Holly Leaf Miners

As part of our fieldwork last week, we were investigating the abundance of holly leaf miners (Phytomyza ilicis) in a stretch of holly trees in some of the woodland on campus. In this post, I wanted to explore some of the fascinating biology of these organisms, as well as explain how we decided to survey them to estimate their population.

Biology

Holly leaf miners are part of the Agromyzidae family. This is a group of small flies whose larvae mine leaves for food. Many other groups of organisms, however, have leaf-mining larvae as well, including other flies, moths, sawflies and beetles.

They have a fascinating life cycle, where adult flies lay eggs on new holly foliage in May or June. The eggs are laid on the midriff of the leaf, on the protected underside of the leaf. The larvae tunnel inside the leaves, feeding on the plant tissues, first the midriff during the autumn months, followed by burrowing into the mesophyll. The following spring, they pupate inside the cuticle of the holly leaf, which is retained over the pupa as a protective covering, before finally emerging as adult flies. The larvae can even be further parasitised by parasitoid wasps!

Pests?

The holly leaf miner causes yellowish blotches on the upper surface of holly leaves. While this obviously causes a loss of photosynthetic efficiency of the leaf, it generally doesn't have a major effect upon the holly tree as a whole. However, in other areas of the world, such as Canada, holly leaf miners are invasive pests which can have a more severe impact on the population of holly trees. In fact, investigations have been done into using the parasites of the holly leaf miners themselves as a form of biological control.

Surveying

To determine whether the miners are likely to have a serious detrimental impact on the holly trees, its helpful to know just how many miners there actually are. And while it would be practically impossible to count all of this tiny species of fly in a woodland, it is relatively easy to count the number of holly leaves scarred by their mining activities. Again however, unless you have endless time and patience (which I definitely do not!), you can't examine all of the leaves of all the trees. So instead, we came up with a method of selecting a sample of leaves to examine.

It all relies on the magic number - 30. This is the number of observations you need to have a relatively reliable estimate of whatever you're studying. So we decided to survey 30 trees, and on each tree, choose (at random) 3 branches. We then counted the number of affected leaves on each branch on each tree that we counted. It's easy to modify this method to work out two things:
- The percentage of leaves affected by the miners - by also counting the total number of leaves looked at
- The total abundance of affected leaves - by estimating the number of branches per tree and trees in the wood, and use these to scale up the average amount of mined leaves that we found

This demonstrates how important various surveying and sampling methods can be when dealing with very very large populations! It was also a fascinating insight into the lives of some tiny but interesting critters.


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