Ennomos subsignaria (the elm spanworm) is a North American (Canada and USA) moth which intermittently reaches outbreak densities, causing serious defoliation of hardwood forests in its native distribution.

Risk of entry
Wood in the rough and fuel wood were rated as moderately likely pathways on which this pest could enter the UK. These ratings were made with medium confidence due to the lack of detail in the trade data including information about processing. Broadleaved trees for planting, and cut branches and foliage were rated as unlikely pathways with high and medium confidence respectively. Other plants for planting not falling into the broadleaved tree category and cut flowers were both rated as very unlikely pathways with high confidence. Isolated bark/bark chips and contaminating pest/hitchhiker pathways were also both rated as very unlikely pathways but with medium confidence.

Risk of establishment
Risk of establishment outdoors was rated as likely. This pest is highly polyphagous, feeding on many broadleaved genera and species present in the UK, and UK 2 temperatures sit within the extremes of the pest’s current distribution. This risk was rated with medium confidence due to few lifecycle parameters being published for this pest.

Economic, environmental and social impact
All impacts were based on the assumption that established E. subsignaria populations could reach outbreak numbers in the UK. Potential economic, environmental and social impacts were all rated as large but with low confidence due to the uncertainty over this pest’s ability to outbreak in a novel climate and environment.

Endangered area
The endangered area was considered to be broadleaved trees in urban and rural
environments across the UK. 

Risk management options
Many of the host genera of this pest are listed as ‘high-risk plants’ in the regulations and as such are prohibited from import as plants for planting pending a risk assessment. Bacillus thuringiensis (Bt) appears to be the most common means of control in North America (USA and Canada). Biopesticides with B. thuringiensis are available for use in the UK, but, if required, large-scale or aerial spraying of trees might be controversial. Other management options are discussed but have many uncertainties.

Key uncertainties and topics that would benefit from further investigation
Main uncertainties: Causes of drastic increases in E. subsignaria populations and the environmental impacts.
Further investigation: Parasitoids of Lepidoptera and more specifically Geometridae which are present in Europe and their potential to parasitize E. subsignaria.