A. Sivapragasam*
Malaysian encounters with invasive alien species (IAS) are described. There is still little information on how IAS have impacted on the endemic biodiversity and altered the structure and functions of the inherent ecosystems. There has also been no comprehensive or coordinated monitoring to properly document or catalogue introduced IAS. There are four options or steps for dealing with alien species: (1) prevention, (2) early detection, (3) eradication and (4) control. Details on how Malaysia has put these steps into practice are provided. Studies on the ecology and management of three recent invasive species of horticultural crops – beet army worm, leafminers and the whiteflies’ complex are elaborated. Suggestions are given for further strengthening of national capacity to deal with invasive pest problems in the future.
There are many definitions of invasive alien species (IAS). The Convention on Biological Diversity (CBD) has defined an alien species as a species, subspecies or lower taxon (includes any of its part that might survive and reproduce) introduced outside its natural past or present distribution (Roger 2003). It is invasive if it causes any form of damage. In the context of the CBD, it is a species that threatens biological diversity but in the broader context it has negative impacts in any areas such as agriculture, human development, human health as well as biodiversity (Roger 2003). The issues of IAS thus cut across a multisector dimension. The problem of insect pests, whether endemic or exotic, is irrevocably a major concern in agriculture. More recently, the IAS problem has acquired greater significance owing to global interest in biodiversity issues. The force behind IAS is driven largely by the apparent breakdown of biogeographical borders owing to increasing international trade and globalization exacerbated by modern modes of transportation that enhance the probability of biological invasions.
The biological process of colonization or invasion by alien organisms can be divided into four steps (Kiritani 1998): (i) Introduction, (ii) Establishment, (iii) Spread and (iv) Naturalization. The initiation of the process through the introduction of invasives can occur through: (i) Long distance migrations or movements (e.g. the brown planthopper, Nilaparvata lugens in rice), (ii) transportation and (iii) human activities. With increased international travel, the movement and incidences of exotic species have increased in both number and variety. Of particular concern are agricultural products, especially fresh produce such as vegetables, ornamentals, stored grains and timber. Fortunately, the rates of successful introduction, colonization and subsequent naturalization of an invasive species in a new habitat are remarkably low. Environmental sieves and dispersal constraints, natural disasters and human interventions have considerable influence. For example, in the United Kingdom studies showed that only 0.53 percent of the 220 000 imported species became naturalized and not all of them were invasive (Crawley et al. 1996). Such low incidences of successful naturalization are attributable to the low base-rate probability of invasions (Williamson 1998).
By 1979, several serious IAS had reached Malaysia already and collectively impacted on its agricultural landscape and economy. Examples of such pests are given in Table 1. There is still little information on how the IAS have impacted on the endemic biodiversity and altered the structure and functions of the inherent ecosystems. There has also been no comprehensive or coordinated monitoring to properly document or catalogue introduced IAS.
Table 1. Important IAS detected in Malaysia (Asna et al. 2001)
|
Crop |
Pest species |
Year recorded |
Suspected country of origin |
|
Cocoa |
Conopomorpha cramerella |
1986 |
Indonesia |
| Vegetables |
Chromatomyia horticola Spodoptera exigua Trialeurodes vaporariorum |
1986
1996 n.a*. |
The Netherlands Thailand The Netherlands |
| Cut flowers |
Liriomyza huidobrensis |
1991 |
The Netherlands |
| Turf |
Nematode, Meloidogyne graminis |
1980 |
USA |
| Citrus |
Nematode, Tylenchlus semipenetrans |
1996 |
Indonesia |
| Paddy |
Golden apple snail, Pomacea canaliculata Echinochloa crusgalli |
1984
1979 |
Philippines Australia |
| *n.a. = not available | |||
Malaysia also monitors several other alien pests which are yet to be detected but are considered to be a threat to economic crops. These invasive pests include thrips species (Scirtothrips dorsalis, Chaetanaphothrips signipennis and Frankliniella bispinosa) for vegetables and fruits; the Mediterranean fruit fly (Ceratitis capitata) and the San Jose Scale (Quadraspidiotus perniciosus) for citrus; the larger grain borer (Prostephanus truncatus) for stored products and the palm weevil (Rhynchoporus palmarum) for oil palm (Asna et al. 2001).
Malaysia has also been the source of invasives based on interceptions by importing countries. Some of the recent interceptions were: (i) Aquatic plants infested with the whitefly, Bemisi tabaci and the snail, Achatina fulica by France and Japan, respectively; (ii) Fresh chrysanthemums infested with thrips, Frankliniella occidentalis by Taiwan; and (iii) Carambola fruits infested with fruit flies by France.
