The aphid Sitobion avenae (F.), parasitized by Aphelinus abdominalis (Dalman), tends to leave either the feeding sites gathering in particular parts of its host-plant or even the host-plant. This behavior partially explains the phenomenon of reduced parasitism contribution to the overall mortality in several aphid population studies as interpreted by Hughes' method.

The potential rate of increase (PIR), used in Hughes' time-specific life table analysis for aphid populations as a multiplication factor for the instar-period, was studied in populations of Sitobion avenae (F.) in the absence and presence of the parasite Aphelinus abdominalis (Dalman) under controlled conditions. Two factors were mainly found to alter PIR values in the presence of parasites. These were the feeding preference of adult A. abdominalis for the first instar aphids and the prolonged instar duration of the third instar of aphids which had been parasitized by an adult parasite at the first instar. These two factors contributed to lower values of PIR and as a consequence to an underestimation of the expected (potential) population for the next instar-period.