparently exclusive to a Brazilian population. Y-Tube bioassays confirmed the presence of a male-produced aggregation pheromone and showed that all components need to be present in a similar ratio and concentration as emitted by male A. diaperinus to produce a positive chemotactic response.