Biological Control of Diamondback Pacific

The diamondback moth Plutella xylostella (L.) is the major pest of brassicas in the oceanic Pacific, where it is often associated with the cabbage cluster caterpillar Crocidolomia pavonana (F.) and, less frequently, with the cabbage webworms Hellula undalis (F.) and H. rogatalis Hulst. Cotesia plutellae Kurdjumov (and, at most, no more than one other major parasitoid in any one country) is present in the Pacific, but only in 4 of the 20 or so countries. Diadegma semiclausum Hellen, possibly the most effective parasitoid of diamondback moth elsewhere, is not yet established in the Pacific, although there are current introductions to Cook Islands and Fiji. There are good reasons for believing that wider dispersal of those parasitoids present and the establishment of additional species would confer important benefits. The biological control of all important members of the brassica pest complex deserves early attention in order to minimize the need to apply insecticides, with their generally adverse effects on natural enemies. Introduction By far the most important insect pest of cabbages and other brassicas in the oceanic Pacific is the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Yponomeutidae). Although extremely widespread, it is still apparently absent from Kiribati, Tokelau, Tuvalu, Wallis and Futuna (Waterhouse 1985), the Federated States of Micronesia (Esguerra et al. 1990) and also from the Marshall and Caroline Islands. Cabbage is the most important brassica grown in the Pacific. The cabbage cluster caterpillar Crocidolomia pavonana (F) (= C. binotalis) is also a widespread pest of brassicas, but is less important than DBM. Somewhat less important still are the two cabbage webworm species Hellula rogatalis Hulst (in Micronesia) and H. undalis (F.) (throughout most of the Pacific). Most of the other insects associated with cabbage only attain pest status locally and occasionally except, perhaps, for highly polyphagous species such as the corn earworm Helicoverpa armigera (Hübner), cutworms of the genus Agrotis and, less frequently, the armyworm Spodoptera litura (F) (Waterhouse and Norris 1987). The significance to the biological control of DBM of other insects attacking brassicas lies of course in the fact that, if insecticides need to be employed against them, these insecticides, unless selective, are likely to have serious adverse effects on the natural enemies of DBM. The Present Position The distribution and importance ratings assigned by country experts to the more damaging pests of cabbage in the Pacific are shown in Table 1. The damage ratings are largely subjective and different authorities from the same country do not always agree. Nevertheless, the ratings