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THE WART-BITER BUSH-CRICKET OLIVER D. CHEESMAN On a global scale, the Orthoptera (grasshoppers, crickets, bush-crickets [katydids] and closely allied insects) represent an important taxon in the functioning of certain ecosystems (Samways, 1997). This ecological significance adds weight to arguments for the conservation of a group which contains many rare and threatened species, but which also supports a number of notorious pests (Samways & Lockwood, 1998). Orthoptera may also be attractive candidates for conservation by virtue of their relatively large size, and the spectacular morphological and behavioural characteristics of many species (eg. see Preston-Mafham, 1990; Rentz, 1996), which enhance their potential to act as ‘flagships’ for wider conservation activities (Samways, 1997).

The UK orthopteran fauna consists of around 30 native species and a further ten or so naturalised species or occasional migrants (Marshall & Haes, 1988; Haes & Harding, 1997). Many of these are confined to a more or less southerly distribution in the UK, where colonies often represent populations at the edge of a species’ wider geographical range. Such is the case with the wart-biter bush-cricket Decticus verrucivorus (L.) (Orthoptera: Tettigoniidae).

As with other insect species, it appears that these range-edge populations exhibit more precise ecological requirements than those found closer to the centre of the species’ range (Cherrill & Brown, 1992), potentially rendering them more vulnerable to extinction.

The wart-biter bush-cricket, D. verrucivorus In the UK, D. verrucivorus has probably always been scarce, and confined to southern England (Marshall & Haes, 1988). However, only five populations are currently known here, one of which represents a successful re-introduction.

D. verrucivorus is protected under Schedule 5 of the Wildlife & Countryside Act (1981), was one of the first species to receive attention from English Nature’s Species Recovery Programme, and is now included in the UK Biodiversity Action Plan (BAP) (UK Biodiversity Group, 1999). It occurs throughout much of Europe, but appears to be declining in parts of its continental range (Holst, 1986; Hjermann & Imms, 1996; Kleukers & van Nieukerken, 1996).

D. verrucivorus (see Plate 7) is a large, striking insect, typically darkish green with more or less dark brown mottling, providing excellent camouflage amongst vegetation. Hatchlings emerge in mid-late April, and there are seven nymphal instars. Adults are present in the field from July to early October, and females oviposit into the soil from about mid-August. Eggs generally remain below ground for two winters, but may remain dormant for a number of years (Ingrisch, 1984).

The ecology of D. verrucivorus at its main stronghold site in southern England was characterised in detail by Cherrill & Brown (1990a, b). These studies indicated that the species has highly specific requirements within its calcareous grassland habitat, which have limited the scope for re-introduction

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attempts. For a site to be favourable, the sward should be herb rich, and support a diversity of small invertebrates, providing suitable resources for the species’ omnivorous habit. The structure of the sward is also critical, with a mosaic required, consisting of areas of bare ground and short turf (for oviposition, see Cherrill, Shaughnessy & Brown (1991), and nymphal basking), as well as tussocks of taller vegetation (within which adults can hide from predators, and which provide adult males with perches for stridulation).

A more or less southerly aspect is also required, to ensure sufficient warmth for the species to complete its development in the relatively short UK summer.

Re-introductions of Orthoptera Some general ecological considerations for insect re-introductions are given

by Thomas (1989). Samways (1997) notes that Orthoptera generally lend themselves well to re-introduction, using either captively bred or translocated stock. Historically, re-introductions of UK Orthoptera have certainly been carried out, either officially, eg. the introduction of the large marsh grasshopper Stethophyma grossum to Thursley Common, Surrey in 1967 (see Baldock, 1999), or through unofficial translocations. Re-introduction is currently a recognised approach towards the conservation of Orthoptera in the UK. For all four species listed under the UK BAP (D. verrucivorus, S. grossum, the mole cricket Gryllotalpa gryllotalpa, and the field cricket Gryllus campestris), re-introduction is included or implied as an Action Plan objective/target (UK Biodiversity Steering Group, 1995; UK Biodiversity Group, 1999). Concerted attempts at re-introduction have been made for two of these species, D. verrucivorus and G. campestris. In support of these attempts, captive rearing programmes have been established at the Zoological Society of London (London Zoo) (Pearce-Kelly et al., 1998; Jones, PearceKelly & Cheesman, 1999).

Re-introductions of D. verrucivorus in the UK The re-introduction programme for D. verrucivorus in the UK has been guided by the general principles laid down by JCCBI (1986), IUCN (1987, 1998) and English Nature (1995). New colonies of D. verrucivorus are most unlikely to be established in the UK through “natural” dispersal of individuals from extant populations. This is because patches of favourable habitat tend not to occur in the vicinity of existing colonies, and because the species is poorly adapted for dispersal over long distances, problems which face other UK insect species which might be considered for re-introduction programmes (Thomas, 1989).

Re-introduction efforts for D. verrucivorus have drawn on individuals from the captive breeding programme (derived from UK founder stock) and on individuals translocated directly from the strongest extant UK colony. The latter measure has only been undertaken at times when it has been judged that removal of individuals would not significantly weaken the donor population.

Although available stock from either source has been limited, the greatest constraint on the re-introduction programme has been the lack of potential recipient sites with suitably high quality habitat. For Orthoptera in general, Samways (1997) notes that eggs may provide the most suitable developmental stage for re-introductions, but that adults generally translocate well, and that

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decisions on the most appropriate life-stage for re-introductions must be made on a species-by-species basis. Thomas (1989) stresses the advantages of using adults, or (in some cases) larvae, as the release stage for insect reintroductions. Re-introductions of D. verrucivorus under the UK conservation programme have involved releases of late instar nymphs and adults.

