Phytophagous insects that deposit eggs directly on hosts confront a plant world of variable quality. Some studies to date provide evidence for positive relationships between the site of oviposition and offspring success. as expected if females choose sites of high quality or if they or their offspring manipulate host physiology. We studied relationships between oviposition site and larval success for the anthomyiid fly Hylemya sp., a predispersal seed predator of the confamilial montane plants Polemonium foliosissimum and Ipomopsis aggregata. The intensity of oviposition on a given P. foliosissimum plant was a good predictor of attack in the following year, suggesting choosiness of female flies. However, larval survival did not decline detectably when eggs were transferred among flowers within or between plants, compared to survival where the female fly originally had deposited the eggs. Also, P. foliosissimum flowers with eggs did not have a higher probability of setting fruit than those without. In contrast, I. aggregata flowers on which Hylemya laid eggs had a significantly higher probability of setting fruit than flowers chosen at random; this probability did not decline if eggs were removed experimentally. These results suggest that female flies are choosing among I. aggregata flowers in an adaptive manner, or manipulating their quality on behalf of offspring.