In Malaysia there are four options or steps for dealing with alien species (Wittenberg and Cock 2001): (1) prevention; (2) early detection; (3) eradication and (4) control. Four major programmes are implemented by the Department of Agriculture under the Malaysian Plant Quarantine Act to prevent the entry and spread of IAS: prevention, detection and monitoring, containment and eradication and control. Prevention and eradication are fraught with difficulties: prevention, because of the complexity of the problem facing the front-line quarantine service and the degree of vigilance required to filter invasives before they enter the country (see below); and eradication because of the exorbitant costs involved. In most cases, the invasives are detected only after they have entered the country. Thus, the management strategy centres around control to minimize further risks.
Prevention is the first line of defense against any invasive pest. The Malaysian Plant Quarantine Act, 1976 (hereafter the Quarantine Act) and Plant Quarantine Regulation 1981 (hereafter the Quarantine Regulation) are the two laws that aim to prevent the entry and spread of noxious plants and plant pests that threaten agriculture. Besides IAS, the Quarantine Act also has provision to control outbreaks of endemic pests. In regulating the IAS, the Quarantine Regulation has listed 240 species of insects, fungus, viruses and weeds that are prohibited from importation or possession. This List is gazetted in the Fourth Schedule of the Regulation. Thus, all goods that harbour pests in the List will be treated or destroyed immediately. The Quarantine Act and the Quarantine Regulation also have provision to contain, eradicate or control any dangerous pest, either foreign or endemic, found in the country. This provision allows the Department of Agriculture (DoA) to call relevant bodies or agencies to collaborate and enforce any action to eradicate or control a pest.
However, despite vigilance, IAS still overcome quarantine barriers, as evidenced by the number of recent Malaysian interceptions (Table 1). In 2000, a total of 580 agricultural consignments with pests were intercepted. However, only 82 species of arthropods belonging to eight orders and 41 families were identified. Out of this total, three species (Trogoderma granarium, Ceratitis capitata and Diatrea sp.) were pests gazetted under the present quarantine act (Wan Normah and Asna 2001). The recent growth of the highland (temperate) horticultural industry has also been conducive towards the increase in the number of IAS, particularly greenhouse pests such as whiteflies and leafminers. Commercial activities enhance transnational spread. For example, Tan and Lim (1985) found that between 18.5 and 38.9 percent of English cabbage heads imported from Indonesia were infested.
Early detection of introductions and quick, coordinated response can eradicate or contain IAS at much lower cost than long-term control. Apart from inspection at entry points, there is no comprehensive national system for detecting incipient invasions of IAS. However, a few programmes are in place for rice, rubber and oil palm. The Pest Surveillance and Forecasting System was set up in 1979 in Peninsular Malaysia by the DoA to detect and monitor rice pests, particularly naturally invasive pests. Pests and crops are monitored regularly through field scouting, light traps, net traps and mobile nurseries. For rubber and oil palm, the Malaysian Rubber Board (MRB) and Malaysian Oil Palm Board (MPOB) with the assistance of the DoA are carrying out a bi-annual national monitoring survey for early detection of dangerous pests of rubber and oil palm. Apart from these measures, invasive pests are also detected through normal agricultural extension activities and reports of pest incidence by the public. For monitoring, pheromone traps have been used for pests such as the beet armyworm, S. exigua and T. granarium.
In the past, programmes were initiated to eradicate dangerous exotic pests and contingency plans were developed after the pests were detected. The eradication programme for Cocoa Pod Borer (CPB) was unsuccessful as the pest had spread beyond control. The CPB is currently managed under the IPM approach.
Another programme, targeting the khapra beetle (Trogoderma granarium) was successful as the beetle was only found in several isolated rice warehouses. The model emergency action programme of Ganapathi et al. (1992) entailed: (i) physical destruction of infested plant products and (ii) fumigation of products, warehouses or stored rooms with methyl bromide at 80g/m3 for 48 hours depending on the temperature or with phosphine at 2g/m3 for three days. The other chemical approach is spraying the warehouse and contaminated materials with chemical insecticides. Storing of the products below 9 percent moisture content or less was also effective. Otherwise, no other attempts have been made to eradicate any IAS, even though numerous incursions have taken place, because many of these pests (e.g. leafminers, whiteflies) were only detected when they were already widespread throughout the country.
