FUNCTIONAL-RESPONSE OF DIAERETIELLA-RAPAE (MCINTOSH) (HYM, APHIDIIDAE) TO DIURAPHIS-NOXIA (MORDWILKO) (HOM, APHIDIDAE) HOSTS

Functional-response experiments involving the parasitoid Diaeretiella rapae (McIntosh) (Hym., Aphidiidae) and Diuraphis noxia (Mordwilko) (Homo., Aphididae) hosts were conducted. The host densities used were 10, 20, 40, 80, and 160 hosts per arena per day. Searching bouts lasted 24 h, and female parasitoids were allowed to search over two consecutive days. New hosts were offered to parasitoids after the first 24 h. Temperature was maintained at ca. 21-degrees-C, photoperiod was set at 14L:10D, and humidity ranged from 50 to 70% RH. In general, the offspring sex ratio did not appear to vary significantly with a host density of up to 80 hosts per female per day. However, the proportion of female offspring increased when female parasites were offered 160 hosts per day. In addition, the overall proportion of female offspring produced across all host densities offered decreased on day 2 (0.582 +/- 0.037) relative to day 1 (0.661 +/- 0.023). Diaeretiella rapae displayed a Type 2 functional response to D. noxia hosts during the first 24 h. The functional response displayed during the second 24 h was unlike any response commonly reported in the literature, with an initial rise followed by a sharp decline in numbers of mummies produced when more than 40 hosts were offered. This was presumably due to the greater depletion of the female parasitoid's egg complement after 24-h exposures to high host densities (80 and 160 hosts), rather than to lower host densities (10, 20 and 40 hosts). The functional response parameters for day 1 were a' = 1.341 arenas per day and Th = 0.017 days per mummy. The maximum attack rate per 24-h period (i.e. 1/Th) was 59.5 mummies.