Earlham Park: Holly Leaf Miner

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The holly leaf miner (Phytomyza ilicis) is a fly that lays its eggs between the upper and lower surface of a holly leaf. Initial damage is caused to the leaves by the female leaf miners piercing the leaf to deposit the eggs. The larvae feed on the leaf tissue which causes the mines which are pale blemishes on the leaf surface. Mines ruin the aesthetics of the leaf and may cause partial defoliation.


In June-July, the eggs are laid on the younger, softer leaves. The older leaves on the trees have a thick and tough cuticle that the flies find it more difficult to break through. Leaf miners like to lay their eggs here because it provides a protected zone and a food source when the larvae emerge.
The larvae first tunnel through the midrib of the leaf and feed outwards through the tissue for the winter. By March, the larvae should turn into pupae. The miners will emerge as adult flies in May and June.

The success of holly leaf miners is limited by predation from birds, such as blue tits. Parasitic wasps (Chrysocharis gemma) insert an egg through the leaf and into the body of the miner. The wasp larva will hatch inside the body of the leaf miner and feed on it, killing the leaf miner larva. 

Although the leaf mines are not attractive to look at, the growth and health of the leaf is barely affected.

One of our spring practical sessions was spent in Earlham Park observing these leaf mines. We were given the task of quantifying how many leaf mines were in the area. This was with the aim of measuring the population of leaf miners over time. Each year's growth could be defined from the last as the branches have joints where the new growth has emerged from the mature branch. 

We had to consider factors like how we defined a holly tree, what height range we should look at branches and how many branches we should pick from each tree. We decided to walk 5 metres between sampling sites and pick 3 branches from each sample sites. A limitation of this data is that the eye is naturally drawn to the branches with more leaf mines on them so we were more likely to pick branches that would yield higher numbers of mines in our results. Our results found that when a holly branch had any leaf mines, they tended to range from having 6-10 mines each.



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