Field surveys showed previously that mummies of Diuraphis noxia (Mordvilko) parasitized by Aphelinus asychis Walker were S-fold more abundant than those parasitized by Aphidius matricariae Haliday in a region where these species are endemic. We carried out a series of olfactometer experiments to test whether this difference in parasitoid abundance might arise from differences in attraction to host or host-plant odors: We used a 4-way olfactometer to measure choices among 4 odor sources-filtered air, barley (Hordeum vulgare L.), D. noxia, and D. noxia plus barley-and the time spent in these 4 odor fields. Field-collected and laboratory reared A. asychis females did not move preferentially toward D. noxia or D. noxia-barley odors compared with filtered air or barley odors. This lack of preference was not affected by experience in which parasitoid females were allowed to oviposit in D. noxia on barley before being tested. However, after such experience, A. asychis females spent more time in the D. noxia-barley odor field than in the other odor fields. In contrast, naive A. matricariae females chose D. noxia-barley odors over other odors, and experienced A. matricariae females chose odors from D. noxia alone, as well as odors from barley plus D. noxia, over filtered air and barley odors. A. matricariae females spent more time in the D. noxia odor field than in the other odor fields, whether or not they were experienced with the plant-host complex. These results suggest that greater attraction to D. noxia cannot explain the greater abundance of A. asychis found on this host in the field.
