Neodiprion abietis (the balsam fir sawfly) is a sawfly native to North America that attacks coniferous species, including fir, spruce, and pine. Described as a serious pest in Canada, N. abietis feeds on host needles, leading to defoliation, reduced growth, and mortality of the host plant. Historically, outbreaks in Canada are localised and periodically last 3-4 years however, in recent years, the outbreaks are larger scale and last much longer. It is expected that this pest could establish in the UK and cause significant damage.

Risk of entry 

Various life stages of this pest could be associated with the host plants for planting or host branches and cut trees, however these pathways were rated very unlikely with a high confidence due to the existing prohibitions on these commodities from non-European countries. Natural spread was given a similar rating, given the pest’s current distribution. Finally, hitchhiking on non-host plants, as previously intercepted in the Netherlands, was rated as unlikely with medium confidence, considering its strong host preference for oviposition.

Risk of establishment 

Outdoor establishment of N. abietis was rated likely with medium confidence. The balsam fir sawfly has multiple hosts that are present in the UK, though some uncertainty remains regarding its host range. In addition, the current distribution of N. abietis spans climates that are similar to the UK.

 By comparison, establishment under protection was rated very unlikely with high confidence, given that its hosts are cultivated outdoors. No infestations of the pest have been reported indoors.

Economic, environmental and social impact 

Neodiprion abietis could have a significant economic impact in the UK, given that multiple known hosts are grown for forestry, in estate woodlands or plantations for timber, or for ornamental purposes. The economic impact was rated medium, given that a pest outbreak could impact survival or growth of these trees. A low confidence was provided with this rating, as there is some uncertainty surrounding the host range of the balsam fir sawfly. As defoliated ornamental species would affect the aesthetic appeal of parks, botanic collections and gardens, the social impact of this pest was rated as small with high confidence.  

Any defoliation or tree mortality could negatively impact native species that rely on the trees for their survival. As tree mortality is less frequently reported in the literature, the environmental impacts were rated small. Medium confidence was provided, again reflecting the uncertain host range.

Endangered area 

Woodland areas and plantations of conifer hosts are most at risk from this pest, though urban spaces with these host species are also threatened. One study suggests that only certain parts of the UK, including Wales, Scotland, and northern areas of England, are at risk of this pest due to the presence of host species combined with other environmental factors.

Risk management options

 Given the damaging nature of this pest, and the presence of many of its hosts in the UK, some of which play significant roles in the forestry industry, exclusion of N. abietis is the preferred option. There is already an existing prohibition on the import of plants for planting, branches and cut trees of the confirmed hosts (Abies, Larix, Picea, Pinus, Pseudotsuga and Tsuga) from non-European countries, including all the countries identified within the distribution of the balsam fir sawfly.

If a pest outbreak occurred in the UK, eradication should be attempted with the destruction of all associated material, and the application of general pesticides to eliminate any remaining sawflies, followed by re-inspections of the site over time.

Key uncertainties and topics that would benefit from further investigation

A key uncertainty is regarding the host range of this pest and whether it would attack species deemed uncertain hosts, or European conifer species. This could impact the rating for establishment and potential impact of the balsam fir sawfly in the UK. 

If this pest spread to a European country, it would also be useful to understand the natural capability of N. abietis to disperse.