Ann. uppl. Bid. zyxwvutsrqpon (2000), zyxwvutsrqpo 137539-97 zyxwvutsrqp Printed zyxwvutsrqp in zyxwvutsrqponmlk Greut Britain 89 Density, distribution and dispersal of the carabid beetle Nebria brevicollis in two adjacent cereal fields THOMAS** By AVELINA F E R N h D E Z GARCIA', GEORGIANNE J K GRIFFITHS* and C F GEORGE IACR-Long Ashton Research Station, Department of Agricultural Sciences, University of Bristol, Long Ashton, Bristol BS41 9AF, UK (Accepted 16 August 2000; Received 12 May 2000) Summary The distribution of the carabid beetle Nebria brevicollis was monitored in the summer during a period of declining activity associated with aestivation in a hedgerow. After emergence from aestivation, population density, distribution and dispersal of N. brevicollis were studied during autumn 1994 in a mark-recapture experiment. 3560 beetles were marked and 1887 were recaptured in a grid of pitfall traps spanning a hedgerow and extending approximately 32 m either side into two recently harvested cereal fields. Population size, estimated from a Lincoln index, increased slightly with time with a mean population density of approximately 0.9 beetles m-2. Activity-density varied during the experiment and was significantly related to maximum temperature. The population was aggregated within the hedgerow during aestivation and in several spatially stable hot-spots of activity-density within the field during autumn. There was considerable movement within fields but the hedgerow was a significant barrier to dispersal between fields, with potential effects on the metapopulation structure of the species. Key words: Aggregation, aphid predator, field population density Introduction The development of sustainable farming is cur- rently promoted in many European Union countries through support for agri-environment schemes that shift agricultural production towards less intensive, low input systems (Ovenden, Swash zyxwvuts & Smallshire, 1998). Restricting the use of pesticides and enhancing pest control by natural enemies is one of the principal goals of low-input systems. In recent years, methods to enhance beneficial invertebrate populations in temperate arable crops have included beetle banks (Thomas, Wratten & Sotherton, 1991 ), conservation headlands (Sotherton, 1991 ; Hawthorne, Hassall & Sotherton, 1998), field margins (Dennis & Fry, 1992; Thomas & Marshall, 1999), weed strips (Frank, 1997) and other forms of strip-management (Lys & Nentwig, 1992). These methods act by improving colonisation of large fields, protecting vulnerable areas from pesticide applications and drift, enhancing overwintering survival, and providing nectar sources and overwintering sites for a broad assemblage of predatory species, and have been well reviewed by k o m p (1999). Much research has focused on the role of natural enemies in preventing the build-up of cereal aphid populations in the summer to damaging levels (Hol- "Corresponding Author E-mail: gthomas@plymouth.ac.uk margin, hedgerow, integrated crop management, land & Thomas, 1997). However, nearly 3 million ha of winter cereals are grown in Great Britain of which approximately 75% are treated with insecticides at some time during the year. In 1998, at least 69% of the treated area of wheat, and 86% of winter barley were sprayed with foliar insecticides, principally pyrethroids, to control the introduction and spread of Barley Yellow Dwarf Virus (BYDV) in crops by aphids (Garthwaite & Thomas, 1999). Most of these insecticides are applied in the autumn at a time when most beneficial species are overwintering or hibernat- ing in refugia. However, autumn cereal fields are still populated by linyphiid spiders. Insecticide applica- tions at this time can cause population reductions of 95% that persist for several months following spraying (Thomas & Jepson, 1997). The only other predator present in crops in the autumn and winter in significant numbers is the winter active carabid beetle Nebria brevicollis (F.) (Desender & Pollet, 1987). This species, therefore, may also be vulnerable to insecticide applications at this time of year. The biology and ecology of N. brevicollis are well known and documented in the literature (Nelemans, 1987a, 1988; Nelemans, den Boer & Spee, 1989). In contrast to most other farmland carabids, N. brevi- collis has a bimodal period of activity. Adults appear in May and June. After a brief spell of activity present -Addresses: 'University of Plymouth, Seale-Hayne Faculty of Agriculture, Food & Land Use, Newton Abbot, Devon TQ12 6NQ, UK Bundesforchungsanstalt fur Ernahrung, Haid-und-Neustr. 9, 7613 1 Karlsruhe, Germany zyxwv 0 2000 Association of Applied Biologists