After introducing plants, herbivorous insects and insectivorous birds needed in ecological experiments of tritrophic interactions, it’s time to tell more details of the actual experiments. This post will tell how to use artificial plasticine caterpillars to measure predation rate of insectivorous birds. The basic principle is to attach the artificial caterpillars on experimental trees and then regularly check those for pecking marks. Photos of intact caterpillars are on the top row and pecked ones on the lower row.
It is surprisingly easy to recognize pecking marks by birds but you can also record attacks from mammal, reptile and arthropod predators, and even parasitoids (Low et al. 2014). Using artificial caterpillars has become a quite common method in various kinds of ornithological experiments. For example, Sam et al. 2015a studied predation rate in tropical mountains, Muiruri et al. 2016 in different tree diversity areas and Loiselle et al. 2002 in forest canopy and understory. I have used plasticine caterpillars to study attraction of insectivorous birds to herbivore-damaged birches (Betula spp.) when the birds couldn’t see the real caterpillars (Mäntylä et al. 2008a, 2014) or when birds were fooled with artificial chemicals (Mäntylä et al. 2014; Koski et al. 2015). The real caterpillars in my experiments were autumnal moths (Epirrita autumnata) that are mainly green and thus my plasticine caterpillars were made of a green plasticine close to that colour. The artificial plasticine caterpillars are an easy way to study also conspicuousness but I will not go into that in this post (e.g. Mänd et al. 2007; Rowland et al. 2007; Hossie & Sherratt 2012).
Material: I have used plasticine (modelling clay) that is soft and doesn’t get harder when taken out of the package. There are several companies producing these. The plasticine is not really toxic but it is also possible to use edible pastry as material. Sam et al. 2015b have compared these two materials. I have used a thin metal wire to attach the caterpillars on branches (see the photos of this post) but it is possible to also glue the caterpillars on the leaves or branches (e.g. Howe et al. 2009). Then make sure that the glue is not too bad-smelling and doesn’t cause any unwanted reactions with the plant. In 2006 I tried to use freeze-dried autumnal moth caterpillars glued to birch leaves but that didn’t work at all. The caterpillars lost all their shape and colour in that process. 😉
Placing: In my experiments I have usually placed 10 plasticine caterpillars per experimental tree. The exact place for each caterpillar was semi-random so that they were throughout the branches of the tree. The caterpillars shouldn’t be too visible but also not hidden under big leaves. I always checked the caterpillars daily and replaced the damaged ones with new caterpillars attached to a slightly different place. Then there were on each day the same amount of intact caterpillars. Even though it is easy to recognize the bird pecking marks on the caterpillars, there can be some differences between people. Therefore, it is important that throughout the experiment the same person is checking the caterpillars or all of the people are trained to record the damaged caterpillars in the same way. One other thing is that use only one colour when marking your experimental trees with ribbons (or something similar). If you use a different colour for each treatment, the birds may learn that and cause unwanted error in the results.
Duration: In my experiments the birds learned within two weeks that the plasticine caterpillars were not edible and lost their interest (see figures below). Therefore, this is a useful method for experiments of a rather short duration.
Bird monitoring: Of course, it is also interesting to know exactly what bird species peck the plasticine caterpillars. You could get that information with video or trail cameras but it could be quite expensive and a lot of work to install these next to several experimental trees. But you can monitor at least a couple of trees to get a general idea (as in Muiruri et al. 2016). In my experiments I wrote down all the birds (species and number) I saw and heard at and around the study area each study day.
This is a brief overview of some important points when using artificial plasticine caterpillars. You can find more information, for example, from this article (Barber 2012).
PS. When the experiment is over you should try to find all the plasticine caterpillars and remove them. But sometimes you miss a few and find them next spring (photo below). 😉
© Elina Mäntylä (firstname.lastname@example.org), 29 June 2016