The study of the distribution of wood-boring pests and their gallery construction behavior is key to exploring their survival and reproductive mechanisms and population variation, as well as the associated disaster prevention and control planning (Zhao, 2005). In this study, our CT scan results showed that the M. alternatus galleries were single, that is, each gallery had only one individual larva, and the galleries were independent of each other. The three-dimensional reconstruction results showed that the majority of galleries were C-shaped, which was consistent with the results obtained by dissection of damaged wood (Gao et al., 2022). In addition, the results of 3D reconstruction showed that a few of the M. alternatus galleries were S-shaped or Y-shaped, which also reflected the intuitive nature and comprehensiveness of the three-dimensional reconstruction method. Gallery structures vary, because in the process of boring, larvae need to make a detour when meeting host plant tree knots, or change their boring direction when encountering other larvae to avoid each other. This boring behavior not only feed conveniently, it also reduces unnecessary energy consumption in the region of obstacles (Hao et al., 2005). In this study, there were no significant differences in gallery parameters such as tunneling depth and boring volume (Table 1), which may be because of the fixed energy input of larvae while boring the gallery and constructing the pupa chamber after entering the xylem. In addition, the change of boring direction may be related to the nutrients in the regions with different xylem height or diameters (Yang, 2010). Intraspecific and interspecific aggregations have been analyzed via longhorn beetle numbers and the spatial layout of galleries in a given space (Gregoryet, 2015). In this study, the whole section of damaged wood was 70 cm long, containing nearly ten M. alternatus galleries, and the regional population was large. Previous studies have shown that M. alternatus clusters in the host tree trunk (Gao et al., 2015; Wen et al., 2018), which may indicate that longhorn beetles choose the central area to lay eggs to avoid natural enemies and human interference during laying (Yang, 2010). However, the direction of boring and the distribution of galleries were coordinated and orderly, and did not disturb each other. That meant when insect density reached a certain level, the movement of the aggregation distribution center was blocked (mainly in the form of intraspecies competition), leading longhorn beetles to expand their space more to areas with less competition (Johnson et al., 2001). Some studies have shown that longhorn beetle larvae have formed a special communication mechanism during their evolutionary history. The hardened body wall tissue makes sound when rubbed against the gallery wall, and the abdominal side plate has an organ that detects and produces sounds, and this organ senses the sound of nearby longhorn beetle larvae. Using these organs, longhorn beetle larvae locate the movements of nearby larvae by means of the mechanism of sending and receiving sounds within the dark host trunk (Tang et al., 2005). To avoid entry by individuals of the same species, the residents of the galleries use the sound warning of rodents, thus playing an important role in regulating the degree of population aggregation, so as to effectively use the space and resources of host plants (Jiang, 1989; Wang, 2005).
The galleries formed by wood-boring pests, as represented by the M. alternatus, have been shown to be stereoscopic. Through dissection of the damaged wood, only local conditions can be observed. In this study, CT scanning technology clearly identified the insect body and the boring conditions of M. alternatus, which enabled the complete structure and distribution of galleries to be reconstructed via 3D reconstruction. Continuous scanning and three-dimensional reconstruction of galleries in different life stages better reflects the beetles’ biological characteristics (e.g., food consumption, boring habit, overwintering and pupation characteristics). In this study, one or two generations of M. alternatus occurred in one year (Wang, 1992; Anbutsu et al., 1997). Larvae in the xylem stage had a small boring volume (approximately 30 mm3), and the structure and type of galleries were relatively simple and easy to distinguish and identify through CT scanning and three-dimensional reconstruction. Therefore, the structural characteristics and spatial distribution pattern of galleries were used as identification methods for larval species. However, the generations of species such as Apriona swainsoni, Batocera horsfieldi, Massicus raddei and Aromia bungii span 2–3 years, with longer feeding times, increased food intake, larger gallery volumes and more complex structures (Liu et al., 1999; Liu et al., 2009; Yang et al., 2010; Tang et al., 2011). In addition, some longhorn beetles with more generations may also have a variety of gallery types, or irregular galleries, including galleries that connect with each other, such as seen in Xylotrechus quadripes, which has two generations per year and a considerable overlap in generations. Within the same trunk, the borers’ ages were inconsistent, and the galleries in the trunk occurred at different times, which led to subsequent larvae entering established galleries (Chai et al., 2020). Therefore, in future studies, sample sizes should be increased, and a database established to determine the gallery structure for each wood-boring pest species, so as to increase the identification accuracy of subsequent testing.
