Bean seed fly (Delia platura, Delia florilega) and onion fly (Delia antiqua) incidence in England and an evaluation of chemical and biological control options

Bean seed fly and onion fly are significant pests of alliaceous crops in the UK. Their activity was monitored using yellow water traps at three field sites in England in 2002 and 2003. Bean seed fly were not split between Delia platura or Delia florilega because from the growers point of view control measures are independent of species. The traps were effective at catching bean seed fly, which was present from April until September. A total of 1729 bean seed fly were trapped in 2002 and 4501 in 2003, with peak activity in May in both years. In 2003, there appeared to be three to four peaks in abundance of the pest. Only 113 onion flies were trapped in 2002 and 23 in 2003. More male onion fly were trapped than females. Pot experiments were carried out to evaluate efficacy of a range of insecticides, garlic and two parasitic nematode species (Steinernemafeltiae and Steinernema kraussei) against bean seed fly and onion fly. Pots of salad onions were exposed to natural oviposition by bean seed fly, but the onion fly experiment was carried out in a glasshouse with eggs of the pest being inoculated into the pots. Tefluthrin seed treatment appeared to be especially effective at preventing bean seed fly damage and produced the most robust seedlings but did not appear to kill the larvae. A drench of chlorpyrifos at the ‘crook’ stage gave 100% control of bean seed fly larvae. A chlorpyrifos drench was the only treatment to give effective control of onion fly. There was some evidence that the parasitic nematode S. feltiae reduced numbers of bean seed fly larvae by about 50%. Guidelines for control of both bean seed fly and onion fly are discussed.