Evidence of leaf-cutter bees using plastic flagging as nesting material

*Author note: We originally published these findings in an online journal called "Science Matters." The journal has since been discontinued and the online version of the paper was removed with the collapse of the journal. I have talked to a few editors of other journals but none seem interested in publishing these interesting findings. We feel that it is important to have the information out there, especially given the rising awareness of the negative impacts of plastic pollution in ecosystems around the world. We decided just to publish our findings here on our blog as they appeared in the original Science Matters article. Please feel free to share this post.

Evidence of leaf-cutter bees using plastic flagging as nesting material

Joseph S Wilson, Sussy I Jones, Scott McCleve, Olivia M Carril

Abstract

The important role native bees play in the pollination of natural and agricultural landscapes is becoming more widely known. Bees in the genus Megachile (leaf-cutter bees) are among the most recognizable solitary bees due to their habit of cutting out small circles of leaves to use in nest construction. Several recent studies report some leaf-cutter bees using cut pieces of plastic along with leaves used in nest construction. Here we report additional evidence of leaf-cutter bees potentially using plastic in nest construction. We provide a discussion on the negative impact plastic use may have on solitary bees.

Introduction

Most bee species are solitary and construct nests in cavities, either in the ground or in pre-existing tunnels in wood. Those nesting in the wood often augments their nests with plant material[1]. In particular, leaf-cutter bees in the genus Megachile are known for cutting semi-circular pieces out of leaves to line their nests. While leaves from a variety of plant species are used for nesting material by Megachile[2], several recent publications have highlighted observations of these bees using plastic waste instead[3][4][5].

Objective

Here we report two additional observations that indicate Megachile might be using plastic, particularly plastic flagging, in nest construction.

Figure 1. Examples of leaf-cutter bee use of a sumac leave and two pieces of plastic flagging.

Results & Discussion

Yellow and orange flagging were found in Douglas, Arizona, the USA that had clear circular pieces removed from the margins of the flagging (Fig. 1). These markings are nearly identical to the markings made by Megachile leaf-cutter bees when cutting out pieces of leaves to use in nesting material (Fig. 1). We can therefore speculate that the plastic material was cut by Megachile bees, presumably to use in their nests. The first observation occurred in July of 2009, where yellow flagging was found with 18 distinct putative bee-made holes. The second observation was in September of 2019, where orange flagging was found with eight semicircles removed, and two pieces partially removed.

The impact of this plastic flagging on leaf-cutter bee larvae is not directly known as adult bees from these nests have not been observed. One prior publication suggested that plastic use could be an “ecologically adaptive trait” in response to increasing anthropogenic plastic pollution[3]. We hesitate to draw a similar conclusion— An ecological adaptation, by definition, should increase the fitness of the bee exhibiting the trait, compared to that of bees without the trait. Because experiments comparing fitness levels have yet to be performed, it seems early to consider this an adaptation. Moreover, what scant evidence we do have about bees and plastic hint at the possibility that this behavior is not advantageous at all— it has been shown that when Megachile built their nests in plastic straws they experience up to 90% mortality because of moisture problems that resulted in increased mold growth[6]. Plastic use by bees appears uncommon, though reports of it are increasing. Because its effects are unknown, it would be beneficial if citizen scientists and others could watch for additional instances and record them in digital inventorying databases (e.g. iNaturalist.org), as a precursor to more in-depth analyses.

Limitations

While no direct observations of Megachile bees cutting plastic flagging for nesting materials was made, we are fairly positive that the marks made on the plastic flagging were indeed made by leaf-cutter bees. Furthermore, these observations are limited as we were unable to examine the larvae from these nests or even the nest cells themselves. It is therefore unknown if the entire nest cell was constructed of plastic, which would likely have a detrimental effect on the larva inside. Alternatively, plastic could have been used in multiple different nest cells, intermixed with leaf pieces, which may have limited the negative effects of plastics and moisture accumulation.

Conjectures

We suggest that as an alternative to plastic flagging, researchers might consider using fabric ribbons made from natural fibers to mark field sites. These fibers will be biodegradable and, if used by bees, will likely avoid the negative moisture capturing effects that plastic can have.

Methods

Plastic flagging was collected in Douglas, Arizona, the USA at an elevation of ~1240 m on July 30, 2009, and September 10, 2019.

Funding Statement

This project was not funded.

Acknowledgements

We would like to thank Marshall Topham for enabling this collaboration.

Conflict of interest

The authors declare no conflicts of interest.

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