Methods papers from PhDs: pan traps for pollinators

My most-cited paper so far (although not really the most-cited when you take years of publication into account) is an entomological field methods paper. It was also an unplanned paper. It came out of my PhD data, but wasn’t one of my research questions.

Methods papers are great contributions to the literature, and I highly recommend PhD students consider writing one, especially if they are working on understudied systems, or find some interesting patterns during data collection. Methods papers have much broader application to diverse fields and sub-disciplines than the PhD results themselves might.

Some background: my PhD research investigated the unmanaged insect pollinators living in and around almond plantations in the beautiful mallee region of north-west Victoria, which is the centre of almond production in Australia. (See posts here, here, here, here). Almond is Australia’s top nut crop and relies 100% on insect pollination to produce commercial yields. Almond trees flower in late winter, before peak insect activity, and European honey bees are rented in to orchards during flowering to pollinate the almonds, with huge costs to growers. Although almonds are one of the top horticulture industries in Australia, no research had been published on what native insects might be available to pollinate almonds. This was interesting to me, because most large plantations are managed as monocultures, which is more efficient for management and harvesting, but isn’t ideal habitat for keeping beneficial insect communities around.

An almond plantation in north-west Victoria.

So, I knew what research questions I wanted to answer, but how was I going to collect the insects I needed to answer those questions?

There are lots of ways to collect insects outdoors (and see Simon Leather’s Entomological Classics tag on his blog for posts on the history and use of different types of traps). The question is not ‘What’s the best trapping method?’ but ‘What type of insect are you trying to collect and what part of its ecology do you want to know about?’.

I wanted to collect flying insects that visit flowers (mostly bees, flies and wasps) and I didn’t have much field assistance; so I wanted a cheap, effective trap that I could deploy and collect quickly and easily at lots of sites across a large study region, often with long drives between sites (traps need to be deployed/collected within a short timeframe to be comparable).

Pan traps ticked all the boxes. They are plastic picnic bowls painted in different UV-reflective colours, filled with water and a drop of detergent to break surface tension. They are then left in a visible location at the site for most of the day. Flying insects are attracted to the colours and fall into the water, where they can be collected later.

I followed the standards for pan trapping from previous published studies, and at each site used the three main colours (yellow, white, blue) that are known to be attractive to my target insect groups. Yellow and white are considered very attractive to flies and wasps, while blue is considered to attract bees. In fact, blue is thought to be so attractive to bees, that a few entomologists I asked for advice during my project design told me that I should be using blue-vane traps if I was looking for pollinators. But they were really expensive and had only been tested in the northern hemisphere. Plus I wanted to collect a range of insect pollinator taxa, not just bees, so I stuck with the pan traps.

When I started going through my first round of data, I noticed something interesting. Yellow traps caught the highest number of insects overall, including bees. But blue was supposed to be the bee colour! At my sites, European honey bees were caught more often in blue traps than native bees. I also noticed that the distribution of insect taxa in each trap colour varied depending on the habitat of the site – in this experiment I was comparing intensive monoculture plantations, organic orchards with high plant diversity, and native mallee vegetation.

Organic almond orchard with ground cover.
Native mallee woodland.

Once I analysed the data as proportions across each habitat type, I found that blue traps were more attractive to pollinators in the native mallee vegetation compared to either orchard type, while white traps were less attractive in mallee. I was pretty excited about this, so published the results with my lead supervisor in Austral Entomology.

Is there something going on with light intensity in different parts of the world (the majority of historical studies on insect colour vision and attraction have been done in labs or field conditions in the northern hemisphere, but light frequency and intensity is quite different in the southern hemisphere)? Or maybe it’s something about different habitats that influences the attractiveness of colours to different taxa. We discuss these possibilities in the paper!

Since we published the paper in 2012, a lot of cool studies have explored this concept in other systems. A few recommended ones below:

Heneberg & Bogusch (2014) found that there may be sex differences in attraction to different colours.

Larsen et al. (2014) looked at colour differences in apple orchards in New Zealand.

Ernst et al. (2016) compared beetle and spider samples between pan traps and pitfall traps across arctic, subarctic and boreal zones, with some interesting results.

Moreira et al. (2016) compare colour differences in Brazil, with similar results to ours (southern hemisphere!!).

In summary, if you want to sample a wide range of insect taxa across different habitat types, use multiple colours of pan traps, or (even better) multiple trapping methods. And we need more research on how northern/southern hemisphere factors influence ecological interactions!

© Manu Saunders 2017

2 thoughts on “Methods papers from PhDs: pan traps for pollinators

  1. sleather2012 July 14, 2017 / 8:10 PM

    Nice post and not just because you tag my entomological classics series 🙂 Have drawn to the notice of one of my PhD students who is looking a colour preferences of leaf hoppers

    Liked by 1 person

  2. standingoutinmyfield July 19, 2017 / 8:59 PM

    In Kenya and Pennsylvania, I had more bees in yellow too! But I got some different and interesting bees in blue and white, so I like using all three colours. Some bee species were only ever found in blue for example.

    Liked by 1 person

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