Abstract. 1. For the first time, long-term changes in total aerial insect biomass have been estimated for a wide area of Southern Britain.
2. Various indices of biomass were created for standardised samples from four of the Rothamsted Insect Survey 12.2 m tall suction traps for the 30 years from 1973 to 2002.
3. There was a significant decline in total biomass at Hereford but not at three other sites: Rothamsted, Starcross and Wye.
4. For the Hereford samples, many insects were identified at least to order level, some to family or species level. These samples were then used to investigate the taxa involved in the decline in biomass at Hereford.
5. The Hereford samples were dominated by large Diptera, particularly Dilophus febrilis, which showed a significant decline in abundance.
6. Changes in agricultural practice that could have contributed to the observed declines are discussed, as are potential implications for farmland birds, with suggestions for further work to investigate both cause and effect.