The Lesser Silver Water Beetle - Hydrochara caraboides
Survey and habitats in North West England
The Lesser Silver Water Beetle (Hydrochara caraboides) appeared in Cheshire in the 1990s but does not appear to have colonised ponds in Greater Manchester, Lancashire or North Merseyside. It is a legally protected animal which was hitherto confined to sites in southern England. As it can fly it is capable of finding most wetlands although it may be found prospecting a new habitat this does not mean that the place in which it is found should automatically be considered its breeding site (e.g. In Northwich it visited a translocated marshy pond habitat but it did not colonise it). It favours "late succession ponds" and ditches or more properly stable old ponds and ditches with a fluctuating water level. A tendency to acidity is not so important as for the Mud Snail but these two rare species are sometimes found together. New ponds are avoided. There is also a strong association between the occurrence of the beetle and Great Crested Newts. The beetle requires warm, open ponds, with up to knee deep open water adjacent floating mats or rafts of vegetation. It avoids ponds with any sort of fish and also avoids very shaded ponds. The beetle's prey animals include molluscs, particularly those with wide apertures and small invertebrates.
The Lesser Silver Water Beetle makes egg cocoons using folded leaves and their own silk - Forget-me-not, Bittersweet, Greater Reedmace and also Crack Willow leaves have been used. The leaf is folded into the shape of a pyramid, of which one face is silken and white. A silk mast extends upwards from the apex. The cocoon is about the size of a sugar lump. One to five cocoons are typically found in May in breeding ponds in quiet areas of the pond, floating on the water-surface. This is the time when non-specialist ecological consultancies send out their trainee ecologists to go crashing about in ponds setting and removing bottle traps up to 6 times, netting the pond just looking for newts, and squelching around up to 6 times at night doing torchlight newt surveys. This seems like good way of destroying egg-cocoons and removing evidence of the species. Your ecological survey is compromised.
Other survey methods which have turned up Lesser Silver Water Beetles are netting for adults, torchlight surveys and bottle-trapping finding adults, and netting and sieving for beetle larvae. The adult beetle itself can easily be confused with other black beetles of a similar size whilst the larvae resemble a myriad of other larvae and groups. The cocoons look like any other bit of pond vegetation. Finding Lesser Silver Water Beetles is a skilled task and declaring the beetle as absent from a particular pond can only be done by a fully experienced wetland ecologist who works with all aquatic invertebrate species and who understands the growth and decay of individual plant species.
Lesser Silver Water Beetle Lesser Silver Water Beetle Cocoon (preserved) Cocoon against Alisma leaf
The Lesser Silver Water Beetle has fairly exacting habitat requirements and hence can be driven out by inappropriate pond management. At two ponds surveyed by Dave Bentley they have departed due to a change in the grazing regime, one pond being stripped by horses and another being churned from a fen to a muddy pool by young cattle.
Single visit surveys are generally carried out by Dave Bentley Ecology Services as part of a general Pond Biodiversity Survey.
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Text and images copyright of Dave Bentley 2007 & 2011. Banner panorama photo H Groth-Andersen 2006.