Agricultural intensification and greater production of Brassica vegetable and oilseed crops over the past two decades have increased the pest status of the diamondback moth (DBM), Plutella xylostella L., and it is now estimated to cost the world economy US$4–5 billion annually. Our understanding of some fundamental aspects of DBM biology and ecology, particularly host plant relationships, tritrophic interactions, and migration, has improved considerably but knowledge of other aspects, e.g., its global distribution and relative abundance, remains surprisingly limited. Biological control still focuses almost exclusively on a few species of hymenopteran parasitoids. Although these can be remarkably effective, insecticides continue to form the basis of management; their inappropriate use disrupts parasitoids and has resulted in field resistance to all available products. Improved ecological understanding and the availability of a series of highly effective selective insecticides throughout the 1990s provided the basis for sustainable and economically viable integrated pest management (IPM) approaches. However, repeated reversion to scheduled insecticide applications has resulted in resistance to these and more recently introduced compounds and the breakdown of IPM programs. Proven technologies for the sustainable management of DBM currently exist, but overcoming the barriers to their sustained adoption remains an enormous challenge.
Twenty years have passed since Talekar & Shelton (168) published their landmark review of the diamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Plutellidae), and we now know considerably more about many aspects of its biology (34, 147, 155, 161), ecology (125, 129), and genetics (12). Despite these advances, DBM has retained its status as the most destructive member of the different insect pest complexes that attack Brassica vegetable crops in various parts of the world (34, 147, 155, 161, 165, 167), and it is increasingly considered a significant, if sporadic, threat to canola production (45).
Between 1993 and 2009 the global area of Brassica vegetable crops increased by 39%, and in 2009 an estimated 3.4 million hectares were grown worldwide (38). Concomitant with this change was an intensification of farming practices, with cabbage yields increasing by 27%, and Brassica vegetables now contribute more than US$26 billion to the world economy (38). Over the same period, the area of oilseed rape planted increased by 59%, and in 2009 more than 31 million hectares were cultivated worldwide (38), often in regions where the crop was not previously grown. These changes have resulted in considerable modifications to many local landscapes (45) and provide revised challenges for DBM management (143).
Talekar & Shelton (168) stressed the importance of the proceedings of the international workshops on DBM held in 1985 and 1990 (165, 167). An additional four workshops convened between 1996 and 2011 and each published valuable proceedings on contemporary DBM research (34, 147, 155, 161). In consideration of this body of work, of the many publications in the academic literature, and of the changes that have occurred in Brassica crop production since 1993, it is timely to review our knowledge of DBM ecology and management and to set it in the current context.
DBM DISTRIBUTION, CURRENT MANAGEMENT, AND ECONOMIC COSTS
Despite the pest status of DBM and assertions that it has the most extensive distribution of all Lepidoptera (168), current understanding of its global distribution and relative abundance is limited (187). The original distribution map (25) is a composite of incomplete distribution records, and this has recently been superseded by a version that simply records countries where DBM has been reported (16).
Zalucki & Furlong (187) developed and validated a bioclimatic model for DBM that predicts its core distribution, where it persists year-round, as well as regions where it can be a seasonal pest (Figure 1). The model also predicts the variable seasonal phenologies exhibited by DBM across its vast range (189), illustrating the population-limiting effects of high rainfall and extreme temperatures (82) that can preclude year-round persistence (55, 58). Further, it identifies regions that are seasonally suitable for DBM population growth, where pest outbreaks can be promoted following DBM influxes (31).
Despite the considerable effort to develop integrated approaches to DBM management (39, 51), the vast majority of Brassica crops are treated prophylactically with insecticides (51). This practice is most acute in tropical countries, where nonselective products, which are often mixed, are typically applied 1 to 2 times per week (5, 51, 96); however, excessive use of insecticides against DBM is not restricted to the developing world or the tropics (95). Such practices promote the selection for insecticide resistance (81), destroy natural enemies (44), and contaminate the environment. They are also expensive, and a recent study (189) estimated that annual DBM control in Brassica vegetable crops alone costs US$1.4 billion worldwide, rising to US$2.7 billion if yield losses are included and to $4–5 billion if DBM losses and control costs to the worldwide canola industry are added.
DBM BIOLOGY AND ECOLOGY: ADVANCES SINCE 1993
Natural Enemies of DBM
A wide range of natural enemies, including parasitoids, arthropod predators, viruses, microsporidia, pathogenic fungi, and bacteria, attack DBM. Considerable basic and applied research has focused on these organisms over the past two decades, particularly on classical biological control programs utilizing hymenopteran parasitoids (129).
Hymenopteran parasitoids. Classical biological control for DBM began in 1936 when the larvalpupal parasitoid Diadegma semiclausum (Hellen) (Hymenoptera: Ichneumonidae) and the pupal ´ parasitoid Diadromus collaris (Gravenhorst) (Hymenoptera: Ichneumonidae) were successfully introduced into New Zealand from the United Kingdom (168), leading to further introductions of both species from New Zealand into Indonesia (180), Australia (186), and Malaysia (104). Following importation of D. semiclausum from Indonesia, the parasitoid was established in Taiwan (169), providing stock material for subsequent successful introductions into the Philippines (177), India (21), Laos, Vietnam, China (166), and Kenya (89). In 2005, D. semiclausum from Malaysia successfully established in the highland regions of Thailand (174). Hence, the provenance of the D. semiclausum populations that have established throughout Asia, Australasia, and parts of Africa is clear and well documented and all ultimately originate from the United Kingdom. Similarly, the larval parasitoid Cotesia vestalis (=plutellae) Haliday (Hymenoptera: Braconidae) has been the subject of more than 20 classical biological control introductions, and many of them have been successful (28, 166). In addition, this parasitoid appears to have a wider natural distribution than D. semiclausum, and it has been recorded to attack DBM in many regions [e.g., Malaysia (104), Taiwan (166), Vietnam (166), China (87), and Japan (3)] with no records of introductions. The larval-pupal parasitoid Oomyzus sokolowksii Kurdjumov (Hymenoptera: Eulophidae) has also been recorded in countries with no records of introduction [e.g., Australia (46), Japan (166), South Africa (71), and North Korea (39)]; however it has been deliberately introduced into China and Taiwan (166). Similarly, the DBM pupal parasitoids D. collaris and Diadromus subtilicornis (Gravenhorst) (Hymenoptera: Ichneumonidae) have widespread natural distributions (28). Since the initial release of D. collaris in New Zealand, it has been introduced into many countries (28), but D. subtilicornis has never been introduced for biological control (28).