Use of the parasitoid Diaeretiella rapae (Mclntoch) to control the cabbage aphid Brevicoryne brassicae (L.)

The parasitoid Diaeretiella rapae accepted all stages of the cabbage aphid Brevicoryne brassicae. The average number of mummies produced per female parasitoid was 42.8. Diaeretiella rapae did not show preference between young (first–second instars) and old (fourth instars) nymphs of the aphid. Nymph aphids younger than 4 days (third instars) produced no offspring after parasitism. The average number of offspring per aphid that was parasitized at day 5–6 (third–fourth instars) and day 7 (adults) were 3.1 and 15.3, respectively, which were much less compared with the aphid that was not parasitized (38.1). The average reproduction period for the aphids that were parasitized at day 5–6 and day 7 were 1.9 and 4.4 days, respectively and that for non-parasitized aphids was 14.9 days. Cage experiments showed that at the ratio of 1.2 : 1 (six female wasps : five aphid adults), with three releases on day 1, 3 and 6, the percent parasitism of the offspring on day 20 was 88.9%, and the aphids were successfully controlled by the second generation of the released parasitoids. Release of cabbage plants that included both mummies and young aphids in a broccoli field, with a dose of about 2 mummies/m2, resulted in higher percentage of the broccoli plants with mummies in the treated plot compared with the control plot. The percent parasitism of the aphids on broccoli plants in the treated plot was also, with one exception, significantly higher. Seven weeks after the release of the parasitoid, the percentage of the broccoli plants with mummies was 93.3% in the treated plot and 56.7% in the control plot; the percent parasitism of the aphids were 6.7 and 1.4% in the treated and control plots, respectively. These results indicated that one release of the cabbage plants with mummies enhanced the spread of the parasitoid D. rapae and increased parasitism of the aphids in the field. More releases of the parasitoid are needed to control the aphid effectively.