The vegetarian water beetles in the Hydrophilidae family number about 40 aquatic species. Others in the family are dung and detritus beetles on land. They are mostly 2-8mm long but some are larger. While some species can swim through the water, most crawl around on vegetation or in algal mats close to the margins of ponds. The adult beetles feed on foliage and plant debris but the larvae are predators of small pond creatures. These beetles come to the surface head first when they need to breathe and they gather an air bubble with the aid of their clubbed antennae.
We have 20 species of Helophorid beetles, between 2 and 7mm long, often called water scavenger beetles. The common Helophorus brevipalpis has a metallic green colour and strongly ridged thorax. They do not swim but crawl through vegetation, feeding on plants and decomposing material.
The Scirtidae or marsh beetles are very small and poorly known, associated with damp vegetation. The aquatic larvae favour rather stagnant conditions. Some adults can jump using their hindmost legs, rather like fleas.
Pond beetles usually lay their eggs on submerged plants. Vegetarian water beetles deposit their eggs in silk packets that are attached to plants. The larvae pass through several instars before they are fully fed. When ready to pupate, they burrow into mud near the pond margin. Most pond beetles overwinter as pupae and emerge as adults between late spring to late summer. During warm weather, pond beetles will emerge from the water and fly away to colonise new ponds and somesuch as the Scirtidae are caught in moth traps.
Role of pond beetles in gardens
The predatory habits of many large pond beetles may make them unpopular with pond owners who treasure their fish and tadpoles. However, they are part of the predatory fauna that occurs in a healthy pond habitat and so should be tolerated. Predatory pond beetles and their larvae also help reduce the numbers of mosquitoes and other blood-sucking flies that breed in ponds.
1. Jennifer Owen 2010 Wildlife of a garden, a thirty year study. RHS publications
Other sources of information
Foster, G. N., Bilton, D. T. & Friday, L. E. (2014) Handbooks for the Identification of British Insects. Vol 4 part 5b Keys to adults of the water beetles of Britain and Ireland (part 2). Royal Entomological Society
Foster, G. N. & Friday, L. E. (2011) Handbooks for the Identification of British Insects. Vol 4 part 5 Keys to adults of the water beetles of Britain and Ireland (part 1). Royal Entomological Society
Friday, L. E. (1988) A key to the adults of British water beetles. A Field Studies Council AIDGAP key