Semiochemicals

All organisms produce chemical signals, and all organisms respond to these signals. The chemicals involved in biological communication are collectively referred to as informational chemicals (Wang and Yan 2023). The category of semiochemicals is further divided into two major groups: pheromones and allelochemicals, depending on whether the interaction is within a species or between species, respectively (Nordlundd and Lewis 1976). Insect pheromones are extremely small amounts of chemical substances secreted and released by insects to regulate population behavior and development. These pheromones are typically categorized as sex pheromones or aggregation pheromones (Jiang et al. 2002, Pitman et al. 1975, Vite et al. 1972, Vite and Pitman 1969, Yan 2006, Yan et al. 2014). Sex pheromones of Tomicus are chemical substances produced by individuals of a specific gender, detected by receptors in both same- and opposite-sex individuals, and trigger reproductive behaviors like mate seeking and mating. These pheromones are emitted from the hindgut of Tomicus and facilitate interactions between individuals. The limited amount of sex pheromones released by Tomicus, whether in natural or controlled settings, hinders research progress on these compounds. Additionally, aggregation pheromones released by Tomicus within their bodies induce aggregation behavior in individuals of the same species, contributing to the damage caused by beetles in the Yunnan region.

The pheromones secreted by bark beetles regulate their aggregation and harmful behavior (Byers 1989, Byers et al. 1985, Poland et al. 2003). The attraction of Tomicus to ethanol was weaker compared with its attraction to a monoterpene mix, (±)-α-pinene, (+)-3-carene, terpinolene. However, a combination of (−)-(S)-α-pinene, (+)-(R)-α-pinene, (+)-3-carene, and terpinolene, or each alone, was effective in attracting Tomicus (Byers 1992). Pinene is not only a volatile compound found in pine trees but also a crucial component of aggregation pheromones in bark beetles. Alongside carene, limonene also plays a significant role in aggregation pheromones.

Verbenol, myrtenal, and myrtenol were detected in the hindguts of male and female adults of T. yunnanensis, whereas verbenone was only detected in female adults (Yan et al. 2009, Zhou et al. 1997). During the shoot-feeding stage, β-pinene, trans-verbenol, and myrtenol exhibited strong attraction to T. minor and T. brevipilosus. Additionally, myrtenol was found to have attractive effects on T. yunnanensis. During the trunk-breeding phase, α-pinene, 3-carene, trans-verbenol, and myrtenol were strongly attractive to T. minor. Furthermore, α-pinene, β-pinene, trans-verbenol, cis-verbenol, and myrtenol were strongly attractive to T. yunnanensis. Last, α-pinene, β-pinene, myrcene, 3-carene, trans-verbenol, cis-verbenol, and myrtenol were strongly attractive to T. brevipilosus (Wang 2015). Additional studies have confirmed this conclusion, demonstrating that the effect of verbenol is dose dependent and that verbenone has repellent activity against Tomicus. The hindguts of female and male T. minor and T. yunnanensis adults contained the following amounts of the compounds: 0.19, 0.09, 0.22, and 0.05 ng/individual of (−)-trans-verbenol; 0.16, 0.06, 0.03, and 0.05 ng/individual of verbenone; and the most attractive levels of cis-verbenol and (−)-trans-verbenol for T. yunnanensis and T. minor were 0.1 and 1.0 ng/μl, respectively. Verbenone was not attractive at any concentration, and its addition to cis-verbenol or (−)-trans-verbenol reduced the attraction response. Therefore, it can be concluded that (−)-trans-verbenol produced by these two pine shoot beetles is attractive at low concentrations and repellent at high concentrations (Wu et al. 2019). Additional research results have confirmed this. During the initial phase, researchers collected volatiles from the hindgut of the beetles in the laboratory. Through detection, analysis, and field trapping experiments, it was determined that verbenol and myrcenol are likely the primary components of the sex pheromones of Tomicus.

In recent years, research has clarified the effects of verbenol and verbenone on the control of Tomicus. The hindguts of female and male T. minor and T. yunnanensis adults contained (−)-trans-verbenol, and this level increased to 16.74–292.71 ng/individual in T. minor females after interactions with other beetles. The levels of cis-verbenol and (−)-trans-verbenol were most attractive to T. yunnanensis and T. minor when the content was 0.1 and 1.0 ng/μl, respectively. Verbenone was not attractive at any concentration to these two beetles, but it had a repellent effect when added to verbenol. The research suggests that the presence of aggregation pheromone (−)-trans-verbenol in the hindgut of T. yunnanensis regulates population behavior (Wu 2019). However, the use of verbenone as an inhibitor to control Tomicus damage has not been satisfactory (Frühbrodt et al. 2024). Tomicus are small, and the amounts of verbenalenol and verbenalenone detected in their hindgut volatiles are also very small. During the field trapping experiment, the artificial attractant configured according to the test results needs to be continuously adjusted according to the trapping results. This is also one of the reasons why research on the application of artificial attractants in the field to control beetles has been relatively slow. Compared with verbenalenol, verbenalenone has a stronger attracting effect on Tomicus.

