Rare Water Beetle Turns Up at Trust HQ

On 1 June, Jim Hollinshead from Liverpool John Moores University, was surveying the ponds at Bickley Hall Farm for amphibians. By luck, he had recently had a conversation with Andy Harmer, the LBAP co-ordinator for the Lesser Silver Water Beetle (Hydrochara caraboides), so he was in little doubt about what he had found when he came across an egg cocoon of this species in Black Sheep Pond (fig.1). It was at this point that his camera decided that it had had enough for the day. Jim (first having planted his pond net to mark the spot) dashed over to the farmhouse where he was able to obtain a camera and record his discovery (fig. 2). The following week, an adult beetle was netted.

Distribution

In Eurasia, Hydrochara caraboides extends as far as Siberia, though it is absent from Iberia and most of Scandinavia. Between 1938 and 1990, it was unknown, in the UK, outside the Somerset Levels. Since the first record in 1990 there have been further finds, of cocoons, larvae and adults, in Cheshire and over the border near Wrexham, but it is by no means common. The Bickley find brings to 45 the number of Cheshire sites at which breeding has been confirmed.

Status

Hydrochara caraboides is listed as Endangered (RDB1, the most threatened category) in the British Red Data Book; it also receives full protection under Schedule 5 of the Wildlife and Countryside Act. It is a priority species in the UK Biodiversity Action Plan.

Life Cycle

The adult beetle is one of our largest water beetles, 14 to 18 mm in length. The underside is densely coated with thick, felty hairs which trap an air bubble to provide oxygen during a dive. It is the silvery reflection from this bubble which gives the beetle its common name. It is a herbivore.

Adult beetles are most likely to be found from March to June or July, when they are breeding. During this time, the female lays her eggs (40 – 100) in a silken cocoon which is wrapped in a leaf. In Cheshire, leaves of the Broadleaved Pondweed are most commonly, but not exclusively, used. The cocoon is laid within a couple of metres of the pond margin. It floats on the water and has a `mast` sticking up from one end (fig. 2). Cocoons have been found from the middle of May to the middle of July.

The larvae usually emerge in early to mid June, two weeks after laying. (However, recent finds of fresh cocoons in July imply that larvae could be around as late as August.) In contrast to their parents, larvae are voracious predators, and have been observed to take water lice (Asellus sp.), mayfly larvae and even small rams horn snails. They are reported to attach themselves to emergent vegetation and ambush their prey. (Water lice are probably an item in the diet of many aquatic predators, but were not found when Black Sheep Pond was sampled in April of this year; however, this is true also of some of the Hydrochara-inhabited ponds surveyed by Jonathan Guest in the mid-nineties.)

After three moults, the larva leaves the water and pupates. In the ditches of the Somerset levels, it is known to burrow into the peaty bank to pupate, but little is known of this process in the ponds of the Cheshire plain; floating rafts of vegetation may be favoured, though not essential. The pupa changes into an adult over the following two weeks, and, after hatching, the adults may disperse from their birth ponds. Adults are thought to overwinter in ponds. (Adults are said to be strong fliers, though there are no reports of their being taken in flight – in moth traps, for instance.)

Habitat Requirements

Shallow water seems to be essential for successful development, possibly because it warms up quickly in spring. Some vegetation, especially the smaller sweet grasses, Glyceria spp., also seems to favour the presence of cocoons, providing cover against discovery by predators; however, if the vegetation is too dense, it shades and cools the water. Cattle, by their trampling, produce shallow depressions which fill with water; and their grazing restricts vegetation growth. It follows that fencing of ponds is to be avoided. (Sheep, it is thought, do not have the weight to produce depressions; while their ability to crop vegetation very short might deprive the cocoons and larvae of cover.)

Some source of broad leaves for the construction of cocoons is obviously needed. The use of broad-leaved tree leaves has been described from Somerset, but not Cheshire.

Black Sheep Pond

Black Sheep Pond is approximately 27 metres long and 17 metres wide at its longest and widest; it has a raft of floating vegetation over water of unknown depth. The extent of this raft can be clearly seen in the photograph taken in April of this year (fig. 3). By June (fig. 1), a mass of vegetation has grown out of the raft, and at the same time, the pond has shrunk in area. Around the margins, extending across a radius of about 3 metres are the grassy, water-filled hollows (fig. 4) which form such desirable sites for Hydrochara cocoons. Sampling in April yielded 18 other species of aquatic invertebrate, conspicuous among which was the wandering snail, Lymnaea peregra. Further sampling will be carried out when we can be confident of not disturbing breeding Hydrochara. Great crested newt eggs and larvae were found in another pond nearby, so there is every chance that they occur also in Black Sheep Pond.

Having a pond with breeding Hydrochara on our doorstep presents a unique opportunity for long-term study of the beetle. Furthermore, whereas other sites may be at the mercy of the whims of landowners with different priorities from ours, Black Sheep Pond can be assured of management sympathetic to the continued presence of the Lesser Silver Water Beetle.

Much of the general information on the biology and ecology of Hydrochara which appears above has been drawn from leaflets and reports by Andy Harmer, Jonathan Guest and David Boyce, which we gratefully acknowledge .

Mike Tynen, Aquatic Invertebrates Officer