Oviposition preferences of a herbivore, the wheat midge Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), on wheat were investigated in relation to two hypotheses: female preferences are adapted to offspring performance; plants may evolve independent defenses that both deter oviposition and reduce offspring performance. Variation in egg density and larval performance were measured for three plant genotypes: a preferred, susceptible wheat; a less preferred, susceptible wheat; a less preferred wheat defended against larval feeding. Oviposition peaked 10–11 d after emergence of the inflorescences and then declined sharply on all three wheat genotypes, although the inflorescences of the genotypes developed at different rates. On the preferred, susceptible wheat, larval performance was high for oviposition that occurred until pollination and low later. On a less preferred wheat, larval performance was high when eggs were laid before or after pollination. On a defended wheat, larval performance was always low. Oviposition preference was associated with larval performance that varied with plant developmental stage, but imperfectly, possibly because females do not detect cues for seed development. Females deposited eggs further from larval feeding sites when ovipositing on less preferred wheats, regardless of whether larval performance on the wheat was high or low. A low preference in combination with a shift in oviposition site supports the hypothesis that some wheats have evolved a defense that deters oviposition. This defense against oviposition is independent of a defense that reduces larval performance, which causes an apparent failure in the expected preference-performance relationship.
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