Biological Control of Diamondback Moth in the Pacific D.F. Waterhouse Australian Centre for International Agricultural Research, GPO Box 1571, Canberra ACT 2601, Australia Abstract 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 213 214 W aterhouse Table I. Distribution and importance of brassica pests in the oceanic Pacific. Major pests Occasional Pests DBM Crocidolomia Hellula Helicoverpa Agrotis sp. pavonana undalis rogatalis armigera Cook Islands +++* ++ ++ ? ++ Fiji +++* ++ + ++ ++ French Polynesia ++* + P + + Federated States of Micronesia + ++ Guam +* +++ +++ +++* Hawaii +++ ++ + Kiribati + P Marshall Islands New Caledonia +++* + + +++* + Niue +++* +++* P Northern Mariana + + + * ++ +++ +++ Islands Palau +++* +++* +++* +++* Papua New Guinea + + + +++* +++ ++ American Samoa +++* ++ P Western Samoa +++* + Solomon Islands ++ +++ ++* ++ Tokelau P Tonga +++* + ++* P +++* + Tuvalu P P Vanuatu ++ ++ ++ + Wallis and Futuna 'One of the country's top IO insect pests; + + + widespread, causing important damage every year; + + less widespread, but of great importance; + important locally; P present, but unimportant. do provide the best available guide to the relative importance of the pests. The same conventions have been used as in 'Biological Control: Pacific Prospects' (Waterhouse and Norris 1987). New data have been provided from a 1990 survey carried out by B.M. Thistleton, Biological Control Officer, South Pacific Commission, and from my recent correspondence with Pacific officials. A workshop on biological control in the Pacific, held in Tonga in 1985 and attended by representatives of 15 Pacific nations, unanimously placed DBM at the top of the priority list of pests causing serious problems and deserving of an attempt at biological control. No improvement in pest status has taken place over the past 5 years, as shown in Table 1, that would alter this priority. Indeed, with the exception of a modest, continuing program in Fiji and a very recent introduction to Cook Islands (see later), there have been no active biological control projects involving DBM in this period. This situation may well change, however, since, at an SPC/GTZ Biological Control Planning Meeting in Fiji in July 1989, DBM was tentatively selected as the priority target for a major new initiative in the Pacific to commence during August 1990. It is thus timely to review what is known about the natural enemies of DBM in the Pacific, the history of past introductions of agents to the region and also to consider what the prospects are for successful biological control. Biological Control in the Pacific 215 Since both Australia and New Zealand have benefited significantly from biological control of DBM, brief overviews are first presented to assist in setting the scene for consideration of the position in the oceanic Pacific. Australia DBM became a troublesome pest in Australia soon after it was first reported in 1893. It has since been the target of a number of attempts at biological control and three major parasitoids have been established (Table 2). It is also attacked by a number of native parasitoids (Waterhouse and Norris 1987). As a result, there has been a marked reduction in damage in many areas and particularly in the Australian Capital Territory and South Australia, where DBM now seldom needs to be treated with pesticides. Elsewhere, and particularly in Queensland, damage to crops for human consumption may require treatment but, for forage crops, less frequently than before. However, even if DBM were to become no problem, much the same regime of insecticide treatments would still be required against other brassica pests, including Pieris rapae (L.), C. pavonana, Helicoverpa armigera, Hellula hydralis Guene and Brevicoryne brassicae (L.). It is probable that the current insecticide regime exercises a significant adverse effect on the abundance of DBM parasitoids. Therefore, any renewed attempts at biological control should also involve the introduction of agents for all important members of the pest complex. Even at the time of greatest pest pressure it is generally only necessary to spray brassicas at 7-14-day intervals and sometimes, as in winter, even less frequently. There has been an increase in Bacillus thuringiensis Berliner usage, but no growers appear to be relying on it entirely. Table 2. Major parasitoids of DBM establishedin Oceania, Australia and New Zealand. Cotesia Diadegma Diadegma Diadromus Oomyzus plutellae insulare semiclausum collaris sokolowskii Cook Islands + + Fiji + + Hawaii + + Papua New Guinea + Australia + + + New Zealand + + New Zealand In spite of the fact that they are attacked by hyperparasitoids, the introduction and establishment of Diadegma semiclausum (= D. eucerophaga) and Diadromus collaris (Table 2) has provided adequate control of DBM in North Island brassica crops, but insecticide applications are still sometimes necessary in the South Island. The fungus Zoophthora radicans Brefeld (= Entomophthora sphaerosperma) also exerts important control when weather conditions in autumn are suitable and moth populations high. In neither island do parasitoids keep damage at an acceptably low level in brassicas (especially cabbages) for human consumption. The major insecticide groups are still effective and the commercial use of B. thuringiensis preparations is rare. The establishment of additional parasitoids is under consideration (Thomas and Ferguson 1989). Cook Islands The two lepidopterous pests of cabbage in Cook Islands are DBM and Crocidolomia pavonana, the latter being of lesser importance. Diadegma semiclausum, Diadromus collaris and Trichogramma sp. were introduced in 1974-75 from New Zealand but, in 1978, only D. 216 Waterhouse collaris could be recovered (Table 3) (Walker and Deitz 1979). Diadegma semiclausum was again introduced from New Zealand in 1990, but there is no information yet on its establishment (G. Hill pers. comm. 1990). Although it has not been intentionally introduced to Cook Islands C. plutellae was found on Raratonga (and possibly Mauke) in 1988 and, together with D. collaris, was responsible for moderate to low levels of parasitization. In addition, a male of Apanteles sp. (ultor group) was found in association with cabbages on Raratonga (G. Hill pers. comm. 1988). An Apanteles sp. of the same group has been reported from C. pavonana in Pakistan (CIBC 1981, 1982). Resistance to organophosphorus insecticides emerged in the late 1970s and to synthetic pyrethroids in the mid 1980s. Trials with insect growth regulators have given good results, but these materials are not yet available. Bacillus thuringiensis has given variable results and more effective formulations are required (1990 Report of the Totokoitu Research Station). Fiji DBM continues to be a major pest in Fiji, where it is resistant to all insecticides that have been used against it for any