Robert May Prize Shortlisted Article

Post provided by Femke Batsleer

Digger wasp (Bembix rostrata). Credit: Femke Batsleer.

Each year Methods in Ecology and Evolution awards the Robert May Prize to the best paper in the journal by an author at the start of their career. Femke Batsleer has been shortlisted for her article ‘The neglected impact of tracking devices on arthropods‘. In this blog, Femke discusses how her paper came to be and the outcomes of the review.

How the idea originated: a digger wasp and tons of questions

For my PhD, I mainly study the spatial ecology of a threatened solitary digger wasp (Bembix rostrata), which digs nests in sandy soils. It is one of the few solitary arthropods that show brood care, regularly feeding flies to its larvae while they develop. I wanted to get a better understanding of their feeding behaviour: does it change in the presence of brood-parasitic flies, with the spatial position of the nests, across seasons, during the development of the nest? To partially automate the process, I looked into the use of PIT (Passive Integrated Transponder) tags on insects. These are passive tags of which the ID is read when they pass through a circular antenna, basically like microchips used in pets. The literature I found regarding the use of such techniques on insects was promising and barely mentioned any inconveniences. However, I still wanted to check the impact on the wasps’ behaviour, as one of my colleagues and co-author on the article was struggling with radio-tracking bumblebees.

Bumblebee equipped with a radio-tag. Credit: Steven Goossens.

In radio-tracking, an active tag is used that emits a signal that can be captured by a hand-held antenna. The tags themselves hold a battery, which makes them heavier, but a few promising articles did report their use on bumblebees. Nevertheless, as my colleague found out, radio-tracking bumblebees was not as easy as it was suggested in literature. Another colleague, who has experience with radio-tracking birds and bats, had been involved during his MSc in a project PIT-tagging hornets. However, the glue seemed to have an effect on the social behaviour of the hornets in the nest, so the technique was abandoned a never published or reported anywhere.

A pilot study to quantify the impact of a tracking device

Performing the pilot-study with students in the dunes: observing the behaviour of paired wasps, each in an insect-rearing cage. Credit: Femke Batsleer.

In the summer of 2018, I set up a pilot study to quantify the impact of PIT tags on the behaviour (i.e. time allocation to flying, resting, etc.) of the digger wasps. I organized this as a student project, involving 4 BSc students. We caught about a hundred wasps and paired them: one wasp of each pair was tagged, the other one handled in a similar way but not tagged. We recorded the behaviour during a fixed time period before and after tagging.

This pilot study went quite efficiently and, together with a postdoc and the four students, we collected the data in about three and half days. I remember talking to the postdoc during the field work, who suggested publishing this small study somewhere. We found that the tag indeed had an impact on the wasps’ behaviour, and I abandoned the idea of using this method for my studies and ended up monitoring feeding behaviour with cameras at the nests. However, our results, and mainly the discrepancy with the optimism I noticed in the literature, lingered in my head.

Turning a challenge into an opportunity

I talked with colleagues from my lab about this discrepancy and the idea grew to do a review or even a meta-analysis on the impact of tracking devices on insects. I pitched my idea to my supervisors, and they were immediately very enthusiastic and encouraged me to put my time and energy into this idea. At the 2018 annual meeting of the British Ecological Society in Birmingham I could pitch my idea to one of the editors of Methods in Ecology and Evolution during a ‘speed review’. Thereafter, I was completely convinced and motivated to dive into this review.

I involved other PhD-students at our lab, my two supervisors, and the postdoc who helped in the field. I also asked the students who participated in the fieldwork for the pilot study whether they would like to be involved in this follow-up of their student project, and two enthusiastically agreed. We performed a systematic search of the literature, and included all extra literature sources we could find. We then collected any information in these articles regarding the possible impact of tracking devices on the behaviour or ecology of arthropods.

Hornet with a PIT tag. Credit: Daan Dekeukeleire.

One third of the insect’s mass… or was it something else?

I found the results of our literature study staggering: forty percent of the articles did not mention anything, not a word or hint, about a possible impact or bias due to the tagging. 12% did measure some kind of impact, but due to the various methods used to quantify these side effects, drawing general conclusions proved difficult. We also noticed that often-cited rules of thumb had no empirical basis or were misconstrued. For example, several studies used the rule: “One third of a beetle’s mass can be used as a tag mass”. Yet, in the original article cited,  the authors stated that tags should weigh no more than circa thirty percent of the ‘acceptable extra loading’. If you carefully read the paper, this translates into a tag-to-body mass ratio of around two percent (for the particular study species they worked with).One misinterpreted citation got the ball rolling and the idea that a thirty percent tag-to-body mass ratio is acceptable became a rule of thumb.

The need for including impact assessments, critical evaluation, and transparent reporting

Conceptual diagram from our original paper. The possible effects of tags on arthropods and the bias they can create in research results for different research fields. Credit: Femke Batsleer.

It is clear to me that in the field of arthropod movement ecology, the possible biases and impact caused by tracking devices are still too often neglected. The initial optimism (‘this is a cool technique, and the insects can still kind of crawl/fly with it’) is still lingering and backed up by some promising articles that were the first to test these techniques. If we want to raise these techniques to the next level and employ them for a wide range of ecological questions and conservation purposes, we also need to advance our ethical and scientific critical attitude.

We need comparable and transparent impact assessments and a critical evaluation of what each form of impact means for interpreting movement data. We included our pilot study on the digger wasps as an example in the paper, as it underscores that we should not shy away from abandoning tracking methods if undesired side effects outweigh potential benefits. I hope my review provides the needed background to researchers who use or want to use tagging techniques on arthropods. I hope it encourages researchers to include impact assessments in future studies and leads to a more balanced and critical attitude towards this method.

Limitations and possibilities

Even more advanced technologies, such as biotelemetry (measuring physiological or energetic variables remotely) or even cyborg insects, will become more commonly utilized in entomological research. But we should always be aware of –and critical about– their limitations. Tracking devices have given us invaluable insights into the movement and broader ecology of animals. I am sure they will keep doing so in the future, as limitations also define their inherent counterpart: possibilities.

Find out more about the articles that were shortlisted for this year’s Robert May Prize here.