During the gallery initiation stage, the hindgut extracts of beetles not only contained higher amounts of myrtenol, cis-verbenol, trans-verbenol, and verbenone, but also exhibited candidate aggregation pheromone compounds in the form of myrtenol and trans-verbenol. Furthermore, a blend of these two components with S-(−)-α-pinene and S-(−)-β-pinene was found to attract more T. brevipilosus individuals in a field bioassay (Liu et al. 2019). Therefore, on the basis of previous research results, it can be inferred that verbenol, myrtenol, and pinene are the main pheromone chemical components of Tomicus. This provides great support for the future deployment of artificial attractants. These can be combined with indoor bioassays and, through the results of field trapping experiments, used to develop more effective artificial attractants to prevent and manage the damage of bark beetles.

Artificial Attractants

In recent years, researchers have made progress in enhancing the effectiveness of artificial attractants for bark beetles by collecting field samples, conducting indoor chemical composition analysis, and integrating biological measurements. Despite these advancements, the low levels of semiochemicals in bark beetles pose challenges for detection. The composition remains incompletely understood, leading to limited effectiveness of current artificial attractants.

The bioassay results using the blend of V(α-pinene):V(β-pinene):V(β-phellandrene):V(γ-careen) = 20.6:4.7:6.6:1 demonstrated a significant attraction effect on T. yunnanensis. However, when tested with individual components, or blends of α-pinene and terpinolene or α-terpinen and γ-careen, no significant attraction was observed (Yin et al. 2002). This shows that, in many information chemical species, not one component works alone, but a variety of chemicals works together. It was further found in the bioassay that the attraction rate of α-pinene, γ-careen, and trans-verbenol exceeded 95%, whereas cis-verbenol only achieved an 83.4% rate. Additionally, a forest-based bioassay revealed that α-pinene and trans-verbenol had superior attraction to T. yunnanensis compared with other compounds (Li et al. 1993). The test result is consistent with the previous ones, indicating that α-pinene and trans-verbenol attract the beetles, but there should be other unknown chemical substances. Although the content of these substances should not be substantial, they also play an important role, and research on the detection and identification of related substances should be increased in the future. Studies have shown that artificial attractants 1-6 were developed through bioassays to identify the main components as monoterpenes and monoterpenols (Wang 2015). The results showed artificial attractants 1-4 effectively attracted T. minor and T. yunnanensis in field trapping experiments; however, attractants 5 and 6 trapped only a few T. brevipilosus individuals, and the effect was not satisfactory (Wang 2015). The study results reveal that T. minor, T. yunnanensis, and T. brevipilosus are found in the Yunnan region. Although the primary components of artificial attractants are monoterpenes and monoterpenols, their efficacy in attracting the three species of beetles varies. Therefore, it is essential to explore and create distinct types of artificial attractants tailored to the species of Tomicus.

The attractant trans-verbenol was mixed with α-pinene in a volume ratio of 1:20 to test its attraction on Tomicus in the forest. The average number of adults captured by each trap was 96.33 (Zhou et al. 1997). When using the artificial attractants formulated with S-α-pinene, myrcene, R-α-pinene, trans-verbenol, cis-verbenol, and myrtenol, the highest number of T. yunnanensis was captured in the forest. Additionally, the average insect infestation rate in the forest decreased from 65.25% to 26.88% (Zuo and Huang 2018). In Xiangyun County, Yunnan Province, artificial attractants were prepared using 10 ml of α-pinene, 1 ml of 3-carene, 1 ml of myrtenol, 0.1 ml of ipsenol, and 0.1 ml of verbenol. Forty-five artificial traps were used to trap T. minor, and a maximum of 486 individuals was caught in 5 d (Fang et al. 2019). Myrtenol and trans-verbenol were identified as candidate aggregation pheromone compounds. Furthermore, a blend of these two components with S-(−)-α-pinene and S-(−)-β-pinene attracted a greater number of T. brevipilosus individuals in a field bioassay. However, there was no significant difference in the number of traps captured by the two attractants (Liu et al. 2019). These research results further demonstrate that, in addition to the predominant chemical components like verbenol, myrtenol, and pinene, there are also myrcene, carene, ipsenol, and other compounds that exhibit significant effects on bark beetles. Ongoing research on artificial attractants is in progress, with a focus on enhancing the ratio of each component and exploring unknown chemical compounds to develop increasingly effective attractants in the future.