When an IAS appears to be permanently established and widespread, control is the most effective action to prevent their spread or lessen their impacts. The IPM approach is recommended. At this stage, pest control is the responsibility of the farmers themselves or individual owners and government agencies provide advisory services and assistance to control the pests effectively. Experiences with diamondback moth, fruitflies, leafminers and whiteflies are notable examples.
The beet armyworm, a recently reported invasive pest in Malaysia (Palasubramaniam et al. 2000; Palasubramaniam and Sivapragasam 2001), is a subtropical and tropical species found in many tropical and temperate regions of the world. Spodoptera exigua has become an important pest of various economic crops in the last ten years in Malaysia and has caused extensive damage to crops such as onions, brinjal, legumes and crucifers. In the United States, where it is major pest for cotton, S. exigua is considered a migratory species originating in Southeast Asia. In the southern states such as Arizona, Georgia, Florida and Texas, it has become a very important pest causing millions of dollars of damage to cotton. It is suggested that for S. exigua, migration has evolved as a major component of its life history especially during its pre-oviposition period to allow for egg maturation. Thus, heavy infestations of S. exigua may occur suddenly owing to the migration of many adults. French (1969) and Mitchell (1979) reported moths capable of migrating over large distances in a single night when the weather is favourable. In Malaysia, it is interesting to note that before 1996, S. exigua was never reported as a pest, unlike its congeneric species S. litura. However, rather surprisingly, outbreaks of S. exigua began emerging in 1996 (Palasubramaniam et al. 2000). Migration of adults is a possibility based on the proximity of Malaysia to neighbouring countries such as Thailand, Indonesia, Viet Nam and Myanmar where the pest is endemic and prevalent. The other possible reason could be through transnational commercial activities (Ng et al. 1999).
Figure 1. Trapping study of S. exigua using pheromone traps in Kuala Terla
The basis for long distance flights and large-scale dispersal of moths could be derived from pheromone trap catches. Trapping studies were conducted in the highlands of Malaysia (Kuala Terla) with two types of pheromones (from Taiwan and Agrisense, UK) to ascertain the dynamics of adult S. exigua populations in the field (Figure 1). Despite the continually high number of males caught in the traps, random sampling of host plants around the trap area revealed only a very few S. exigua larvae. The sampling of weeds around the trap area also revealed the absence of larvae; the placing of emergence cages did not trap any adults since S. exigua pupates in the soil. The latter experiments implied that the adults captured in the traps were not local and could have been attracted to the traps from outside the area. Long distance flight activity has been reported as a common phenomenon in S. exigua as part of its life history strategy to allow for egg maturation.
Spodoptera exigua is currently a major pest for vegetables. It is very difficult to control as it has developed resistance to most insecticides. The invasive nature of S. exigua has shifted the focus from other endemic pests of vegetables. In the lowlands of Malaysia, damage assessment results showed that damage by this pest was almost 100 percent in some crops. Almost all types of vegetable crops grown were found to be infested with this pest. Chilli and shallot were the most seriously affected crops and losses were significant.
Farmers currently use a range of insecticides to control S. exigua. The cost is currently very high and in instances of heavy damage some farmers decided to abandon the crops for that season. Some farmers also claimed that despite frequent applications of insecticides, they can only obtain less than one-half of the expected crop yield. Many farmers still use light traps (12 watt blue–black light bulb) to capture adults. However, unlike pheromones, low catches were experienced.
The effect of various commercial Bacillus thuringiensis products on S. exigua was evaluated in the laboratory (Figure 2). Amongst the biopesticides tested, ProtectR (Bacillus thuringiensis aizawai) gave the highest mortality of the larvae. Bacillus thuringiensis aizawai (ProtectR) was also very effective against the fourth instar of S. exigua.
Figure 2. Cumulative mortality of 5-day-old S. exigua using B. thuringiensis products
Natural enemies play a key role in suppressing S. exigua populations (Sivapragasam et al. 2001). In Malaysia, two larval parasitoid species were bred from field-collected samples: the braconid, Microplitis manilae and the tachinid, Peribaea orbata. The parasitization levels of these parasitoids, especially P. orbata, was as high as 45 percent in the field. Studies in the laboratory suggested the potential of Trichogrammatoidea bactrae fumata to parasitize eggs of S. exigua. However, the generalist nature of these parasitoids limits them as effective biological control agents. Generalist predators too play an important role contributing to the mortality of S. exigua in the field as shown by recent predator-exclusion studies. Survival was highest in caged cabbage plants (11.3 percent) compared to uncaged plants (3.8 percent). The major predators in the field were the red ant, Solenopsis sp. and some species of birds. Other potential predators include the pentatomid bug, Cantheconidea furcellata, reduviids, Cosmolestis picticeps and Rhinocoris flavipes, and the mirid, Cyrtopeltis tenius.
