Rapid Pest Risk Analysis (PRA) for: Corythucha ciliata
[Draft for stakeholder comments up to 2023-06-28]
Corythucha ciliata is a hemipteran insect, native to North America and now also present in mainland Europe. It is not known to be present in the UK. It mainly feeds on Platanus (plane) trees.
Risk of entry
Platanus trees are the preferred host, with very little data on movement of Corythucha ciliata on other tree species. Entry on the pathway plants for planting has been assessed as moderately likely with medium confidence. The highest risk would come with adults overwintering under loose bark of mature trees or loose-leaf litter in large pots but as most imported species of Platanus plants for planting would be immature, this is a lower risk.
Entry on wood and wood products with bark associated has also been assessed as moderately likely with medium confidence.
Hitchhiking on transport has been assessed as likely with medium confidence due to patterns of dispersal seen in Europe. This has been lowered from very likely to likely due to the fact it has not yet established in the UK though it was initially found in 2006 in the UK.
Risk of establishment
The temperate climate of the UK is not thought to be a barrier to the pest establishing outdoors and so establishment outdoors has been rated likely with medium confidence. This has been lowered from highly likely due to populations found in the UK previously not establishing.
There are few of the host species grown under protection, so establishment indoors has been rated unlikely with high confidence.
Economic, environmental, and social impact
Economic impacts have been rated as small with medium confidence, as it is thought that the impact would be minimal as the pest may put trees under further stress but not be the primary cause of decline.
Environmental impacts have been rated as small with medium confidence, due to the low numbers of plane trees in wider environment and impacts in urban areas are taken into account under economic and social impacts.
Social impacts have been rated as medium with medium confidence, as the pest would possibly cause minor skin irritation due to biting and general decrease in shade provided by trees in urban areas though this is likely to only occur in very hot weather.
Areas of the UK where plane trees are found, particularly those in urban areas where there is an increase in plane tree density and the trees tend to be more stressed may be particularly susceptible. The most prominent example of high density of plane trees planted is London which has 1.6 million trees planted as of 2015 of which 64,000 are London plane trees (4% of total trees) (Kenton Rogers 2015).
Risk management options
One option could be to exclude C. ciliata and to classify it as a quarantine pest. This could possibly require plants for planting and wood of plane trees in C. ciliata infested regions to meet specific requirements before entering the country. Action in the event of an outbreak could be eradication by pesticides (as was conducted last time) or containment via sticky traps, or cardboard collars, which could be beneficial in both early detection and keeping populations low.
Key uncertainties and topics that would benefit from further investigation
In the UK, there was an initial finding of populations on numerous plane trees at two nurseries in Bedfordshire and a stand of plane trees 50 m away from one of the nurseries. These were treated by spraying with a pyrethroid insecticide but due to the number of Corythucha ciliata found at the time, was assumed to be established and no further statutory action was taken. However, no further findings have been found at these sites or elsewhere in the UK since this initial finding. Therefore, it appears that treatment was effective, and the population was unable to establish. It is unknown why C. ciliata has not established since then and this uncertainty is worth further discussion and research. This could perhaps be due to lack of plane trees in the immediate vicinity at the time.
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- United Kingdom