Insects commonly utilize multicomponent informational chemicals for intraspecific chemical communication, rather than relying on single compounds alone (Zhao and Zhang 1993). Despite the increasing application of attractants in insect management in recent years, the regulatory effect of a single compound on insect behavior remains limited (Wang and Yan 2023). The chemical components that effectively trap bark beetles for control should be a combination of multiple compounds, rather than a single substance. Further research is needed to determine the optimal ratio of these different components, presenting a key scientific challenge that must be addressed urgently in the chemical ecological management by trapping Tomicus.

Host Plant Volatiles

The three species of bark beetles damage host plants at different times, which may be related to the different attracting effects of the host plant’s volatiles on the beetles. Tomicus brevipilosus breeding attacks in P. yunnanensis generally start in early March and end in early June in Anning County, Yunnan. From early March to mid-April, T. brevipilosus bred preferentially in the trunks of Yunnan pine trees that were already infested by T. yunnanensis and T. minor; from about mid-April to early June, when there were no Yunnan pine trees newly infested by T. yunnanensis and T. minor, T. brevipilosus attacked Yunnan pine by itself (Chen et al. 2015). The behavior of the three species should be influenced by the volatilization of the host plant and volatiles of other bark beetles. The application of volatile emissions from host plants in trapping and killing bark beetles has a certain impact on their prevention and control (Wang and Zhang 2019). The aggregation of bark beetles is primarily influenced by the volatile emissions of the host plant, as the volatiles in the phloem of P. yunnanensis encourage beetle aggregation. Additionally, trap log bundles are attractive to beetles, and untreated bundles are heavily attacked; verbenone, either alone or in combination with nonhost volatiles (three green leaf C₆ alcohols and one bark C₈ alcohol), significantly reduces attacks on the bundles (Sun et al. 2005). Notably, monoterpenes emitted by pines serve as signaling substances for Tomicus host location, and they are a significant component of pheromones produced by certain bark beetles (Yan 2006). The volatiles of the host plant have an attracting effect on the beetles that harms the host first, and the volatiles of this beetle may have an attracting or repelling effect on other beetles. They attract other beetles to harm the host; on the other hand, they avoid them to prevent overpopulation.

The trap log bundles proved to be highly attractive to beetles, with untreated bundles experiencing heavy infestation (Sun et al. 2005). Tomicus yunnanensis exhibited a strong attraction response to both P. yunnanensis needle and branch material, with a more significant response observed toward branches from which the needles had been removed. α-Pinene, the primary volatile emitted by P. yunnanensis, is believed to be the key chemical attractant for T. yunnanensis. When exposed to α-pinene, the female adults of T. yunnanensis exhibited an increase in both the length of the main tunnel and larval tunnel, indicating that α-pinene serves as a feeding stimulus factor that promotes feeding behavior and leads to an increase in the body length and weight of larvae (Liu 2019).

Tomicus minor, T. yunnanensis, and T. brevipilosus were found to be attracted to shoot extracts and dynamic headspace volatiles from shoots damaged by the same species and sex. Furthermore, female T. minor and male T. yunnanensis demonstrated attraction to dynamic headspace volatiles from shoots damaged by both sexes of T. brevipilosus. These results suggest that specific semiochemicals induced or produced by T. brevipilosus also attract T. minor and T. yunnanensis (Wang et al. 2015). When the trunks of P. yunnanensis are damaged by T. yunnanensis, the amount of α-pinene in the phloem decreases, whereas myrcene and 3-carene increase. This promotes the release of terpinolene and 4-allylanisole from Yunnan pine. Healthy pine trees typically contain a higher concentration of 4-allylanisole, which generally inhibits the attraction of host volatiles to beetles, compared with damaged pine trees (Joseph et al. 2001, Nebeker et al. 1995).

