Britain’s most common native evergreen is Ilex aquifolium, the holly tree. It can reach 15 metres in height and live for around 300 years. Its glossy dark green leaves and bright winter berries make it one of the most recognisable trees in the country, especially when female trees are in fruit.
Those sharp leaf edges that many of us associate with holly are not always present. Research has shown that the tree produces spines as a defence response when its leaves are grazed.
Image courtesy of Pete Hillman Nature Photography via Nature Journeys
Holly has long been steeped in symbolism. Early Pagan traditions viewed it as a tree with protective qualities, and later it became linked with Christianity, with its red berries representing the blood of Christ. Today it is simply a winter icon, used widely in festive decorations.
Yet holly is much more than a seasonal ornament. Its berries are an important food source for birds during the harshest months of the year, and the tree supports a variety of insects that use its dense foliage for shelter and breeding.
One of the most specialised of these is the holly leaf miner fly Phytomyza ilicis. This tiny insect relies entirely on holly for its survival. The larvae can only feed and develop within holly leaves, making the species completely dependent on the tree. Despite this, the fly is widespread across Britain, though the adults are only around three millimetres long and rarely noticed when they visit holly in late spring.
Their presence becomes obvious later in the year. Eggs are laid along the petioles of new leaves, and within a couple of weeks the larvae burrow into the mid vein where they begin feeding on the internal leaf tissue. Over the next five months they tunnel through the leaf, creating pale green, transparent blotches as they grow.
Image courtesy of Pete Hillman Nature Photography via Nature Journeys
In winter the larvae stop feeding and lie dormant inside the leaves, protected from frost. Feeding begins again in early spring before pupation takes place within the cavities they have formed. Two or three weeks later, a new generation of flies emerges from the leaf.
Despite the cosmetic damage, this activity causes little real harm to the tree. However, if you harvest holly for Christmas decorations, it is worth checking for the telltale blotches and avoiding affected leaves. It helps preserve the look of your festive displays and leaves the overwintering larvae undisturbed.
Discover more fascinating insights from Dr Ian Bedford in our ongoing Bug of the Month series, exclusive to Fargro Home and Garden.
About Dr Ian Bedford
Ian has been fascinated by the bug world for as long as he can remember. From studying butterflies on the South Downs as a youngster, he went on to pursue a career in Research Entomology and ran the Entomology Dept at the John Innes Centre in Norwich up until his recent retirement.
'Bug of the Month'
Every month Ian will share his knowledge on how to protect your plants and gardens from preventable pest invasions while providing valuable insights into the insects regularly found in our gardens.