Resistance to Sitodiplosis mosellana (Diptera : Cecidomyiidae) in spring wheat (Gramineae)

Cultivars of winter wheat, Triticum aestivum L., previously identified as p ossible sources of resistance to wheat midge, Sitodiplosis mosellana (Gehin ), were crossed with spring wheat to produce lines with a spring growth hab it and assure synchrony between insect and plant. Many of the lines showed low levels of infestation by wheat midge in the field, and 21 of these were tested for resistance in the laboratory. All test lines exhibited resistan ce, ranging from 58 to 100% suppression of larvae and 70 to 100% suppressio n of seed damage, compared with a susceptible line. Larval development was delayed and survival was reduced on all lines. This antibiosis was associat ed with a hypersensitive reaction in the seed surface. The hypersensitive r eaction, or feeding damage by young larvae before they died, reduced the bi omass of some infested resistant seeds by 28% compared with over 60% for in fested susceptible: seeds. Some lines also reduced the level of infestation either through oviposition deterrence or a resistance which prevented newl y hatched larvae from establishing on the seed surface. A few lines also re duced the hatching rate of wheat midge eggs. The resistance was equally eff ective in field trials during two consecutive summers in Manitoba and Saska tchewan, with at least a 20-times difference in the level of infestation be tween susceptible and resistant wheats. No larvae could develop to maturity on some resistant lines. Large plots of one resistant line produced less t han 1% as many larvae as a typical susceptible wheat, and the larvae that d id survive produced few, small adults. This resistance is the first documen ted case of a high level of true resistance to wheat midge in spring wheat, distinct from asynchrony between the insect and susceptible stage of the p lant. The antibiosis component of the resistance is currently being incorpo rated in cultivars suitable for production in western Canada.