Agromyzid leafminers are pests of economic importance for several vegetables and ornamentals both in temperate and tropical regions. In Malaysia, the predominant species found infesting vegetables such as sugar peas and crucifers prior to the 1990s were Chromatomyia (Phytomyza) horticola in the highlands and Liriomyza brassicae in the lowlands (Ooi 1979; Sivapragasam et al. 1992). However, in the mid-1990s, various other species of Liriomyza such as L. huidobrensis in the highlands and L. sativae in the lowlands were reported (Sivapragasam and Syed 1999). Liriomyza spp. are native to the Americas (possibly Nearctic and Neotropical in origin) and are major pests on vegetables such as tomatoes and celery. It is believed that the introductions of temperate cut flower planting materials from Europe could have introduced the leafminers into the Cameron Highlands (Myint 1997). This situation is mirrored in other countries like Japan where the distribution of leafminers has been attributed largely to the failure of quarantine procedures to detect and prevent its entry into the country.
Leafminers are major pests of vegetables and ornamental plants in the Cameron Highlands. Yield losses of up to 30 percent were reported on most vegetables crops such as sugar peas, tomatoes, Chinese cabbage, capsicum and french beans if no control measures were undertaken. On chrysanthemums, farmers also reported losses of up to 50 percent due to leafminer infestation, particularly by C. horticola (Sivapragasam and Syed 1999). About 17 percent of the farmers abandoned vegetable cultivation because of these invasive pests.
Insecticides are generally used to control outbreaks followed by the use of yellow sticky traps. Under natural conditions, leafminers are attacked by a suite of parasitoids (Myint 1997; Sivapragasam and Syed 1999; Sivapragasam et al. 1995a; Sivapragasam et al. 1999a). However, the non-specificity of these parasitoids precludes them as effective biological control agents in the conventional sense. The IPM approach is currently used to manage leafminers in sugar peas (Sivapragasam et al. 1995b; Myint 1997) and chryanthemums (Sivapragasam et al. 1999b).
Two species of whiteflies were recently found in vegetables in Malaysia. The lowland species is the sweet potato whitefly, Bemisia tabaci (Gennadius) and the highland species is the greenhouse whitefly, Trialeurodes vaporariorum (Westwood). They are probably recent introductions from Europe through importation of ornamental plants (Syed et al. 2000). Another species of whitefly, the spiraling whitefly, Aleurodicus dispersus (Rusell) is endemic and found in several ornamental and fruit crops.
Recently, the whitefly infestation was serious, particularly for tomato, bell pepper and coyote; crop losses of up to 50 percent were reported. A high degree of insecticide resistance has already developed within many field populations of B. tabaci associated with agricultural crops (Martin 1999). The problem is further compounded by the ability of B. tabaci to transmit 60 different types of plant geminiviruses (Syed et al. 2000) and the presence of a new strain of B. tabaci, i.e. the B strain which has spread to most parts of the world. Bellows et al. (1994) described this strain as a new species, B. argentifolii, which is morphologically similar to B. tabaci. Since 1991, B. argentifolii (known as the silverleaf whitefly) has caused annual losses of US$500 million in the United States for various crops like cotton and crucifers. To date, in Malaysia, 21 different biotypes of B. tabaci have been observed based on allozyme patterns. The Malaysian population belongs to the Asia 2a group which has also been found in Indonesia, Sri Lanka, North India, Thailand and Bangladesh (Mohd. Roff, MARDI, pers. comm.).
The main method of control is the use of pesticides – but efficacy problems have already been manifested. A few predators have been recorded (Syed et al. 2000), viz., a mired bug, Macrolophus sp.; the green lace wing, Chrysopa sp. and a coccinellid, Delphastus sp. The parasitoid, Encarsia sp., was recovered from a few field specimens. Currently, the IPM approach is been investigated.
Based on the evidence to date, there are many gaps and needs in the current management strategy against invasives. There are many areas that need to be strengthened to manage the problem effectively. Against the backdrop of resource limitations, some of the urgent areas that need to be pursued in future are:
With the exception of PRA for key economic pests such as the South American Leaf Blight problem in rubber, there is no specific protocol to date for invasive insect pests. The PRA exercise will identify properly potential dangerous foreign species, economic consequences and their potential biological control agents. The review should assess their pathways of introduction and probability of entry, establishment and spread. In addition, the PRA must include control measures to prevent their introduction, establishment and spread.
