Wheat Bulb Fly (Delia coarctata Fallén, Diptera: Anthomyiidae) is an important pest of winter wheat in the eastern half of Britain, and in northern and eastern Europe. There is one generation per year; eggs are laid in bare soil from late July to September. The eggs enter diapause which is broken after mid-January, when soil temperatures rise above 0°C. Neonate larvae must find a host plant and invade a tiller soon after hatching.
Winter wheat (Triticum aestivum L.) is the preferred cereal host, but other winter cereals and related grasses may also be attacked. All are annuals, except for the perennials couch grass (Elytrigia repens (L.) Nevski syn. Elymus repens (L.) Gould, Agropyron repens (L.) Beauv.) and perennial ryegrass (Lolium perenne L.). On wheat, high larval mortality occurs when neonate larvae fail to find a host, and when developing larvae kill their host plants. The geographical distribution and phenology of WBF are matched more closely with those of couch than with those of other hosts. These factors suggest that couch, and not wheat, is the preferred host. Aspects of this hypothesis were tested in the laboratory, glasshouse and field.
In choice test bioassays neonate larvae chose couch seedlings and their exudates over wheat seedlings and their exudates, and couch rhizome exudates over controls. Couch seedling exudates had attractant properties, whereas wheat exudates had attractant and arrestant properties, when compared with controls. The larvae were photophobic and positively geotactic.
In a pot trial, symptoms of infestation appeared earlier in couch than in wheat. Attacked plants responded by producing extra shoots, which were also killed by larvae; this response was greater in couch than in wheat. After 5 weeks, infested plants suffered a relative reduction in number of shoots, but uninfested neighbouring plants, especially wheat, compensated for this by producing more shoots themselves.
Larvae raised on couch emerged as adults earlier than those raised on wheat. They thus develop more rapidly, and use more resources, on couch than on wheat, i.e they are better adapted to couch as a food source. Earlier eclosión would allow adults to make better use of favourable weather conditions, and to live longer, mate more often, and produce more eggs. Older eggs developed more rapidly to adulthood
In laboratory and field adult WBF preferred to rest on couch than on wheat. Buried couch rhizomes did not encourage WBF oviposition in the laboratory or the field.
These findings support the hypothesis that couch is the preferred host of WBF, provide a partial explanation of high larval mortalities on wheat, and suggest that attractants isolated from couch and arrestants isolated from wheat could be used in WBF control programmes. The ecological implications of a preference for couch as a host are discussed.