Plane lace bugs, enemy of London plane trees, were recently confirmed in the UK for the first time since 2006. So what threat do they pose?

RECENT findings of plane lace bugs (Corythucha ciliata) on central London’s plane trees (Platanus x hispanica) do not bode well for greenery across England’s capital city.

These sucking pests are not immediately life-threatening to the 100,000-plus plane trees in London, but are pernicious and debilitating, especially over consecutive years. This emerging problem has a lot in common with the effects of horse chestnut leaf miner (Cameraria ohridella) on white-flowering horse chestnut (Aesculus hippocastanum), with the first finding in Wimbledon in 2002. 

Notwithstanding that the adults and nymphs of one (lace bug) damage the trees by sucking the sap from the leaves, while the larvae of the other (leaf miner) chew their way through the internal tissues of the leaf, there is much in common. Either way, the results do not bode well for white-flowering horse chestnut or London plane. Both are important amenity/landscape trees for which looks are all important, especially a pristine leaf canopy.

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Both are alien pests attacking exotic and primarily amenity/landscape trees. Horse chestnut leaf miner originates in the Balkan countries of south-east Europe, although plane lace bug arrived in Britain via a more circuitous route. Plane lace bug is a native of North America and was first discovered in Europe at Padova, Italy, in 1964. The insect pest has since spread throughout most of mainland Europe and as far east as Uzbekistan in Central Asia. White-flowering horse chestnut is an exotic tree, arriving in the UK from Turkey as far back as the mid-1500s. 

The origin of the London plane tree is equally exotic, but measurably more complicated, as the result of a natural hybridisation between Oriental plane (Platanus orientalis) from southern Europe and the American sycamore (Platanus occidentalis). The natural hybridisation is variously thought to have occurred during the 17th century in Spain (hence the name) or a garden environment in Oxford.

One in five trees in London is a London plane.One in five trees in London is a London plane. (Image: Getty)

There is no doubting how horse chestnut leaf miner arrived in the UK – inside the dead leaves of white-flowering horse chestnut attached to vehicular traffic arriving from Europe. There is no definitive answer to the arrival of plane lace bug, but the ability of the insect pest to attach itself to transportation including ships, trains, lorries and cars is historically well established. Other established means and modes of entry include imports of wood and timber, with the insect able to attach itself to such plant materials. Interestingly, little has been said about the risk posed by imports of London plane tree planting material from Europe. However, since historically plant nurseries were advised to spray their young trees with insecticide, it seems transfer on tree-planting material poses a real risk.

Corythucha ciliata was first found in the UK way back in 2006 at two plant nurseries in Bedfordshire, one of which was close to a stand of mature London plane trees. Further investigation showed London plane and Oriental plane trees imported from France and Italy, and on site at the nurseries, were infested, as were the mature plane trees alongside one of the nurseries.

According to DEFRA documents from that time, the level of pest infestation and its distribution on more mature plane trees found at multiple sites led to the assumption that C. ciliata was already established in the UK, so statutory action against the pest was deemed inappropriate. As recently as 2023, DEFRA said: “17 years later, there have been no further reported findings of Corythucha ciliata in UK and it is deemed to be absent.” They clearly spoke too soon, because the pest was found in 2024.

According to DEFRA documents, the insect has other host plants, including Fraxinus sp, as if the poor old common ash tree has not got enough to contend with already.

BIOLOGY OF THE INSECT PEST

The common name (plane lace bug) refers to the insect’s host tree preference and the appearance of its lacy white exoskeleton. Several species of lace bug can be found in the UK, although Corythucha ciliata is the only one that feeds on the leaves of the London plane tree, where it is invariably found on the abaxial surface (underside of the leaf). That’s because, like most other sucking insect pest species, lace bugs directly tap the prominent veins on the under-leaf surface to access, suck and secure the sugar-rich sap.

The adults are extremely small (maximum 4 mm in length) and are therefore difficult to see and identify on the leaf surface with the naked eye. Both nymphs and adults feed on the underside of leaves, initially near the veins and then progressively outwards, to affect the entire leaf. 

This results in a chlorotic stippling of the upper (adaxial) surface of the leaf ,which may then drop off prematurely. Adult females have the capacity to lay up to several hundred eggs along the leaf veins during the warmer months of the year, with up to three generations of the insect pest occurring during one year. 

The optimum temperature for insect development and a complete life cycle is 19–33°C, although adult plane lace bugs have been known to survive winter temperatures down as low as -24°C, by overwintering in loose bark on the trunk. Ironically, the exfoliating nature of London plane bark, critical in this amenity tree’s ability to withstand urban atmospheric pollution, has proven highly advantageous to the plane lace bug. The closely related Corythucha arcuata (oak lace bug) has been known to overwinter under raised bark, in bark crevices and branch forks, covered by leaf litter and other debris, so it is likely Corythucha ciliata does the same. 

The exfoliating bark of the London plane provides a convenient hiding place for plane lace bugs.The exfoliating bark of the London plane provides a convenient hiding place for plane lace bugs. (Image: Getty)

The first sign of feeding damage is a stippling of small, yellow spots on the upper leaf surface, often concentrating around the leaf veins. As the lace bug populations and damage increase, the chlorotic spots coalesce to produce large, yellow- and bronze-coloured areas on the upper leaf surface. Heavy infestations can also cause premature leaf drop. If there is a severe infestation, whole leaves become whitish and the tree may lose all leaves in late summer. Perhaps the easiest way of finding and identifying the presence of plane lace bug is to peel off the bark during the autumn, winter or early spring, to locate the overwintering adults. 

ECONOMIC IMPACT

Consecutive years of insect feeding, especially in high numbers, weakens plane trees, which are now vulnerable to opportunistic fungal pathogens causing disease. Such situations are likely to require the replacement of affected trees, with localised environmental impact for many years until the replacement trees mature. 

Accumulative severe damage caused by Corythucha ciliata, combined with other environmental influences, especially physiological stress caused by high temperature and drought, may actually kill affected trees. That said, unless lace bug feeding is combined with other factors, the only noticeable and tangible effect will be bronzed foliage and early leaf drop. In this context there is clearly a potential exacerbating effect due to ongoing climate change. Observing the plane lace bug situation in countries with measurably warmer climates than our own may permit an accurate prediction of the likely effects should it become widely established in the UK. 

Other impacts, including the social impact of insect biting and the environmental/social impact of shade tree loss in urban environments could also become economic issues. Biting has been recorded in Paris in France, Piedmont in Italy, and Bucharest in Romania, which incidentally has roughly similar climatic conditions to those experienced in the southern half of England.

Experience shows that biting only occurs in areas where insect populations have progressively built up to give a high density of adult lace bugs on plane trees, most often occurring during the hot summer months. Of course, the loss of trees in urban environments will push temperatures even higher to aggravate such problems. As a general rule there is a drop of 2°C in air temperature under leafy tree canopies in urban environments.

Last but not least, I must return to the comparison of plane lace bug/London plane tree and horse chestnut leaf miner/white-flowering horse chestnut. 

The effects of horse chestnut leaf miner, as bad as they were for the UK’s number one amenity/landscape tree, were totally eclipsed when bacterial bleeding canker disease (Pseudomonas syringae pv aesculi) raised its head. This bacterial pathogen has essentially destroyed white-flowering horse chestnut as an economically viable amenity/landscape tree. 

Don’t forget there is a disease of plane trees called plane wilt, caused by the fungal pathogen called Ceratocystis platani, causing destruction of plane trees in Europe. This disease has the potential to do to London plane what chalara ash dieback is currently doing to common ash.