Biology of Diaeretiella rapae (Mc Intosh) (Hymenoptera: Aphidiidae) on Cabbage Aphid ( Brevicoryne brassicae Linnaeus) and Influence of Host Age on the Developmental Duration

The biological parameters of Diaeretiella rapae and influence of the host age on the life cycle of the D. rapae. The fecundity of Diaeretiella rapae was 120-140 eggs per female. Eggs were transparent white but appeared light brownish under high magnification. Average incubation period was 1.90 days. Larvae of D. rapae underwent four moults. The first instar larva was transparent white, mandibulate, caudate and measured 0.57 mm in length and 0.14 mm in width and was active. The second instar larvae were creamy white and curved slightly, lacked mandibles, salivary gland and were hymenopteriform. They measured 0.77 mm in length and 0.19 mm in width. Third instar larvae were light yellow in colour, hymenopteriform and measured 1.22 mm in length and 0.43 mm in width. Fourth instar larvae measured 2.76 mm in length and 0.87 mm in width. The average total larval duration of D. rapae was 7.50 days. When the aphid host died, its body changed to hardened exoskeleton called mummy. Pupa measured 2.0 mm in length and 0.80 mm in width with average pupal duration was 5.60 days. Under laboratory condition, egg to adult development of D. rapae ranged from 13.75 to 16.00 days with an average of 15.0 days. The adult female parasitoid was bigger than male. Adult female measured 2.13 mm in length while male 1.78 mm in length. Longevity of adult was 6.4 days when 10% honey solution was provided and 4.4 days without honey solution. The life cycle parameters of parasitoid varied with the age of host insect. On 48 hrs old nymph of B. brassicae, the total duration required to form mummy was 13.6 days, in comparison to 14.6 days in case of 72 hrs old nymph and 11.5 days in case of 96 hours old nymphs. Total life cycle duration of the parasitoid was completed in 19.3 days, 20.1 days and 17.2 days when reared on 48 hrs, 72 hrs, and 96 hrs old nymph of B. brassicae respectively which indicated that the days required to form mummy, pupal period and duration from egg to adult emergence was comparatively shorter when D. rapae was reared on 96 hours old B. brassicae nymphs.