Occurrence of diamondback moth and its parasitoids within cabbage heads

G. S. Lim and C. L. Tan

Abstract

In investigating the possible occurrence and survival of the diamondback moth (DBM) (Plutetta xylostella L.) and its parasitoids (particularly Apanteles plutellae Kurdj.) within cabbage heads, two studies were carried out using freshly harvested cabbage heads ready for marketing. The cabbages were individually enclosed for two weeks for emergence of any infesting species and the leaves subsequently peeled to determine the stages of those still remaining within. In spite of the tight and closed conditions of the cabbage heads, both DBM and its parasitoids were found within. The life stages included larvae and pupae of DBM, and cocoons of the parasitoids. Most was found on the first 15 outer leaves while a few as far in as the27th leaf. In unsprayed fields 46.7% of the heads had infestations within while that of sprayed heads 80 per cent. Only a very small percentage of the insects initially trapped within the heads failed to emerge. Many succumbed to diseases as indicated by their watery, foul and disintegrated appearance. The fact that the moth may survive for extended period within cabbage heads has notable implications. This is discussed with respect to carry-over between different croppings, insecticidal usage and the associated problem of insecticide resistance development, and dispersal of the moth and its parasitoids.

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