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Using plastic for ‘Hoverfly Lagoons’

Plastics are incredibly useful synthetic polymers, with their durability being simultaneously one of their greatest strengths, and a massive contributor to a variety of environmental problems. It is estimated that 79% of all plastics ever produced still exist in some form or other (1) and the question of how we can deal with this waste is an extremely complicated one (2,3). Much of the current messaging about what can be done to lessen the impact of plastic waste is to:

Reduce Reuse Recycle

In ‘Hoverfly Lagoons’, we reuse a cut-down plastic milk bottle to make the lagoon. Plastic milk bottles are a single use plastic, meaning they are only designed to be used until that milk portion is consumed, and then to be recycled or disposed of (4,5). They are made of HDPE (High Density Poly Ethylene).

We have been asked a few times why we are asking people to use plastic in a wildlife gardening project, considering the negative environmental impacts of plastic worldwide, and we wanted to address these concerns. For Hoverfly Lagoons, we ask people to use a plastic milk bottle because:

1) It is cheap. It is even likely to be ‘free’, since we assume that no one is buying milk bottles just to use in the project. Where possible, we do not want participants to have to go out and buy extra equipment for our projects, since we want to limit there being any financial barriers to taking part.

2) It is a standard size. When comparing different substrates and positions of lagoons, and how many hoverflies were found, lagoons that are much larger or smaller than others are likely to have very different numbers just based on space available. This broad standardisation allows us to make more scientifically robust conclusions.

3) It is safe to handle. Hoverfly lagoons is a popular project, and is one that we suggest is particularly good to do with children. The protocol requires the lagoons to be emptied out and counted once a month, and making sure that our lagoons will not be too heavy, or dangerous if e.g. dropped, is important. The worst that can happen with a plastic milk bottle is to get messy.

We did not originally specify how long to keep any individual lagoon for, but the instructions from 2021 now request that participants replace the cut-down bottle when they tip out the lagoon for monthly counting.

HDPE is able to be recycled at a fairly high-quality (depending on the technique used; 6,7), so as long as we make sure not to degrade the bottle during the project, it should not affect its recycling. Keeping the bottles in shady spaces, and replacing them frequently, means that the plastic will not get too grubby to be cleaned, or damaged by sunlight, and can re-enter the recycling stream.

In terms of what we put into the lagoon: a mix of fly larvae and plant bits in water is not that far away from milk (a mix of fats and proteins in water, just a bit more… blended), so is unlikely to damage or contaminate the plastic over the duration of project.

Further Information & References

1. Matthews, C., Moran, F. & Jaiswal, A. K. A review on European Union’s strategy for plastics in a circular economy and its impact on food safety. Journal of Cleaner Production 283, 125263 (2021).

2. Our planet is drowning in plastic pollution. This World Environment Day, it’s time for a change.

3. Wilkins, M. More Recycling Won’t Solve Plastic Pollution. Scientific American Blog Network

4. Geueke, B., Groh, K. & Muncke, J. Food packaging in the circular economy: Overview of chemical safety aspects for commonly used materials. Journal of Cleaner Production 193, 491–505 (2018).

5. Nutrition, C. for F. S. and A. Guidance for Industry: Use of Recycled Plastics in Food Packaging (Chemistry Considerations). U.S. Food and Drug Administration (2020).

6. Welle, F. Develop a food grade HDPE recycling process. The waste & Resources Action Programme, Oxon (2005).

7. Pattanakul, C., Selke, S., Lai, C. & Miltz, J. Properties of recycled high density polyethylene from milk bottles. Journal of Applied Polymer Science 43, 2147–2150 (1991).


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