Water Beetles

Water beetles are insects belonging to the Order Coleoptera. There are around 250 species of water beetles in Britain which spend part or all of their life cycle in water. They prefer shallower margins of a wide range of aquatic and semi-aguatic habitats including damp sphagnum moss, peaty pools, streams, canals, coastal rocky shores and salt marshes. Most people would be familiar with “whirligig” beetles which usually swim on the surface of the water if undisturbed. Beetles are relatively common between April and October with the peak in the height of summer. 

Coleoptera means “sheathed wings” as the wings are covered by hard wing cases (elytra). The adult water beetles carry a bubble of air trapped beneath their elytra supplemented by other features like coxal plates on their hind legs. They need to return to the surface to replenish their air supply staying under water for up to 30 minutes. Water beetles have 6 legs, modified to enhance their swimming or submerged crawling ability, and their forewings meet in a straight line down the centre of their back. This distinguishes them from water mites (Hydracarina) which have 8 legs and water bugs (Hemiptera) like corixa. Having wings most water beetles are able to migrate between water bodies, either once warmed by the sun or in the evening. Water beetles vary greatly in size from around 1.5mm to more than 38mm (e.g. great diving beetle Dytiscus marginalis) with most less than 20mm and many around 6-15mm. The diversity of size, shape and colour means that a generic beetle pattern is required.

We fish on Aberystwyth Angling Association waters at Easter and have on several occasions encountered the trout feeding on small reddish water beetles, noted through spooning of rainbow trout stomach contents and watching them take the beetles from the water surface. I have yet to find a fly which has allowed us to target this feeding. But you might try the following if you find the trout are targeting water beetles.

Flies to buy and try if you don’t tie:

• Dry fly – size 14
• Dry fly brown – size 16
• Wet fly – size 10

If you prefer to tie:

Tight Lines

Greg