The development of Tipula oleracea L. (Diptera: Tipulidae) as a pest of winter cereals : the role of oilseed rape (Brassica napus L.)
Studies were carried out to investigate the development of Tipula oleracea as a pest of winter cereals in north-east Scotland. Oilseed rape was identified as an important contributing factor to the appearance of this species in winter cereals. Laboratory experiments showed that T. oleracea preferred to egg-lay into oilseed rape crops rather than winter cereals. Field surveys established that larvae were present during the winter within the oilseed rape. Further experimentation showed that larvae reared on diets of oilseed rape, out-performed those fed on winter cereals, in terms of growth, successful development, and subsequent fecundity of emerging females. Studies on behaviour showed that adults emerging from within the oilseed rape in June were trapped by the oilseed rape canopy and that 'normal' dispersal was restricted. Flies could, however, move between plant stalks underneath the canopy, suggesting that mating and egg-laying could continue. It was confirmed that T.oleracea, usually had two flight periods in north-east Scotland, in June, and again in August-September. Population studies determined that adult emergence was variable, suggesting a useful survival strategy, should peak emergence coincide with poor conditions for larval survival. Studies on growth established that the life-cycle of T. oleracea was responsive to temperature and field studies showed that two generations could be completed during the summer months within the oilseed rape crop. It was also shown that T. oleracea had a high egg-laying capacity and had no requirement for a larval diapause. The population dynamics of this species are suited to the exploitation of short term habitats, such as those created within one year arable rotations.