Introduction and Establishment of Exotic Bark Beetles

F. M. Stephen and J.C. Grégoire

Traits of natural history make bark and ambrosia beetles particularly threatening groups of pests liable to bypass quarantine and colonize new areas, because they can be transported undetected in wood articles. There are many examples of successful invasions and port interceptions of exotic scolytid species.

Bark beetles (Coleoptera, Scolytidae) in general lay their eggs in the phloem of trees, and their larval development entirely occurs in this habitat. Some species live in dead or weakened hosts, but others exclusively colonize live trees, which in most cases are killed by the insects and by pathogenic fungi vectored by the bark beetles in relationships that are frequently obligate.

Ambrosia beetles also belong to the Scolytidae or to the closely related family Platypodidae (Wood 1993). They often bore their galleries into sapwood of host trees and their larvae develop on symbiotic fungi. Some species attack dead or dying trees, but other species attack live trees.

	Great European spruce bark beetle, Dendroctonus micans, a pest that has spread westward across Europe.
	The eight-toothed spruce bark beetle, Ips typographus. An dangerous bark beetle that is intercepted frequently in wood moved internationally from Europe.
	The devastating damage caused by southern pine beetle, Dendroctonus frontalis, in Texas. Populations of this aggressive bark beetle can build up rapidly and lead to extensive tree death.

Successful invasions
The most notorious examples of Scolytidae that have successfully been moved and established as exotics around the world are probably such species as Scolytus multistriatus (a vector of Dutch Elm Disease, first taking the disease to North America, then bringing back a virulent strain to Europe), Ips grandicollis (as a tree-killer of Pinus radiata in Australia), Dendroctonus micans (invaded Britain in the late 1970s, from the European continent), Dendroctonus valens (introduced in central China in the early 1980s and presently killing millions of Pinus tabulaeformis, annually), and Tomicus piniperda (whose recent introduction into North America has stimulated new awareness of exotic scolytids). Mattson et al. (1994) provide a list of 33 scolytid species that have successfully become established in the United States and Canada.

Records concerning establishment of exotic ambrosia beetles are less abundant. Eastern North America, with an overall richer ambrosia beetle fauna, has 10 exotic species that have become established (7 of which are from Asia), and western North America has 4 exotic species (Atkinson et al. 1990). In Europe, the Asiatic species Xylosandrus germanus was observed for the first time in 1952 in Germany and the North American species Gnathotrichus materiarius. was first recorded in 1933 in France. Both species are now widespread over the whole of Western Europe.

Interceptions
According to data collected by Haack and Cavey (2000) about 93% of all insects intercepted on wood articles at U.S. ports of entry between 1985 and 1998 were Coleoptera. Of the nearly 6,500 beetles collected, 67% were Scolytidae. Most of these were true bark beetles, living under bark, rather than ambrosia beetles. Shown in the following table, modified from USDA APHIS data assembled by Haack and Cavey (2000), are interceptions of the top 10 most frequently found Scolytidae at U.S. ports of entry between 1985 and 1998.

Great Britain also has a large record of interceptions at ports. From 1980 to 1988, 28 species of Scolytidae (among which only 1 ambrosia beetle species was present) were recorded from inspection of wooden articles (Tim Winter, Forestry Commission, personal communication), while in 1999 to 2000 16 species belonging to 9 genera (including only 1 ambrosia beetle species) were caught with traps baited with a pheromone for Ips typographus in British ports (source: Forest Research, an Agency of the Forestry Commission). Strikingly, with the exception of Ips typographus, the species caught in 1999 to 2000 were all different from those caught from 1980 to 1988, although some genera (Dryocoetes, Hylastes, Hylurgops, Ips, Orthotomicus, Pityogenes, Polygraphus) were caught in both periods. More generally, introduction of bark beetles is undoubtedly occurring in many countries. Ciesla (1998) reports that the Douglas-fir beetle, Dendroctonus pseudotsugae, was introduced from North America into the People’s Republic of China through importation of whole logs of Pseudotsuga menziesii and Tsuga heterophylla but states that establishment apparently did not occur. A continual flow of scolytids to a variety of countries is evident. Observation of interception record data provided by Beeche (1998) indicates that from 1994 to1996 Scolytidae from 21 countries were intercepted at just one port in Chile.

