Occurrence of Aphidophagous Syrphids in Aphid Colonies on Cabbage (Brassica oleracea) and their Parasitoids

Syrphid (Diptera: Syrphidae) larvae and pupae (149) were collected from an organically grown cabbage field maintained by Department of Agricultural Biology, Faculty of Agriculture, University of Peradeniya. Out of them 15 adults of syrphids emerged and were identified as Allograpta javania and Episyrphus viridaureus (Wiedemann). Most larvae died during rearing. Out of the remaining, 66.7% were parasitized. A significant correlation coefficient (0.83) between the mean number of cabbage aphids and syrphid larvae was noticed. Three species of hoverfly parasitoids, Diplazon orientalis (Cameron) and Diplazon laetatorius (F.) (Ichneumonidae) and Syrphophagus sp. (Encyrtidae) were reared. The Diplazon laetatorius is found to be a new record from Sri lanka. The most frequently found parasitoid was Diplazon laetatorius and the per cent parasitism was 50. The per cent parasitism of Diplazon orientalis and Syrphophagus sp. was 42.3 and 6.3 respectively. Morphological details of each parasitoid species is presented. A significant correlation coefficient (0.91) between total hoverfly larvae and the parasitized hoverfly larvae was noticed. At the beginning of observations, the number of the parasitized larvae was small. As the number of syrphid larvae feeding in cabbage aphid colonies was increasing, the percentage of parasitized larvae increased. During observations in a conventional cabbage field at Paingamuwa area in the Kandy District, 40 syrphid larvae and pupae were collected. Out of them, thirteen adults of syrphids emerged and were identified as Allograpta javania and Episyrphus viridaureus. Most larvae died during rearing. Out of the remaining 10.37% were parasitized. Total number of hoverfly larvae in the insecticide free vegetable field was more than four times higher than the conventionally managed field. Predators of aphids other than syrphids were found at the two sites in low numbers.

DOI: http://dx.doi.org/10.4038/tar.v21i1.2591

Tropical Agricultural Research Vol. 21(1): 99-109 (2009)