RESULTS The number of hosts parasitised by female D. rapae increased with the number of available hosts when D. rapae females foraged alone (P < 0.001) (Figure 1). However, the proportion of hosts parasitised by a foraging female decreased as host density increased (P < 0.001) (Figure 1). Increase in the number of D. rapae females foraging together increased the total amount of parasitism, but the relative contribution of each female (i.e. number of nymphs parasitised) decreased (P = 0.024) (Figure 2). A smaller proportion of parasitoid offspring were females when D. rapae mothers were competing for the same hosts than when females foraged alone (P < 0.001) (Figure 2). INTRODUCTION While foraging haplodiploid parasitoids have a choice between oviposition and mating (Kant et al., 2012a). They encounter hosts of varying densities and can also face competition from conspecifics and other species. When two or more females exploit the same host patch it results in intraspecific competition and interference (Kant et al., 2012b). In response females can manipulate their parasitism rate and sex allocation strategies . Here we examined the oviposition strategies of the parasitic wasp Diaeretiella rapae which is a sole parasitoid of cabbage aphid Brevicoryne brassicae. Oviposition and progeny sex ratio of D. rapae was examined after offering cabbage aphid nymphs (5-day old) at different densities (5, 10, 15, 20, 30 and 50) and when females foraged at three conspecific densities of 2, 3 and 5. REFERENCES: Kant, R., Minor, M.A. & Trewick, S.A. (2012a) Mating or ovipositing? A crucial decision in the life history of the cabbage aphid parasitoid Diaeretiella rapae (M’Intosh). Ecological Entomology 37, 169-74. Kant, R., Minor, M.A. & Trewick, S.A. (2012b) Reproductive strategies of Diaeretiella rapae (Hymenoptera: Aphidiinae) during fluctuating temperatures of spring season in New Zealand. Biocontrol Science and Technology 22, 1-9. Kant, R., Minor, M.A., Trewick, S.A. & Sandanayaka, W.R.M. (2012c) Body size and fitness relation in male and female Diaeretiella rapae. BioControl. DOI: 10.1007/s10526-012-9452-4. PARASITISM BEHAVIOUR OF FEMALE DIAERETIELLA RAPAE IN COMPETITION Rashmi Kant 1 , Maria Minor 1 , Manoharie Sandanayaka 2 and Steve Trewick 1 1 Institute of Natural Resources, Massey University, Palmerston North, New Zealand. 2 The New Zealand Institute for Plant & Food Research Ltd, Auckland, New Zealand. Correspondence: R.Kant@massey.ac.nz Figure 1: (a) Relationship between the numbers of Diaeretiella rapae offspring produced and the density of Brevicoryne brassicae nymphs offered. Cabbage aphid infestation on crucifer plants Diaeretiella rapae parasitising cabbage aphid nymp Superparasitism by Diaeretiella rapae results in upto 8 larvae per host a b c d d y=e (2.92-7.77/x) R 2 = 0.78; P<0.001 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 Number of ofspring produced 0 5 10 15 20 25 30 35 40 45 50 Parasitism Rate (%) 0 20 40 60 80 100 Host Density y=-1.13x + 88.69 R 2 = 0.61; P<0.001 a ab b bc c d Host Density Number of ofspring produced 1 2 3 5 10 12 14 16 18 20 22 a ab bc d Foundress Density Number produced per female Foundress Density Female Male A B BC C a ab b b 1 2 3 5 0 5 10 15 20 Figure 1:(b) Parasitism rates (percentage hosts parasitised) of D. rapae when offered B. brassicae nymphs at different densities. Figure 2: (a) Mean number of Diaeretiella rapae adults produced when Brevicoryne brassicae nymphs (30 individuals per day) were offered to different densities of mated female D. rapae (foundress density). Figure 2: (b) Mean number of male and female offspring emerged per foundress D. rapae from B. brassicae parasitised at different foundress densities (number of females ovipositing together). CONCLUSIONS Reproductive strategies play an important role in the pest suppression ability of parasitoids (Kant et al., 2012c). The results of this study indicate that female D. rapae adjust their reproductive strategies in response to changes in host and foundress densities. Females reduce the parasitism rate and allocation of fertilised eggs in competition. This might be a strategy to save the sperm and eggs for future oviposition; otherwise, these eggs and sperm (when ovipositing fertilised eggs) may be lost through superparasitism. FOR MORE INFO, PLEASE VISIT: http://evolves.massey.ac.nz/Rashmi.htm