The present plant quarantine regulations do not require any import permit for any importation of plants or plant parts for consumption, medicinal, processing or manufacturing purposes and processed agricultural products. Their importation is only subjected to inspection at the entry point upon arrival. This loophole increases the possibility of introduction of exotic pests as inspection at the entry point is limited in effectiveness and mandatory treatment could not be imposed on high risk commodities before entry.
There is an urgent need for human resource development in specific fields such as PRA, inspection, detection, identification, sampling techniques and treatment. Highly trained and efficient quarantine personnel, particularly those working at entry points, will be able to contribute to preventing the entry of exotic pests into the country. There is need for competencies in conducting PRA and impact assessment studies, especially in the context of biodiversity.
Increased awareness reduces the risk of introduction. Therefore, cooperation from the public is required as part of the holistic management strategy to deal with invasives. The public has to be informed or educated about the danger of alien species, to recognize the major alien invasive species and report to authorities accordingly when they suspect their presence. Impact assessment studies could contribute towards realizing the potential dangers of invasives on the local ecosystems.
The author is grateful to the Director-General of MARDI and the Director of the Rice and Industrial Crop Center, MARDI for their permission to participate in and present this paper. The support and invitation by APFISN, FAO, USDA Forest Service and APAFRI is gratefully acknowledged.
Asna, B.O., Mazlan, S. & Ho, H.L. 2001. Plant quarantine regulations and issues pertaining to the prevention of invasive arthropod pests. Paper presented at the Seminar on Invasive Arthropod Pests of Vegetables and Economic Food Crops, Kuala Lumpur, 13 - 14 March, 2001.
Bellows, T.S. Jr., Perring, T.M., Gill, R.J. & Headrick, D.H. 1994. Description of a species of Bemisia (Homoptera: Aleyrodidae). Annals of the Entomological Society of America, 87 (20): 195 - 206.
Crawley, M.J., Harvey, P.H. & Purvis, A. 1996. Comparative ecology of the native and alien floras of the British Isles. Philosophical Transactions of the Royal Society (London), B351: 1251 - 1259.
French, R.A. 1969. Migration of Laphygma exigua (Hubner) to the British Isles in relation to large scale weather systems. J. Anim. Ecol., 38: 199 - 10.
Ganapathi, A., Durai, P.S.S. & Sastroutomo, S.S. 1992. PLANTI’s current and future thrusts on plant quarantine research and development. In P.L. Manalo et al., eds. Proceedings of the international plant quarantine congress, 1991 on plant quarantine in the 90’s. ASEAN PLANTI.
Kiritani, K. 1998. Formation of exotic insect fauna in Japan. In E. Yano, M. Matsuo, M. Shiyomi and D.A. Andow, eds. Biological invasions of ecosystem by pests and beneficial organisms, pp. 49 - 65. Tsukuba, Japan, National Institute of Agro-Environmental Sciences.
Martin, J.H. 1999. The whitefly fauna of Australia (Sternorrhyncha: Aleyrodidae), a taxonomic account and identification guide. CSIRO Entomology Technical paper No. 38, 197 pp.
Mitchell, E.R. 1979. Migration of Spodoptera exigua and S. frugiperda C North American style. In Movement of highly mobile insects. Concepts and methodology in research, pp. 386 - 393. Raleigh, North Carolina, North Carolina State University.
Myint, T. 1997. Ecology and control of leafminers (Diptera: Agromyzidae) on sugarpeas in Cameron Highlands. PhD thesis submitted to University Kebangsaan Malaysia, Bangi, Selangor, Malaysia.
Ng, K.Y., Jamaludin, S. & Mohamed Roff, M.N. 1999. Beet armyworm, Spodoptera exigua Hubner (Lepidoptera: Noctuidae) and its control in Malaysia. In Mohamed et al. (compiled). Proc. national horticultural conference 99, pp. 607 - 611.
Ooi, A.C.P. 1979. An ecological study of the diamondback moth in Cameron Highlands and its possible biological control with introduced parasites. University of Malaya (MSc thesis).
