Factors influencing oviposition by Sitodiplosis mosellana (Diptera : Cecidomyiidae) on wheat spikes (Gramineae)

Factors that might contribute to variability in the densities of wheat midg e eggs, Sitodiplosis mospllana (Gehin), on common and durum wheats, Triticu m aestivum L. and Triticum durum Desf., were investigated to improve the qu antification of oviposition preferences in relation to crop resistance. Egg densities on wheat spikes were highly variable, with a similar contagious distribution in the laboratory and field, although variance was highest in the laboratory. Females laid eggs in small groups, usually of one to six eg gs; most infested spikes had more than one egg group. Females showed no pre ference for ovipositing on different parts of a spike, although spikelets o n one side and at the base often received fewer eggs because these spikelet s were covered by the flag leaf and inaccessible for longer than others. Ov iposition rates varied from night to night, probably related to the weather . Females showed no preference for spikes at different growth stages; from the rime spikes began to emerge until at least flowering. Spike size did no t affect egg density, and spike height was a factor only for spikes deep wi thin or protruding above the canopy. Sources of environmental variation suc h as effects of weather on oviposition rates in the field or spatial phenom ena in cages were measurable but of secondary importance. In the field, com parisons among spikes which emerged on the same day could reduce variation in egg density. In the laboratory, variation in egg density could be reduce d by using arrays of excised spikes arranged at the same height, leaving th e central portion of the array empty. The primary cause of high variability in egg density among spikes was variation in egg-group size and the presen ce of multiple egg groups on a single spike, factors which cannot be experi mentally controlled because they are the result of oviposition behaviour ra ther than environmental heterogeneity.