Field Trip - Sampling leaf miners

As part of our Issues and Approaches in Ecology module we went on an on-campus field trip, and one of the things we did was sampling leaf miners on holly bushes (Ilex aquifolium). A leaf miner is the larva laid on a leaf by an insect, which lives within the leaf and eats the plant tissue. Insects which do this include moths (Lepidoptera) and flies (Diptera). This protects the larva from many predators, and provides them with a source of food. The miner leaves a discoloured blotch on the leaf, which is usually green with some brown marks within, as below:

Photo by David Fenwick

This pest has little impact on the health of the holly bush, and is difficult to control as insecticides are relatively ineffective due to the thick, glossy protective layer the surface of the holly leaf provides. On other plants which are less well protected, however, they can reduce the economical value of a crop, causing problems for farmers, so in this context control of leaf miners is more of a pressing issue.

The task we were set seemed relatively simple, to conduct an experiment to estimate the population of holly bushes and the number of these affected by leaf miners. We began designing our experiment, and in our group of 5 we found that we could easily identify new growths from the last season as it leaves a scar on the branch. Therefore we could estimate the number of leaf miners left in the last growing season, between May and June.

We conducted our survey as follows. We divided the area of woodland we had been given into five separate areas by pacing out five paces along the path, and five paces away from the path, giving us each an area of approximately 25m squared. We decided to sample the holly bushes by taking a measurement of a new growth at a high, medium, and low level (above the head, just below head height, and about knee height). We measured new growths as it would give us flexibility with the data as we would know the average number of leaves on a new growth, whereas measuring the number of leaves would not give us this flexibility and would also be more time consuming. We each aimed to sample 50 new growths.

Bias was an issue we would ideally have overcome by using a random number generator of some sort, but this was not available to us as we were making it up on the spot. The approached this by using a rather amateur method, which included closing our eyes and randomly holding our finger out and sampling the branch we were pointing at. This is a flawed method as it is still subject to bias and means we may not have been pointing at a plant at the right height, but it was the best we could do with the resources available to us.

After sampling at the three heights we moved around the holly bush until we had gone all the way around, measuring each about 5 times, and then moving on to the next one. We recorded the number of leaves on the new growth, and the number of leaves infected by leaf miners. We could then use this information to calculate the percentage of the holly bushes that were infected with leaf miners.

To take this further, we could have used the information to calculate the number of holly bushes infected in the entire wood. To do this we would need to know the area of the wood, but this posed the issue of variation. For example, on one side of the path where we were sampling, there were many holly bushes. On the other side, there were next to none. We would therefore need a larger sample area to calculate the average percentage of woodland covered by holly bushes, and we could then times this by the proportion of leaves that were infected to see how many new growths throughout the wood were infected. However, this would take much more time and planning than just this one field trip allowed.