Hoverfly Larvae Lagoon


Hoverfly larvae are about as pretty as any other maggot, but grow into important pollinators and members of our ecosystems, as well as accomplished mimics of bees and wasps. Their nurseries are in stagnant pools of water that are uncommon in most gardens – however, we can help out with nothing more than some garden waste, water, a milk bottle, and a few minutes cutting.

Hoverflies (called syrphids by entomologists) are a group of true flies (insects in the order Diptera). They occur worldwide, represented in Europe by about 900 different species, and in Ireland by 180 species. You might have seen them hovering around in your garden, visiting flowers or simply drifting about. Perhaps you have also been alarmed by a hoverfly in the same colours as a bee or wasp! You have nothing to worry about from hoverflies however – they do not sting or bite and are simply mimicking bees and wasps for their own protection. Predators know not to mess with stinging insects, and so “dressing up” as one is a pretty good strategy for avoiding getting eaten!

Hoverflies are important to humans in two main ways. Firstly, they are important pollinator species alongside bees, butterflies and moths. 75% of all crops are pollinated by animals, and without our insects, we are in danger of losing not only crop yields, but also the beautiful plant biodiversity that gives our countryside and gardens their character. Secondly, hoverfly larvae are important pest controllers and waste managers – some larvae are carnivorous, and feed voraciously on pests such as greenfly, while others are “saprophytes”, meaning they eat dead and decaying organic material. Say what you like about flies, but without them, we would be up to our ears in waste!

These saprophytic larvae are the ones we can help out by building lagoons. Many of them are sub-aquatic, meaning they live underwater. A very common hoverfly species in Ireland is the drone fly (Eristalis tenax).  In the wild this species’ larvae live in stagnant water, such as in holes in trees that fill with water (called “rot holes”). They feed on the decaying material in these holes and use a long tail-like appendage as a breathing tube – this gives them the rather unpleasant name “rat-tailed maggots”. After growing large enough, these larvae emerge from the water and migrate to drier locations such as leaf litter or just below the surface of soil, where they pupate in a similar way to butterflies and moths. They then emerge as adult hoverflies, which feed on nectar from flowers. After mating, a female lays her eggs on the surface of stagnant water such as in rot holes, and the cycle restarts.

In our well-maintained gardens, we often lack the sorts of spaces that suit hoverflies like Eristalis tenax. We don’t particularly like untidy or “dirty” looking spaces, and try to keep things as clean as possible. However, this isn’t ideal for biodiversity – nature needs a little chaos to work as intended! If you have a relatively sheltered space in your garden or green space, you can help hoverflies out very easily and cheaply. All you will need is an old milk bottle (2 litre ones are great), some leaf litter, grass clippings and twigs, a tray to set your lagoon in, and some tools like scissors or a knife.

  1. Cut the top off your bottle so that you have a deep container with a wide mouth
  2. Poke some holes in the brim of the bottle so that excess water can flow out
  3. Stuff your leaf litter/grass clippings down into the bottle, filling up to the holes you made
  4. Push some twigs into the bottle, so that they reach the bottom of the bottle and stick out of the top (this allows larvae to leave the water when they are ready to pupate)
  5. Fill with water (preferably from a rainwater store, but tap-water is fine!) up to the holes you made, and place another layer of leaf litter on the water (this gives female hoverflies a place to land and lay eggs)
  6. Place some more dry leaf litter in a tray, and carefully place your bottle onto it (the dry leaf litter will be where the larvae pupate)
  7. Place your hoverfly larvae lagoon in a shaded, sheltered space, and check it every so often to see if it needs to be topped up with water

Easy as that, and a great project to get kids involved with. By keeping an eye on the lagoon, you can observe every stage of the hoverfly life cycle, and appreciate these humble yet important insects.


Written by Tom Murphy








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