Ambrosia beetles do not seem to be commonly intercepted at U.S. ports, although Humble et al (1998) and Beeche (1998), respectively, report recent cases in British Columbia and Brazil. As ambrosia beetles are of worldwide distribution and colonize most hardwoods and softwoods, it is probable that they are frequently transported in wood articles. The lack of detection at ports may result from the difficulty in detecting them by visual inspection, and chemically based trapping tactics, such as those used in the United Kingdom, may be required. The commercially available attractant ‘lineatin’ for example, has been shown to attract at least three of the four species of Trypodendron native to Canada (Lindgren et al. 2000), and ethanol may be used as an attractant for some species.

Haack and Cavey (2000) report that, overall, 45% of the 6,952 interceptions on wood articles at U.S. ports of entry between 1985 and 1998 were made on crating, 33% on dunnage, and 6% on pallets. Together these three categories (solid wood packing materials) represented 84% of the total interceptions.

Risks
Although 38 genera of Scolytidae were intercepted during the 1985-1998 period, only 2 species, Tomicus piniperda and Pityogenes bidentatus, have been established in the United States; however, another species so far not intercepted, Hylastes opacus, has also successfully been established in the country (Haack and Cavey, 2000). All the species arriving in a new area obviously do not have the same chances of successful establishment. Speculation as to which species constitute the greatest risks for establishment obviously must include consideration of similarity in climate and biota to the country of origin. However, if climate is not limiting and suitable host tree taxa exist in the introduced region, it is reasonable to ask what biological characteristics of scolytids might be most important in judging their potential for successful establishment.

Factors influencing probability of successful establishment

1. Mass aggregation requirements. Those species that are obligatory tree killers and that require large numbers of conspecifics to overcome tree resistance, such as Dendroctonus brevicomis, although highly dangerous if established, probably have a low risk of initial establishment because large numbers of individuals are needed to establish pioneer populations.

2. Secondary colonizers. Those species that use pheromones or host odors to aggregate at suitable hosts, but that do not need large numbers for colonization of slash, windthrows or highly stressed or weakened trees, such as Ips grandicollis or Ips typographus, would seem to have a good chance of success once initially established. The establishment of I. grandicollis in Australia tends to confirm this conclusion.

3. Those species with special breeding systems, e.g., arrhenotokous reproduction, in which solitary individuals can establish new colonies, would seem to be at advantage in initial establishment. Many ambrosia beetles share these characteristics.

4. Solitary colonizers, such as Dendroctonus micans or D. valens, which can as individuals colonize and reproduce in living host trees would have higher probabilities of establishment, as evidenced by experiences in the United Kindgom and China, respectively.

Atkinson, T. H., R. J. Rabaglia, and D. E. Bright. 1990. Newly detected exotic species of Xyleborus (Coleoptera: Scolytidae) with a revised key to species in eastern North America. Can. Entomol. 122:93-104.

Beeche, M. A. 1998. Control cuarentenario a los embalajes de madera de internacion en Chile. pp. 188-198. In, Proceedings of the Actas Congreso Internacional De Plagas Forestales 18-21 de Agosto de 1997. Corporacion Nacional Forestal. Pucon, IX Region, Chile.

Ciesla, W. M. 1998. Recent introductions of forest insects worldwide, their effects on forest and related resources. pp. 199-208. In, Proceedings of the Actas Congreso Internacional De Plagas Forestales 18-21 de Agosto de 1997. Corp. Nacional For. Pucon, IX Region, Chile.

Flechtmann, C. A. H, A. L. T. Ottati, and C. E. Berisford. 1999. Attraction of ambrosia beetles (Coleoptera: Scolytidae) to different tropical pine species in Brazil. Env. Ent. 28:649-658.

Haack R. A., and Cavey J. F. 2000. Insects intercepted on solid wood packing materials at United States ports-of-entry: 1985-1998. In Proceedings: International conference on quarantine pests for the forestry sector and their effects on foreign trade, 27-28 June 2000, Concepcion, Chile. CORMA, Concepcion, Chile. 16 pp.

Lindgren, B. S., S. E. R. Hoover, and A. M. MacIsaac. 2000. Lineatin enantiomers preference, flight periods, and effect of pheromone concentrations and trap length on three sympatric species of Trypodendron (Coleoptera: Scolytidae). Can. Entomol. 132:877-887.

Mattson, W. J., P. Niemela, I. Millers, and Y. Inguanzo. 1994. Immigrant phytophagous insects on woody plants in the United States and Canada: An annotated list. USDA For. Ser. Gen. Tech. Rep. NC-169. 27 pp.

Wood, S. L. 1993. Revision of the genera of Platypodidae (Coleoptera). Great Basin Naturalist 53:259-281.

Additional links

Bark beetles: Chemical ecology of bark beetles;
FETCH 21

Ambrosia beetles: Interceptions in British Columbia;
Exotic species in the US