In the process of feeding in the galleries, the larvae move forward and push frass and feces to the rear of the galleries by the pressing action of the hardened anterior thoracic plate, dorsal and ventral step vesicles and caudal gluteal plate (Jiang, 1989; Wang, 2005). Among longhorn beetle species, larvae have different ways of treating the frass-feces mixture in the galleries. For example, the larvae of Anoplophora glabripennis and Aromia bungii bite a circular fecal hole and push the feces out of the hole after boring a section of gallery (Niu et al., 2010; Liu, 1999). In view of the similarity between the scanning image data of galleries and wood dust and frass, the boundary between the two must be carefully observed and the galleries carefully selected to distinguish the two successfully. Our results show that in C-shaped galleries, the mixture mainly blocked the main middle section (Fig. 2I), and this blockage is likely generated during the construction of the pupa chamber. However, the mixture in other parts was pushed out of the gallery, which is not completely consistent with the report by Zhao (2005) that the larva of M. alternatus do not push the mixture out of the host, but fill the posterior segment of the gallery. We observed many frass-feces deposits outside the entrance holes, which, according to our many years of experience in forest investigation, would have been pushed out by larvae. Therefore, we conclude that the larva push the mixture outwards while feeding before pupation, and the mixture generated during the pupal chamber construction before overwintering remains in the gallery. To ensure the stability of the population, the borers avoid predation and parasitism by natural enemies via various covert means. For example, the depth of dung beetle tunnels has been shown to be a key variable affecting the rate at which the beetles are parasitized (Arellano et al., 2016). In the interaction between longhorn beetles and their natural enemies, the depth and length of the boring larvae galleries and the blockage by the frass-feces mixture affects their interaction with natural enemies to varying degrees. In some ichneumonid species, females laid eggs directly on host larvae by puncturing branches with their long ovipositor sheaths, and the boring depth of host larvae was a key factor affecting parasitism (Taylor, 1977). Sclerodermus spp. is an important parasitic pest group of longhorn beetles; the adult females penetrate the bark to search within the phloem for larvae to parasitize, but cannot enter the xylem because of the blockage of the frass-feces mixture (Jiang et al., 2015). Before the pupal chamber is constructed by the mature larvae, the fibers bitten by the upper jaw are thick and the gap is relatively large, which provides an opportunity for their smaller natural enemies to successfully colonize or prey on them via the otherwise blocked galleries. The widely used parasitoids of Coleoptera effectively parasitize mature larvae or pupae of M. alternatus after overwintering (Zhang et al., 2014; Gao et al., 2016), probably because the newly hatched larvae (less than 1 mm in length) can penetrate the blocked galleries to complete their parasitism.
The prevention and control of wood-boring pests is an important problem faced by researchers all over the world. At present, CT scanning and 3D reconstruction technology enable the nondestructive detection and identification of such pests, and the degree of damage by pests can be assessed through image processing. However, because of the diversity of pest species and the complexity of the harm they cause to host plants, it is still necessary to expand the amount of data collection and establish a database to ensure the detection accuracy within ecological niches and among similar species. Three-dimensional scanning of the reconstructed gallery structure provides new ideas and methods for studying the interaction between longhorn beetles and exogenous substances in xylem by simulating the gallery environment. In addition, this technology also has great application potential in pest quarantine, the use of exogenous agents, the release of natural enemies, and the research and development of related equipment for the precise control of borer pests.