Gas chromatography–mass spectrometry (GC-MS) and gas chromatography–electroantennographic detection (GC-EAD) methods can effectively detect semiochemicals that attract and repel bark beetles. To investigate the pine volatiles produced by P. yunnanensis infested by T. yunnanensis, dynamic headspace and solvent extraction techniques were utilized. GC-MS coupled with GC-EAD and bioassays were conducted to identify the biologically active compounds that attracted T. yunnanensis to host pine. The results revealed that pine volatiles from the needles and resins of P. yunnanensis consisted of 18 terpenes, and their chemical profiles varied significantly between needles and resins. Mass fraction of needle volatile compounds was primarily monoterpenes (99.98%), with α-pinene (90.82%) being the most abundant, followed by β-pinene (8.78%), d-limonene (4.77%), camphene (2.86%), and β-myrcene (1.42%). In contrast, resin volatiles contained both monoterpenes and diterpenes, with α-pinene (21.38%) being the most prevalent monoterpene, followed by 3-carene (21.42%) and terpinolene (2.78%). The diterpenes present in resin were primarily highly oxidized, with palustric acid accounting for 51.13% of the relative abundance. Electroantennographic responses of T. yunnanensis indicated strong attraction to α-pinene, β-pinene, 3-carene, γ-terpinene, and 4-allylanisole. Behavioral choice experiments further demonstrated that α-pinene, γ-terpinene, and 3-carene significantly enhanced the attraction of T. yunnanensis, whereas β-pinene and 4-allylanisole displayed repellent effects (Yan et al. 2021). Using GC-MS, the volatile compositions of the phloem from healthy and previously attacked T. yunnanensis trees were analyzed. The attraction of these two types of trees to T. yunnanensis was determined through trap log experiments. The results showed that the attraction of logs from previously attacked trees was significantly higher than that of healthy trees at the same time. The content of the main volatile components differed significantly between the two types of trees. The phloem of attacked P. yunnanensis had higher levels of α-pinene, camphene, β-thujene, and β-ocimene compared with healthy P. yunnanensis. Conversely, the levels of β-pinene, β-phellandrene, and myrcene were lower in the attacked trees. The differences in volatile components between healthy and previously attacked pine trees are one of the key factors that influence their attraction to pine shoot beetles (Zhao et al. 2002). The GC-MS analysis of volatiles released from the trunks and branches of P. yunnanensis revealed significant differences between the two types of volatile components. The main component in trunk volatiles was α-pinene, accounting for 81.01% of the total. In shoot volatiles, the main components were α-pinene and β-phellandrene, with percentages of 29.20% and 30.52%, respectively. When compared with shoot volatiles, trunk volatiles had higher levels of α-pinene and 3-carene, whereas shoot volatiles had higher levels of camphene, sabinene, limonene, β-phellandrene, terpinolene, and caryophyllene than trunk volatiles. Tomicus yunnanensis displayed a positive response to the extract of shoots of P. yunnanensis, but a repellent response to the extract of trunks (Yan et al. 2011).

Nonhost Plant Volatiles

The utilization of nonhost plant volatiles primarily serves as a repellent against Tomicus. Currently, common nonhost plants include Cupressus torulosa D. Don ex Lambert, Cinnamomum camphora (L.) J. Presli, Cyclobalanopsis glaucoides Schottky, Alnus ferdinandi-coburgii C.K. Schneider, and A. nepalensis D. Don. Control effects can be achieved by extracting these volatile compounds into biological agents or through intercropping forests and other methods.

The shoots of P. yunnanensis were mixed with those of C. torulosa, Ci. camphora, and Cy. glaucoides, either as single trees, strips, rows, or as groups. The results showed that the single tree mixing method had the greatest effect on the olfactory behavior of T. yunnanensis, followed by the row mixing model. The strip and group mixing methods had lesser effects (Yue and Yang 2011a). When essential oil from two nonhost plants (C. torulosa and Cy. glaucoides) was sprayed on the shoots of P. yunnanensis using single tree, strip, row, and group methods, the total number of T. yunnanensis on the shoots was lowest and the average was lowest with the single tree method, followed by the row, strip, and group methods. This indicated that nonhost plants disrupted the odor characteristics of the original host plants, making it difficult for T. yunnanensis to recognize the host, thereby achieving the goal of protecting the target tree species (Yue et al. 2013). The pine needles of P. yunnanensis were mixed with leaves of C. torulosa, Ci. camphora, and Cy. glaucoides in various ratios, including (in grams) 0:6, 1:5, 2:4, 3:3, 4:2, 5:1, and 6:0. These mixtures were placed in the response arm of a Y-tube olfactometer as the odor source to observe the olfactory behavior of T. yunnanensis. The impact of nonhost plants on olfactory responses of T. yunnanensis was minimal. As the proportion of nonhost leaves gradually increased, the inducement rate of leaf mixtures on T. yunnanensis gradually decreased (Yue and Yang 2011b).

