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Hoverfly Lagoons

This project is open for 2023


The decline of pollinators in the UK and worldwide is of real concern, potentially leading to a pollination crisis and affecting flowering plants and crop yields. Much of the attention and research has been around bees, but other pollinators are similarly important and help enhance the pollination services that our crops and plants receive.  This project focuses on hoverflies - an often overlooked, yet vital part of our wild environment.

Hoverflies are true flies ('Diptera'), more closely related to  bluebottles than to bees, but are often mistaken for the latter because of their yellow-and-black patterns.  This is no accident, since hoverflies mimic bees and wasps for defence, although they cannot sting and do not live in nests.  There are more than 280 hoverfly species in the UK, and Hoverfly Lagoons focuses on those that have an aquatic lifestage, with larvae that live in pools of water or 'rot holes' in trees (particularly the genera Eristalis, Myathropa and Helophilus).

Our gardens often lack these aquatic habitats, providing little egg-laying space for the flies, thus Hoverfly Lagoons encourages you to create small homemade hoverfly havens from discarded milk bottles and fallen leaves. We want to work out what are the best ways to make these lagoons - so we need your help, and gardens!


Lagoons are a quick way to add wildlife habitat to your garden.  Taking part requires no extra knowledge or experience, just a few minutes each month to count larvae and collect pupae, once the lagoon is set up.  Check out the project video below to learn more about lagoons:

Name:            Hoverfly lagoons

Year(s):           2016 -

Focus?           Hoverflies

Status?          Ongoing

Results?        Ongoing analysis

Project lead contact:

Dr Ellen Rotheray


Also check out the blog:

Project summary

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This project is good for kids!

"Lagoons are messy fun, and can be packed full of long-tailed larvae (and other beasties) to discover. Children can get involved collecting pupae and observing them until, like butterflies from the chrysalis, the stunning adults emerge" - Dr Ellie Rotheray (please see the blog at for more inspiration!)

Taking part in Hoverfly Lagoons


The project runs from May - October, which is when the hoverflies are flying and breeding, although setting up your lagoons earlier gives them time to get nicely rotten.  You will need:

  • A 2, 4 or 6-pint plastic milk bottle or alternative container

  • Scissors and a pen, for piercing overflow holes

  • A tray with drainage holes

  • A basin for searching the content

  • A jar or insect tube

  • Sticks, leaf litter and / or grass cuttings.

Once your lagoon is set up we will ask you to check the material once a month for the long-tailed larvae, and pupae in the tray, and record how many individuals you find. We recommend you also clean, recycle and replace the plastic bottle each month (please see FAQs below). The larvae and content should ideally be moved into a new plastic bottle, and pupae transferred to jars until emergence to allow for identification to species level. 


If you're joining the project, also check out Dr Ellen Rotheray's blog, which will continue to develop through the season and includes Lagoons tales and explorations. It is aimed at advising, informing and inspiring adults and children as Lagoons progress each month. In addition, we are asking volunteers to collect data on insects they're observing in gardens, please see here!

Full instructions can be downloaded here, along with a fact sheet about hoverflies and the Lagoon lifecycle:

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Methods now in Welsh!

Ellie demonstrates how to create and survey your lagoons (2016). Featuring some very chunky long-tailed larvae!

Let us know if you are making a Hoverfly Lagoon! 🏖️

Thanks for taking part!

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Frequently Asked Questions

Accidental frog buffet

What can I use as a container for my Lagoon?

Since the project began in 2016 we have asked volunteers to use single-use plastic milk bottles to create their Hoverfly Lagoon, while at the same time encouraging the use of an alternative container of similar size and shape.


This is because we need:

  1. something free and available to most people

  2. something that is safe to use

  3. to standardise the size

Single-use plastic may release chemicals into water and soil as it breaks down, so we recommend replacing the bottles with each survey, and clean and recycle the old ones every month. From 2021 we are also asking volunteers to investigate alternative Lagoon containers, with an aim to move towards a more environmentally friendly solution that doesn't need monthly replacement. For some ideas for alternative Lagoons see; for more information about Lagoons and plastic, see our news article.

When is the best time to create a Hoverfly Lagoon? 


You can create a Lagoon for hoverflies any time of year, but the best time in a new year is early spring, March - April, in time to attract the early-flying adults. If using single-use plastic milk bottles, we recommend that these should be cleaned, recycled and replaced with new bottles each month, but less frequently over winter. 


Lagoons created as late as September have attracted gravid female hoverflies and homed overwintering larvae. 


