Status of bark beetle quarantine of China and research trends abroad

The quantity of imported timber has increased rapidly with the constant economic development of China. Since 1999, the quantity of imported timber is over × 10⁷m³ annually and around 1.3∼1.4 × 10⁷m³ in recent years. Longicorn beetles, bark beetles and termites are important timber pests that might be present in imported timber and introduced to China, which could cause considerable damage to Chinese forestry.

Many species of bark beetles were intercepted from imported timber by the State Administration of Supervision, Inspection and Quarantine of the People's Republic of China in recent years. It was reported that 23 species were intercepted from imported timber and wooden packing from 1993 to 1998, in which were categorized to genera including Xyloterus, Hylurgus, Ips, Scolytoplatypus, Hypothenemus etc (Guan and Sun, 2001). Especially in recent years, the outbreak of the red turpentine beetle, Dendroctonus valens, which spread over China through timber import, occurred in Shanxi, Hebei and Henan Provinces in 1999. At the beginning of 2000, the total area affected was 52580hm² in these three provinces, and more than 6 million Chinese pines, Pinus tabulaeformis, died. In 2001, the damage of this pest was found again in Yan'an city, Shanxi Province, spreading rapidly (Yan et al., 2003). Therefore, it is urgent to strengthen future quarantine efforts to prevent bark beetles, such as the red turpentine beetle, from causing widespread damage in China.

At present in China, the main method of bark beetle quarantine is to check the cargo by hand. It is hard to avoid oversight by only spot-checking the large amount of timber 2 to 3 times, when the container is full of cargo. The effect of quarantine is considerably limited by lack of advanced equipment and methods. New technology for quarantine is therefore needed (Yang, 1993).

In North America and Europe, much research has been done on bark beetle host orientation (Birch et al., 1980), interspecific olfactory communications, interspecific competition (Byers and Wood, 1980), the effect of conifer oleoresin released during bark beetle attack (Birgersson and Bergström, 1989), the effects of host volatiles on bark beetle's reaction to pheromone (Byers, 1992) and integrated management (Evans and Fielding, 1994). In particular, the organs which produce pheromone, and the methods of pheromone extraction and isolation, have been successfully studied, and the components and structures of the pheromones have been identified (Bell and Carde, 1990). Since 1995, research on the application of bark beetle semiochemicals in monitoring the dynamic of exotic species of bark beetles has been carried out in Canada (Leland, 2002). There is no report of applying bark beetle semiochemicals for quarantine in China.

Characteristics of bark beetle semiochemicals

Semiochemicals are chemicals that mediate interactions between organisms. Semiochemicals are subdivided into kairomones and pheromones depending on whether the interactions are interspecific or intraspecific, respectively. Semiochemicals which are used in intraspecific communication are referred to as pheromones. Behavioral responses to pheromones include searching for mates by one sex (e.g., sex pheromones), aggregation of both sexes at a host plant (e.g., attractant or aggregation pheromones), and dispersal of both sexes away from a specific area (e.g., inhibitor or antiaggregation pheromones). Semiochemicals used in interspecific communication are referred to as kairomones when the species receiving the chemical message benefits and allomones when the emitter of the chemical message benefits at the expense of the receiver (Skillen et al., 1997). Semiochemicals from both trees and bark beetles influence many behavioral actions of a bark beetle during its life cycle (Byers, 1995).

Monoterpenoids (such as α-pinene, myrcene, terpinolene, β-pinene) and turpentine play an important role in bark beetle host selection, while the ethanol released by of decaying wood tissues that are infected by microorganisms is also used in host search. These compounds are synergic. Generally speaking, monoterpenoids and turpentine released by the host, or the mixture of these two components, are highly attractive to the bark beetle. Bark beetles can discover damaged trees by a low monoterpenoid release rate and a medium ethanol release rate by the host. If the host's resistance is not strong, “pioneer” bark beetles chew on the tree and stimulate it to release monoterpenoids that attract other bark beetles to aggregate, which results in a mass attack. If the host releases high amounts of ethanol, it indicates that the host has decayed and is inadequate for the survival and propagation of bark beetles (Klimetzek and Köller, 1986).

When “pioneers” bite into the bark, if the host is found to be suitable, they begin releasing the primary aggregation pheromone that attracts other adults of the same species. The male of multi-mate species (such as Ips) attacks the host first, and then the females find the gallery entrance mined by the male and enter it. After the mating, the females mine into the inner bark tissue and oviposit. However, the female of single-mate species (such as Scolytus) act as the “pioneers”. In these two instances, after more bark beetles come, they begin to release aggregation pheromone to attract more and more bark beetles, until the whole tree is mass colonized.

