Post provided by Michael Staab

What are ‘Trap Nests’ and What are They Good For?

When thinking of bees and wasps, most people have social insects living in colonies in mind. But most species are actually solitary. In these species, every female builds her own nest and does not care for the offspring once nest construction is completed. Most of those species nest in the ground. Several thousand species of bees and wasps use pre-existing above-ground cavities though (such as hollow twigs and stems, cracks under bark, or empty galleries of wood-boring insects).

To keep you in suspense, I’ll resolve the importance of studying cavity-nesting species later in this blog post. First, I’ll introduce you to one of the more elegant research methods in ecology: trap nests. To study and collect these cavity-nesting species, you can take advantage of their nesting preferences. By exposing artificial cavities and offering access to an otherwise restricted nesting resource, you can attract females searching for suitable nesting sites.

Building these trap nests is simple, but the design can vary greatly. Many designs and materials can be used to build the artificial nesting sites, such as drilling holes in wooden blocks or packing hollow plant material (e.g. reeds) in plastic tubes. Once females find the trap nest and finish their nest construction, the developing offspring are literally ‘trapped’ in their nests. They can then be collected, their trophic interactions (e.g. food and natural enemies) observed, and the specimens can be reared for identification.

A Brief Look into the History of Trap Nests

Because trap nests let you assess a species’ entire life history, they’ve fascinated biologists for a long time. The first accounts of trap nests (that I’m aware of) are from the French naturalist Jean-Henri Fabre, who studied mason bees nesting in a reed-thatched roof. Continued use of trap nests for studying natural history, especially in the United States, inspired the first comprehensive description of trap nests and the associated species in Karl Krombein’s seminal 1967 volume ‘Trap-nesting wasps and bees: life histories, nests, and associates’. That book sparked many studies, particularly on unravelling natural history and managing bees for crop pollination. Later, in the 1990s, researchers realised the potential of trap nests for comparing species communities. This led to the popularisation of the technique in quantitative ecology and in bioindication studies.

The Origin of Our Article

Our article, ‘Trap nests for bees and wasps to analyse trophic interactions in changing environments—A systematic overview and user guide’ (published recently in Methods in Ecology and Evolution), has a more personal history attached to it. I first learned about trap nests in 2011 at the start of my PhD. My aim was to investigate how tree diversity and forest succession influence trophic interactions in the subtropical forests of south-east China. One of the model communities for this research was host-parasitoid interactions quantified with trap nests.

The overall endeavour was successful (read more here) and we made a spectacular natural history discovery (search online for ‘bone-house wasp’), but the field campaign had some problems. While searching for possible solutions for the initially low success rate of my trap nests, I was surprised that no comprehensive methodological publication was available. To me, it appeared that the practical knowledge on how to successfully use trap nests under challenging field conditions mostly stayed within departments and research groups.

I found this lack of a conceptual methodological standard reference unfortunate, as it made it more difficult for researchers to include trap nests in their research portfolio. This impression was frequently backed-up by reports of problems and failures from other researchers who had tried trap nests. The problems I could see people having encouraged me to start working on the manuscript.

Originally, I only wanted to focus on the methodological part – my paper was going to be a usable and accessible guideline. Relatively quickly the idea matured to combine this methods appraisal with a review of the knowledge we’ve gained from trap nests so far. My plan was partly to summarise and synthesise exiting knowledge, partly to identify research directions that the next generation of trap nest researchers might focus on.

Why Knowledge of Species is Not Enough: The Benefits of Trap Nests

Many research projects in ecology and evolution only sample species. Broadly speaking, they might use the data to investigate, for example, how changes in habitats and ecosystems relate to the occurrence, structure or fitness of single species or entire communities. These approaches are certainly very valuable, but in my opinion they tend to ignore one key idea: species are not isolated entities, they interact with many other species. Of these interactions, trophic (i.e. consumptive) interactions are particularly important.

Many sampling methods for collecting species are not suitable to also capture those interactions. Here lies the big advantage of trap nests: even though the number of species that can be sampled with trap nests is relatively small, the method allows us to closely study trophic interactions of those species and to easily quantify them.

For example, the pollen sources of cavity-nesting bees can be identified by analysing the pollen residues in their nests. Similarly, you could identify the prey organisms of wasps by looking at what they collected to feed their offspring. But, the most ubiquitous interactions are with the natural enemies (mostly parasitoids and kleptoparasites) of the cavity nesters. Every bee and wasp species is ‘host’ to a range of natural enemies attacking their offspring or their food provisions. So far, identification of all species involved relies mostly on morphology, but I expect that molecular identification tools such as barcoding will quickly take over.

The huge amount of interactions that are quantifiable in the same system make trap nests a promising tool to investigate how changing habitats influence species interaction networks. This is a pressing research topic for ecology and evolution and trap nests might be a suitable model system for this endeavour.

To find out more, read the full Methods in Ecology and Evolution article ‘Trap nests for bees and wasps to analyse trophic interactions in changing environments—A systematic overview and user guide’

For more information on the author visit his personal blog.