Palasubramaniam, K. & Sivapragasam, A. 2000. The beet armyworm, Spodoptera exigua: its incidence and control measures used by vegetable farmers in Peninsular Malaysia. Paper presented at the Plant Resource Management Conference, 23 - 24 November, 2000. Organized by MAPPS, SIAS and DOE, Sarawak.
Palasubramaniam, K. Thiagarajan, R. & Asna, B.O. 2000. Status of the beet armyworm (Spodoptera exigua) in Peninsular Malaysia. In S. Zakaria, eds. Proceedings of the plant health seminar 2000, 123 - 126. Malaysian Plant Protection Society.
Roger, D. 2003. Invasive alien species: Global perspectives. In Proceedings of the national workshop on invasive alien species, 20 October 2003, Kuala Lumpur. Organized by the Department of Agriculture, Ministry of Agriculture, Malaysia in collaboration with ASEANET and CAB International – SEA Regional Office.
Sivapragasam, A. & Syed, A.R. 1999. The problem and management of leafminers on vegetables in Malaysia. Paper presented at the CABI-FAO Workshop on Leafminers of Vegetables in South East Asia, 2 - 5 February, 1999.
Sivapragasam, A., Syed, A.R. & Loke, W.H. 1992. The leafminer, Chromatomyia horticola (Goureau) and its increasing menace to farmers in Cameron Highlands. MAPPS Newsletter, 16 (3): 19 - 20.
Sivapragasam, A., Loke, W.H., Syed, A.R. & Ruwaidah, M. 1995a. Dipteran leafminers on highland crops of Malaysia and an integrated approach towards their management. In European Journal of Plant Pathology, XIII International Plant Protection Congress, 2-7 July, 1995. The Hague, Netherlands. Kluwer Academic Publishers.
Sivapragasam, A., Loke, W.H., Syed, A.R. & Ruwaidah, M. 1995b. Agromyzid leafminers and their parasitoids on some ornamentals in Cameron Highlands. In H. Zaharah et al., eds. Proceedings of national seminar and exhibition on integrated production of temperate cut flowers, 27-30 June, 1995, Cameron Highlands, Malaysia.
Sivapragasam, A., Syed, A.R., La Salle, J. & Ruwaida, M. 1999a. Parasitoids of invasive agromyzid leafminers on vegetables in Peninsular Malaysia. In W.H. Loke, et al., eds. Proceedings of the symposium on biological control in the tropics. Malaysia, CAB International South East Asia Regional Center.
Sivapragasam, A., Loke, W.H. & Ruwaida, M. 1999b. A strategy towards managing the agromyzid leafminer Chromatomyia horticola (Goureau) on chrysanthemums through the selective use of insecticides. In Sivapragasam et al., eds. Proceedings of the 5th international conference on plant protection in the tropics, 15-18 March, 1999, Kuala Lumpur, Malaysia, pp. 93 - 95.
Sivapragasam, A., Asna, B.O., Palasubramaniam, K. & Megir, G. 2001. Terminal report of PF project on Spodoptera with emphasis on S. exigua in Malaysia presented at a workshop on Spodoptera in Southeast Asia, 14 - 16 March, 2001, The Legend Hotel, Kuala Lumpur (mimeo).
Syed, A.R., Sivapragasam, A., Loke, W.H. & Mohd. Roff, M.N. 2000. Whiteflies infesting vegetables in Malaysia. In Proceedings of the plant resource management seminar. Organized by MAPPS, DoA Sarawak and SIAS.
Tan, C.L. & Lim, G.S. 1985. Transnational movement of insect pests (and other invertebrates) via imported cabbages. In K.G. Singh and Paul L. Manalo, eds. Proceedings of the regional conference on plant quarantine support for agricultural development. Published by ASEAN PLANTI, Malaysia.
Wan Normah, W.I. & Asna, B.O. 2001. Interception and contingency plans on invasive pests. Paper presented at the Seminar on Invasive Arthropod Pests of Vegetables and Economic Food Crops, Kuala Lumpur, 13–14 March, 2001.
Williamson, M.H. 1998. Measuring the impact of plant invaders in Britain. In U. Starfinger, K.R. Edwards, I. Kowarik and M.H. Williamson, eds. Plant invasions: ecological mechanisms and human responses, pp. 57–68. Leiden, Backhuys Publishers.
Wittenberg, R. & Cock, M.J.W. 2001. In vasive alien species: A toolkit of best prevention and management practices. Wallingford, Oxon, UK, CABI.
* Rice and Industrial Crops Center, MARDI, P.O. Box 12301, Kuala Lumpur, Malaysia . E-mail: [email protected]
