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The insect larva is a surface feeder and with its chewing mouthparts it feeds voraciously on the leaves leaving a papery epidermis intact. This type of damage gives the appearance of transluscent windows or 'shot holes' in the leaf blades. Insect larvae and, in many cases, pupae are found on the damaged leaves. In cases of severe infestation, entire leaves could be lost, leaving only the veins. The larvae nibble the chlorophyll-rich green areas of stems and pods and the damage shows from a distance as an unusual whitening of the crop. The damage is often first evident on plants growing on ridges and knolls in the field (Canola Council of Canada, 2014). Heavily damaged plants appear stunted and in most cases die.
In oilseed rape plants, larvae also feed on flower buds, flowers and young seed pods. The seeds within damaged pods do not fill completely and pods may shatter prematurely. Larvae also chew into pods and consume the developing seeds. Extensive feeding on the reproductive plant parts significantly reduces crop yields (Canola Council of Canada, 2014).
At present, the mainstay of control in all tropical to subtropical developing countries (where small farms dominate vegetable production), is the frequent use of insecticides. In most of these countries, insecticides, all of which are imported from developed countries, are readily available at a reasonable cost. In some countries pesticides are subsidized. These factors lead to over-use and complete dependence on insecticides to control P. xylostella. In tropical countries where crucifers are grown throughout the year, P. xylostella can have up to 20 generations per year. This situation leads to the rapid build-up of insecticide resistance. To overcome resistance, farmers often increase doses of insecticide, use mixtures of several chemicals, and spray more often, sometimes once every 2 days. These high levels of use have caused P. xylostella to become resistant to practically all insecticides in many countries.
Cultural Control (Field Management)
Some of the classical control measures that have been tried with some success are intercropping, use of sprinkler irrigation, trap cropping, rotation and clean cultivation.
Though intercropping is a normal cultivation practice in the tropics it is not presently used for the management of P. xylostella, but rather for horticultural and economic reasons. The earliest successes occurred in Russia where intercropping cabbage with tomato reduced damage to cabbage by several pests (including P. xylostella) (Vostrikov, 1915). However, this practice had only limited success in India (Chelliah and Srinivasan, 1986), the Philippines (Magallona, 1986) and Taiwan (AVRDC, 1987). In Taiwan none of the 54 crops tested for their usefulness in intercropping had any significant impact on the population of P. xylostella on cabbage.
Trials carried out in India showed that planting one row of late season cauliflower with one row of main season tomato significantly reduced the incidence of P. xylostella when the cauliflower was planted 30 days after tomato (Kandoria et al., 1999). Studies on the effects of intraplot mixtures of toxic (genetically engineered with Bacillus thuringiensis) and non-toxic collard (Brassica oleracea var. acephala) plants on the population dynamics of P. xylostella and its natural enemies suggested that intrafield mixtures could decrease the density of a target pest such as the diamondback moth, while not adversely affecting natural enemies (Riggin Bucci and Gould, 1997).
Except for the first instar, all P. xylostella larvae and pupae are exposed on the leaf surface and are influenced by various abiotic factors. Several reports indicate that rainfall is an important mortality factor for P. xylostella (Gray, 1915; Harcourt, 1963; Talekar and Lee, 1985) and thus this pest is serious only during the dry season. Overhead irrigation has been shown to reduce P. xylostella injury to cabbage (Talekar et al., 1986) and watercress (Nakahara et al., 1986). The water droplets are believed to drown or physically dislodge the pest from the plant surface, causing a reduction in their numbers. This operation at dusk also reduced mating-related flight activity and presumably oviposition. Using sprinkler irrigation to control this pest in crops other than watercress, however, is not practical on a commercial farm because of the high cost and probable increase of diseases such as black rot and downy mildew.
