THE LIFE-CYCLE AND DISTRIBUTION OF THE WART-BITER DECTICUS-VERRUCIVORUS (L) (ORTHOPTERA, TETTIGONIIDAE) IN A CHALK GRASSLAND IN SOUTHERN ENGLAND

The Wart-biter Decticus verrucivorus (L.) (Orthoptera, Tettigoniidae) has only four populations in southern England and as such is one of Britain's most endangered insect species. This paper describes aspects of the species' ecology relevant to management recommendations for its conservation. The post-embryonic development and distribution, in relation to microclimate, within a chalk grassland are investigated for the largest population. Egg hatch occurred in mid-April, there were seven nymphal instars and adult emergence occurred in July, although its timing was strongly influenced by weather. From egg hatch to adult emergence, densities fell by 99·3% yet adult survival was high. The study population was estimated to contain at least 290 adults in 1987 and 190 in 1988. After emergence, adult females almost doubled in weight but did not reach their maximum weight (and potential fecundity) until September. No adults survived beyond mid-October. Realised fecundity may thus be severely curtailed by poor weather in September. Adults were smaller and maximum adult densities were 10 to 15 times lower than in typical continental populations. Within the study site, densities were influenced positively by mean temperatures and were highest on a south-facing slope. Mean temperatures were determined by interactions between weather, aspect and vegetation structure. A management policy must ensure that areas of short, open turf are available for the species to raise the body temperature by basking in direct sunlight. However, it is stressed that this is only one component of the species' habitat requirements (albeit crucial). Other studies demonstrate that a mosaic of vegetation structures is required. © 1990.