The pheromone secreted by bark beetles mainly exists in the feces of males. The pheromone of most species is secreted by the alimentary canal, especially the hindgut, and terpenols are the main components (such as ipsenol, ipsdienol, cis-verbenol, trans-verbenol, verbenone), which function in aggregation. However, it was found that verbenone can inhibit the reaction of bark beetles to their pheromones at certain concentrations (Bell and Carde, 1990). On the other hand, damaged trees will release some compounds that attract bark beetles and are used by bark beetles to synthesize pheromones (Bell and Carde, 1990). Species-specific characteristics of the semiochemicals are not distinct.

Some species can utilize interspecific competition to discover adequate hosts by the compounds released during establishment by other species. These compounds are the kairomones of the host or the pheromones of other species (Birch et al., 1980). Many species of bark beetles may exist in the same environment, and they will compete for one resource. The semiochemicals released into the environment by any species may be received by other species that then stimulate aggregation to the same resource (Bell and Carde, 1990).

The research discussed above has established a basis for the development of bark beetle aggregation and application for use in quarantine.

Study Area

The trap trial was carried out in imported timber in Zhangjiagang Port (where timber is imported from South-East Asia, Africa and Oceania) and Taicang Port (where timber is imported from Russia).

Experimental Reagent

Two isomeric compounds of α-pinene, ethanol, trans-verbenol, verbenone, Camphene and Isononylaldehyde were used in these experiments. These reagents were provided by Aldrich ( www.sigmaaldrich.com), and they were all analytical reagents.

Experiment of attractive components selection

The attractant was injected into the core of Lindgren funnelform trap ( www.pherotech.com/lindgren_funnel_trap.html), that were then hung at random every 10 meters among the imported timber on a ship. Each comparison of various lure blends trapped had 10 replications per treatment, and in every treatment, a control trap was set without any attractant. A very large number of traps were used during the experiment. 12 hours later, the bark beetles trapped in every trap were collected, and the numbers of bark beetle trapped was recorded. The percentage trapped by each attractant was calculated relative to total number trapped for each group. Variance analysis and SSR test were done by SAS software. The trapped bark beetles were dried for species identification. After species identification, the relative species-level responses to each attractant between 2 study sites were evaluated separately.

Table 1.

Carrier and septa of controlling released attractants (CRA)

Experiment of controlling release function of attractant

Four controlled released attractants (Table 1) were tested. Formulated lures were placed in the trap core. The bark beetles trapped was determined every 8 hours, until no bark beetles were caught. Bark beetles trapped were released. The length of time that the lures remained active was noted. In the controlled release experiment, for each treatment the dosage of semiochemicals absorbed by the carrier was 5 ml. The controlling released attractants were prepared by National Engineering Research Center for Dyeing and Finishing of Textiles of China.

Table 3.

SSR test of the relative effect of attractants of (5%+/95%-) a-pinene added minor

Species identification of trapped bark beetles

Dry specimens of trapped bark beetles were sent to Institute of Zoology, Chinese Academy of Sciences for species identification.

Experiment of attractive components selection

The result of the experiment of different ratios of two isomeric compounds of α-pinene in the attractant showed that the attraction to (5%+/95%-) α-pinene was higher than for other compounds (Table 2). A significant treatment effect was observed (F_5,54=21.97>F_0.05=2.39, P=6.13E-12).

Table 2.

SSR test of the relative effect of attractants of two isomeric compounds of a-pinene

Table 4.

SSR test of the relative species-level responses to each attractant in Zhangjiagang Port

After adding the minor components α-pinene, ethanol, verbenone, camphene and isononylaldehyde to (5%+/95%-) α -pinene, attraction was higher than for the other compounds, except for the attractant in which trans-verbenol was substituted for verbeone (Table 3). A significant treatment effect was observed (F_5,54=7.54>F_0.05=2.58, P=9.73E-05).

Comparing the response of different species to the attractants, the attractant containing (5%+/95%-) α -pinene, ethanol, verbenone, camphene and isononylaldehyde and the one containing (5%+/95%-) α-pinene, ethanol, trans-verbenol, camphene and isononylaldehyde attracted more bark beetle species than other attractants, including some damaging species, such as Dendroctonus, Scolytus and Ips (Tables 4, 5).

Experiment of controlling release function of attractant

Figure 1 shows that the active time of controlled released attractants using the porous starch and septa of polyvinyl alcohol as carriers was longer than the other carriers.

Species identification of trapped bark beetles

Insects of two subfamilies (Scolytinae and Platypodinae) of Curculionidae were trapped, including eight genera of Scolytinae, (Xyleborus, Cryphalus, Polygraphs, Xyloterus, Ips, Dendroctonus, Orthotomicus and Scolytus), totaling 16 species (Table 6).

Table 5.

SSR test of the relative species-level responses to each attractant in Taicang Port