Pest Risk Management Decision Document – Brown Spruce Longhorn Beetle (Tetropium fuscum)
6.0 Pest risk assessment summary
Information in this section is taken mostly from the regional Pest Risk Assessment produced by the Nova Scotia Department of Natural Resources (NSDNR 2013), as well as the latest update of the official Pest Risk Assessment for Tetropium fuscum, the brown spruce longhorn beetle, completed by the Plant Health Pest Risk Assessment Unit of the CFIA in 2008.
6.1 Pest biology
In its native Eurasia, the brown spruce longhorn beetle is a wood-boring beetle (family Cerambycidae) that typically attacks dying or stressed spruce (Picea spp.) trees. The trees may be stressed as a result of wind, lightning, drought or fire damage, or insect defoliation, and freshly-harvested trees are also frequently attacked (Schwenke 1974; Hanks 1999). BSLB can also build up damaging populations in its native area, and is known to be a primary tree-killer in Europe under outbreak conditions (Juutinen 1955; Freude et al. 1965; Schwenke 1974). In Europe, however, it is mostly recognized as a secondary forest insect (Nüsslin 1905; Crawshay 1907; Saalas 1923; Schimitschek 1953; Juutinen 1955; Lottyyniemi and Uusvaara 1977), 'cleansing' the forest of diseased and weakened trees (e.g. Schimitschek 1929).
In Canada, spruce (red, white, black, and Norway) are attacked with a preference for trees that are physiologically stressed or of 'low vigour' (slow radial growth due to either age or temporary stress) but still apparently healthy. Reports of attacks on other types of conifers in Europe, such as pine (Pinus spp.), larch (Larix spp.) and fir (Abies spp.) are very rare, often outdated and possibly unreliable; more recent tests show that the beetle does not naturally reproduce onthese other conifersin Canada (Sweeney and Smith 2002).
Research in Canada also shows that BSLB prefers stressed trees as it does in its native range (Flaherty et al. 2013), that females preferentially lay eggs on them, that larval mortality is lower, and development time is shorter than on healthy trees (J. Sweeney, Canadian Forest Service, pers. comm.; Flaherty et al. 2011; O'Leary et al. 2003). However the beetle does, when forced, lay eggs in healthy spruce trees which can survive to adulthood (Flaherty et al. 2011), and also lay eggs on less-susceptible hosts (that is, unstressed trees) when they do land on them (Flaherty et al. 2013). Although BSLB generally kills stressed trees in Canada; these same trees may recover after temporary stressors are lifted (drought, defoliation, etc.) and could potentially survive in the absence of the BSLB (O'Leary et al. 2003). Therefore, the tree-mortality this beetle can cause is not simply an alternative to that caused by other sources, but may be, at least partly, an additional source of mortality.
Trunk silhouettes and host volatiles are important in the location and identification of potential hosts on which adults land and mate. The life cycle of the BSLB generally spans one year in central Europe (Schwenke 1974), but two years may be required in colder climates (Cherepanov 1990) or when the nutritional value of the host is poor (Schimitschek 1929; Juutinen 1955), or when the tree is vigorous and healthy (Flaherty et al. 2011). Because generations may overlap, all four life stages may be present on or in the host plant during the summer months in Canada.
Other than central Nova Scotia, Canada, BSLB is widely distributed in northern and central Europe; Austria, Belarus, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, the Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Italy, Lithuania, Latvia, Moldova, the Netherlands, Norway, Poland, Romania, the Russian Federation (European part), Serbia and Montenegro, Slovakia, Slovenia, Sweden, Switzerland, the Ukraine, introduced into but not established in the United Kingdom (Bilý and Mehl 1989, CABI 2012, Duff 2008). It also occurs in parts of Asia: Turkey, Siberia (Russian Federation), Kazakhstan, and Japan (CABI 2012).
6.3 Pathways of spread
Except during the relatively brief adult dispersal period, which peaks (with single or multiple peaks) sometime between late May and mid-July but may extend into August before trailing off (J. Sweeney, CFS, pers. comm.), all life stages of the BSLB are confined to the bark, phloem and sapwood of host trees. During this dispersal period, adult beetles have the capability to fly an average of 2.3 kilometres, based on laboratory flight mill data. Observational evidence suggests that the beetle has moved approximately 4km/year from its presumed point of introduction (Rhainds et al. 2011). It is believed that this combines natural and artificial spread, and that the natural spread has probably been aided by the high volume of susceptible host material remaining after Hurricane Juan of 2003; the most-infested areas of Nova Scotia lie directly in the Hurricane's path across the province (see appendix 1).
Current population levels are variable but low in most areas outside the urban core of the Halifax Regional Municipality as measured by current survey methods. Pathways for human-assisted dispersal include the transport of unprocessed spruce logs or green (i.e., not kiln dried or heat treated) lumber, particularly with attached bark; crates, pallets and dunnage (scrap green spruce lumber used to support loads), as well as spruce firewood. Lower-risk artificial pathways include large wood chips containing attached bark strips (i.e., phloem), and bark. Restricting movement of raw spruce wood products to outside the adult dispersal period mitigates the risk. Manufacturing or processing raw forest products mitigates the risk of specific pathways to acceptable levels of risk to allow movement of spruce articles.
6.4 Overall risk rating
The CFIA's 2008 PRA overall risk rating for Tetropium fuscum is Medium; establishment potential is rated High, natural spread is rated Low, and environmental and economic impact are both rated Medium. The risk components in a risk assessment are combined to give an overall risk summary, which resulted in a Medium ranking. This implies that specific phytosanitary measures are recommended to slow or restrict the spread and introduction of BSLB to uninfested parts of Canada. Spread is expected to continue at a low rate, and regulatory and control actions do play a role in slowing the ongoing artificial spread of this pest.
Although BSLB was initially thought to be a destructive pest of spruce trees because of its ability to attack and kill large, apparently healthy, spruce trees, there is recent evidence it prefers stressed trees, although it does have the ability to colonise healthy spruce trees. Its risk rating in the CFIA PRA was changed from High in 2000, to Medium in 2008.
There is still lingering uncertainty around how it may behave in different environments in Canada, as well as when combined with other tree-stressors such as climate-change and pests such as Spruce budworm and Spruce bark-beetle. The Nova Scotia Department of National Resources (NSDNR) recently produced a regional PRA on BSLB with the overall conclusion that "BSLB is determined to pose a low-moderate risk to the province of Nova Scotia" (NSDNR 2013). The assessment states that the overall risk posed by BSLB to the province is considered low; however, due to uncertainties and information needs, the ranking was raised slightly to 'low-moderate'.
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