You can leave Lagoons in place over winter and the following year (larvae will overwinter in them) but evidence suggests they need fresh organic matter (fallen leaves / grass cuttings) after a year or less. If your Lagoon doesn’t receive leaf fall naturally by being positioned under a tree or bush, just add some more or empty the Lagoon (after searching for larvae!) and start over in spring. 


Where should my Lagoon be positioned? 


The best place to position a Lagoon is in partial or complete shade, ideally under a tree or bush. More exposed Lagoons can work, but ensure plenty of dry leaf litter (or similar) is placed over the top of the water surface to reduce evaporation, and keep it topped up with water. It may still get too hot if in direct sun during the warmer months.


What is the best content for a Hoverfly Lagoon? 


We have tested grass cuttings, nettles, leaf litter, sawdust and compost. While the results are variable, cut grass appears to attract the greatest number and diversity of hoverflies. Other content that has been reported to work well includes waste vegetables, a variety of herbivore poo (e.g. sheep or horse), and comfrey tea. Do ensure there are no plant roots in your Lagoon.


Sawdust is the best-smelling content, but do ensure it’s fresh and untreated.


Why place dry leaf litter on the top of my Lagoon?


Place a thick layer of leaf litter over the water surface of your Lagoon to provide a place for visiting females to crawl over and lay eggs, to stop the water evaporating as quickly, and to reduce the number of mosquitos from inhabiting them. 


What is the best sized container for a Hoverfly Lagoon?


Though results are variable, our evidence suggests size doesn’t matter. We recommend a 2-pint / 1000ml minimum. Water retention and plenty of rotting organic matter (fresh cut grass or leaf fall) is key. If using single-use plastic milk bottles, we recommend that these should be cleaned, recycled and replaced with new bottles each month, but less frequently over winter.  


How can I reduce the number of mosquitos using my Lagoons? 


While mosquitos and other competitors / predators are all part of the natural life cycle in gardens and wild areas, there is evidence that suggests mosquitos compete with hoverfly larvae for food in Lagoons. As mosquitos are not the focal species here, we have experimented with ways of reducing mosquito numbers. The most effective way is by placing a thick layer of dry leaf litter (or similar) over the top of the Lagoon so there is no exposed water surface. 


What predates on hoverfly larvae? 


Birds often use the wet leaf litter / material for nesting, and are regularly observed searching Lagoons for food. The impact of birds can be reduced by using logs positioned over the top of containers, sticks to prevent access, scarecrows or even cages. Exposed Lagoons are the more vulnerable, if you have a secluded area to position your Lagoon this may limit predation. 


Foxes are known to investigate Lagoons and eat hoverfly larvae. 


One of the most effective predators in Lagoons is another species of fly,
a muscid fly called Phaonia exoleta (though this is likely not the only
species, images to come). This muscid larva looks like a hoverfly larva at
first glance, but looking closely you will notice they do not have the
extended breathing tube at their back end, and their front / head end is
pointed, with a black sclerotised mouthpart (which is uses to pierce its
prey). These are thought to have evolved to feed on mosquito larvae, but
will readily feed on hoverfly larvae too. 


Another major predatory group are parasitoid wasps. These wasps locate
the pupal stage of the hoverfly, in the leaf litter tray outside the Lagoon.
The female wasp inserts an egg into the hoverfly pupa, which hatches and
feeds inside, and eventually emerges as an adult wasp. After emergence, tell-tale sign is a small hole in the side of a pupa. An adult hoverfly emerges through the anterior end of the pupa, leaving a large capped hole (images to come). 


 How do I keep the pupae?


Hoverfly pupae are similar to caterpillar cocoons, however they are more fragile so do search and handle with care. They do not move, and they are not soft to the touch like the larvae are. They need to be kept in a shaded area, ideally with a moist piece of kitchen paper or a few damp leaves. They breath through little projections at the head end, so should not be under water. They require airflow so breathing holes should be pierced in the lid of the jar or pot you are keeping it.  

Do hoverflies use ponds?


Ponds are used by some hoverflies, particularly one species The Tiger Hoverfly, Helophilus pendulus. However, more often a pond does not generate the conditions that attract most semi-aquatic hoverflies, that is, the conditions that develop in tree holes or ditches.


The larval stage of Lagoon-dwelling hoverflies requires decaying organic matter, which produces the nutrients that microbes consume. It is the microbes that the larval stage of some hoverflies filter-feed. A pond usually has live, growing plants in it, which use the nutrients to grow so microbe populations are kept low. In addition, tadpoles or fish or other pond predators will likely predate on hoverfly larvae.

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