Owing to its historic scarcity, possibly compounded by under-recording (particularly of low-density populations) as a result of the species’ elusive nature, relatively few specific sites are known from which D. verrucivorus has been lost in the UK. Most such sites are currently unsuitable for the species.

However, UK BAP objectives/targets for the UK Orthoptera listed above refer to re-introductions to sites within the historic range of the species, rather than restricting activities to specific sites at which extinction is known to have occurred. Since 1991, the conservation programme for D. verrucivorus in the UK has included the search for, and on-going monitoring of, potential reintroduction sites for the species. Sites have been assessed on the basis of six criteria: 1. Adequate area of structurally heterogeneous grassland to support a viable population 2. Predominantly southerly aspect 3. High diversity of plants and invertebrates 4. Long-term security of site, and of a sympathetic management regime 5. Approval and support of the land user and owner 6. Lack of serious conflict with other rare or desirable species on the site The first site deemed suitable for re-introduction of D. verrucivorus, a nature reserve in Kent, had supported a population of the species until the mid 1970s, when a combination of factors led to its local extinction. Conditions had improved at the site, under the management of the Kent Trust for Nature Conservation (now the Kent Wildlife Trust), and releases of D. verrucivorus were made here each year from 1993 to 1996. The table below shows the scale of these releases, which all comprised approximately equal numbers of males and females, with the exception of the 1996 stock. These were exclusively adult females, removed from the donor site after the main mating period. The table also shows estimated sizes of field (locally-hatched) populations of adults up to 1999. It is difficult to make precise estimates of these populations for a number of reasons. The 1995 estimate is based on numbers of early instar nymphs, recorded prior to that year’s release of captive reared stock, after which it became impossible to distinguish locally-hatched individuals from those transferred to the site. Estimates from subsequent years do not suffer from this confounding factor, but must be regarded as approximations.

Estimates of colony size must be treated with caution, given the elusive character of this insect, the detectability of which varies considerably with prevailing climatic conditions, even over short time scales. Monitoring of adults using fixed transects (as recommended by Thomas (1989)) has proved

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unreliable for estimating numbers of D. verrucivorus when population density is low (J.P. Shaughnessy, pers. comm.).

The figures presented in the table indicate that the re-introduction attempt has been successful. A small, locally-hatched population was detectable in the first year that offspring from the initial translocated stock might have been expected (1995 offspring from the 1993 release, accounting for the typically two-year life-cycle). As the final transfer of stock to the site was made in 1996, it is likely that most (if not all) of the 1999 population represented offspring from 1997’s locally-hatched individuals. This population thus represented the second generation following release of founder stock at the site. The presence of a strong second generation, and the increasing trend in the size of the local population, bodes well for the successful establishment of D. verrucivorus on this site. Further significant increases in population size may be limited by the available area of suitable habitat, but it appears that the colony now has the potential to be self-sustaining.

INDIVIDUALS RELEASED AND APPROXIMATE FIELD POPULATION RECORDED AT THE D. VERRUCIVORUS RE-INTRODUCTION SITE IN KENT, 1993–1999 Individuals released Translocation Captive-reared stock 54 118 307 14 -

Approximate field population (adults)

1993 1994 1995 <10 1996 10 1997 50 1998 50 1999 200 In addition to activities in Kent, releases of captively reared D. verrucivorus have also been made on two areas of a site in East Sussex. Releases did not begin until 1995, and re-introduction efforts here have been hampered by substantial annual fluctuations in habitat quality. Two further sites are strongly favoured for re-introductions of D. verrucivorus, once habitat management has produced suitable conditions, and the search goes on for additional sites which meet the above criteria.

If the wart-biter bush-cricket is to have a secure future as part of the UK insect fauna, it is likely that further re-introductions will need to be conducted.

Currently occupied sites have NNR or SSSI designation, and the status of the species and its habitat at these sites is monitored closely. The variability in delayed hatching of nymphs (Ingrisch, 1984) may provide some buffer against local extinction, through a “spreading of risk” within each cohort of eggs.

Nonetheless, most of the current, extant colonies are very small and must be regarded as vulnerable. The specificity of the species’ habitat requirements severely limits the number of sites available for re-introduction attempts.

However, our knowledge of the autecology of D. verrucivorus (which is

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extensive relative to knowledge of many insect species) does provide a sound basis for targeting such attempts. The success of the re-introduction reported above is unlikely to have been acheived without this, or without the dedication of those involved in the conservation programme for this species.

Acknowledgements CABI Bioscience wishes to acknowledge the financial support of English Nature. The above account draws on unpublished data collected by John Shaughnessy, and the conservation programme for the wart-biter in the UK has benefited from the guidance of Professor Val Brown and Dr David Sheppard. The efforts of Paul Pearce-Kelly, Dave Clarke, Rachel Jones, Andrew Cunningham and others involved in captive rearing of this species are greatly appreciated, as is the work undertaken by the Kent Wildlife Trust in support of the re-introduction programme.

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O. Cheesman Plate 7: The Wart-biter bush-cricket, Decticus verrucivorus (L.) is protected under Schedule 5 of the Wildlife & Countryside Act (1981), was one of the first species to receive attention from English Natureâ&#x20AC;&#x2122;s Species Recovery Programme, and is now included in the UK Biodiversity Action Plan (p. 31).