Parasitoids and Predators 190 OVIPOSITION STRATEGY OF APHIDIUS ERVI (HYMENOPTERA: APHIDIIDAE) IN RESPONSE TO HOST DENSITY

The reproductive response of Aphidius ervi Haliday to the density of pea aphid, Acyrthosiphon pisum (Harris), was investigated in plastic cylinders (10.5 cm high × 8.5 cm diameter). Mean number of aphids parasitised and eggs laid by a single A. ervi significantly increased with an increase of host density. Numbers of eggs laid per parasitoid reached a plateau at host densities of 75 aphids/cylinder and above. However, the number of eggs laid in each parasitised aphid significantly decreased (from 2.9 to 1.3 eggs) with the increase of host density from 15 to 75 aphids/cylinder, after which no further decrease occurred. These results suggest that the parasitoid adjusts oviposition strategy in response to increasing host density through increasing parasitism and decreasing superparasitism. The proportion of female progeny developed from fertilised eggs increased (up to 70%) with the increase of host density from 15 to 50 or 75 aphids/ cylinder, after which it gradually declined, suggesting that the sperm limit occurs when host density reaches 50 to 75 aphids/cylinder