Pinus armandii Franch, Celtis kunmingensis C.C. Cheng & D.Y. Hong, C. torulosa, and A. ferdinandi-coburgii all had an impact on the host search of T. yunnanensis, with A. ferdinandi-coburgii exhibiting the most significant effect. This species, including its volatile compounds 4-ethylacetophenone, trans-2-hexen-1-ol, (Z)-hexen-4-en-1-ol, and 4-ethylacetophenone, significantly interfered with the postembryonic development of T. yunnanensis. In particular, the two green leaf volatiles, trans-2-hexen-1-ol and (Z)-hexen-4-en-1-ol, directly reduced the body size and weight of pupae and newly emerged adults (Qian et al. 2023). The β-pinene, trans-2-hexen-1-ol, (Z)-hexen-4-en-1-ol, 4-ethylacetophenone, and 4-ethylacetophenone of A. ferdinandi-coburgii exhibited significant repellent action against T. yunnanensis. Among them, 4-ethylacetophenone exhibited the most effective repellent action (Liu 2019).

As nonhost plants for T. yunnanensis, Ce. kunmingensis and A. nepalensis have the potential to interfere with its infestation of P. yunnanensis. Forest surveys have also observed relatively low populations of T. yunnanensis in mixed forests in Yunnan Province (Yue and Yang 2011a). The leaves of Ce. kunmingensis and A. nepalensis emit volatile compounds such as 2-hexenal, 2-hexenol, and 3-hexenol, which can significantly reduce the olfactory behavior and fecundity of T. yunnanensis (D. Wang et al. 2013, 2015). Laboratory olfactory tests using a Y-tube olfactometer showed that the volatiles of P. yunnanensis have an attractive effect on T. yunnanensis, whereas the volatiles of A. nepalensis have a repellent effect (Wang et al. 2016). Nonhost volatiles from Ce. kunmingensis and A. nepalensis can also reduce egg production in T. yunnanensis by regulating the expression of its vitellogenin and vitellogenin receptor genes (Wang et al. 2023). These nonhost plants also decrease the fecundity of T. yunnanensis by reducing the transcription levels of vitellogenin and vitellogenin receptor genes in its leaves (Zhao 2015).

It was concluded that C. torulosa, Michelia alba Figlar, Ce. kunmingensis, and A. nepalensis possess promising taxis effects on T. yunnanensis, making them suitable candidates for mixed planting in P. yunnanensis plantations (Zhou et al. 2012). The repellent activity of essential oils from various plants, including Mentha canadensis L., Artemisia carvifolia Besser, Perilla frutescens (L.) Britton, Eucalyptus globulus Labillardiere, Ci. camphora, Citrus reticulata Blanco, Agastache rugosa Fischer & von Meyer) Kuntze, and Acorus gramineus Alton, was evaluated. The bioassay results indicated that the essential oil from Me. canadensis exhibited the strongest repellent activity and the longest repellent duration. When diluted with acetone at ratios of 10, 40, and 160 times, the repellent rates against adult T. yunnanensis were 100, 100, and 96.55%, respectively. However, when diluted at a ratio of 320 times, the repellent rate decreased to 54.57% (Yang et al. 2003).

Volatiles from nonhost plants are used to prevent bark beetle host location behavior. In laboratory analysis of the chemical composition of volatiles collected from nonhost plants, combined with the bioassay results, we see that it is helpful to plant nonhost plants to prevent and control the damage caused by the beetles. Y-tube olfactometer bioassays revealed that the host location behavior of T. yunnanensis was significantly repelled by the three most abundant green leaf volatiles: (E)-2-hexenal, (E)-2-hexenol, and (Z)-3-hexen-1-ol, which are emitted by nonhost trees. Furthermore, these green leaf volatiles have a negative impact on the feeding behavior of T. yunnanensis in the field, ultimately reducing damage to Yunnan pines (D. Wang et al. 2015). Verbenone, either alone or in combination with nonhost volatiles (three green leaf C₆ alcohols and one bark C₈ alcohol), significantly reduced attacks on the bundles. However, the nonhost volatiles alone did not inhibit attacks (Sun et al. 2005).

Prospects

Research on chemical and ecological management of Tomicus in Yunnan Province is continuously delving into the potential of new chemical components and artificial attractants to gradually improve ecological and environmental protection. This may offer a green and sustainable management method for effectively preventing and controlling the harm caused by bark beetles. The research on chemical and ecological management of Tomicus in Yunnan Province is also helping to avoid the human, material, and financial resources required for cleaning up damaged trees, chemical control of environmental pollution, and the development of pest resistance. In future research, chemical ecology research methods will be further developed and improved, artificial attractant technology will continue to mature, and the trapping effect will gradually improve. This also provides an effective reference for the management of Tomicus and other pests in other regions.