Before the advent of modern organic insecticides, a common practice was to plant strips of an economically less important plant highly preferred by P. xylostella within a commercial crucifer field. The preferred crops, primarily white mustard (Brassica hirta) or Indian mustard (B. juncea) attracted P. xylostella adults, which spared the commercial crop such as cabbage, Brussels sprouts and others from its attack (Kanervo, 1932; Ghesquiere, 1939). Now, because of insecticide resistance problems, trap cropping is becoming a more realistic alternative, especially in developing countries. In India when one row of mustard was alternated with 15-20 rows of cabbage, P. xylostella colonized the mustard and spared the main crop (Srinivasan and Krishna Moorthy, 1992). In order to trap most P. xylostella adults in a field, healthy growing mustard in the vegetative stage must be available throughout the cabbage-growing period. This technique also spares cabbage from attack by Crocidolomia binotalis. Studies in Malaysia (Sivapragasam and Loke, 1997), Hawaii (Luther et al., 1996) and South Africa (Charleston and Kfir, 2000) also suggested that Indian mustard showed potential as a trap crop for P. xylostella. However, Indian mustard may not provide specific advantages to cabbage cultivation if the economics of cultivating the former is considered (Sivapragasam and Loke, 1997; Subrahmanyam, 1998). Conflicting results for Indian mustard were obtained in Indonesia (Omoy et al., 1995; Prabanungrum and Sastrosiswojo, 1995).
Crucifers with a glossy phylloplane are not only attractive for P. xylostella oviposition, but the glossy trait also negatively affects survival and development suggesting that selected glossy cultivars have potential usefulness as trap crops in Brassica vegetable fields. Glossy yellow rocket (Barbarea vulgaris var. arcuata) is a potential candidate for dead-end trap cropping. P. xylostella ovipositional preference was much greater on glossy yellow rocket than on cabbage and oilseed rape but larvae failed to survive on it (Shelton and Nault, 2004). This discovery initiated new interest in trap cropping but there remain questions to be addressed (e.g. competition with the main crop, placement method in the field, weed seed bank in soils, etc.) before yellow rocket could be recommended for extensive field use (Sarfraz et al., 2006).
Rotation and clean cultivation
Crop rotation is rarely practised for control of P. xylostella in intensive vegetable-growing areas of the tropics and subtropics because of the high prices that crucifer vegetables fetch. However, because continuous planting of crucifers allows continuous generations of the pest, which leads to frequent use of insecticides and development of pesticide resistance, crop rotation may become necessary. If crop rotation is followed by all farmers in a locality simultaneously, it will lead to a crucifer-free period that disrupts the pest's breeding cycle and may help control the pest in the crop following the rotation crop.
Clean cultivation can be an important factor in the management of P. xylostella. Planting seedling beds away from production fields, and ploughing down crop residues in seedling beds and production fields, are efficient and easy management practices. Where seedlings are grown in the greenhouse, prevention of infestations by immigrating adults can be accomplished through the use of insect-proof screens.
Tests to determine how undersowing Brassica crops with subterranean clover (Trifolium subterraneum) affected host-plant selection by some pests including P. xylostella indicated that in all cases, 40-90% fewer insect pest stages were found on plants in clover than on those in bare soil (Kienegger et al., 1996). Field experiments comparing two different undersown crops, strawberry clover (Trifolium fragiferum cv. Palestine) and spurrey (Spergula arvensis), revealed that populations of P. xylostella larvae were not as high as in monocropped plots (Theunissen et al., 1996) but the quality of cabbages from the undersown plots was much better. Spurrey is interesting because it is able to suppress pest populations, notably larvae of P. xylostella and thrips, but it could be difficult to integrate in existing cropping practices (Theunissen et al., 1996).
Two types of resistance, normal-bloom cabbage and glossy-leaf cabbage, have been identified by American scientists (Dickson et al., 1990). Hybrid lines of cabbage and cauliflower bred from these resistance sources showing good level of resistance to P. xylostella are available. However, because of the thick leaves in the normal-bloom type and dark green glossy leaves in other lines, these hybrids have not been popular with consumers and thus they are not yet exploited commercially. Factors inducing resistance vary. Ganeshan and Narayanasamy (1997) suggested that high contents of protein, orthodihydroxy phenols and low quantities of sugar (reducing and non-reducing) were factors for resistance in three cauliflower lines, whereas Ramachandran et al. (1998) suggested differing leaf characteristics.
Brassicaceous species differ in their resistance as hosts for P. xylostella. Females preferred to lay eggs on Sinapis alba and Brassica rapa, but development times of larvae and pupae were most rapid on B. juncea and S. alba (Sarfraz et al., 2007). Development was also influenced by varieties within species. Although survival did not vary for P. xylostella reared from egg to pupa on the B. napus varieties Q2, Liberty and Conquest, females deposited more eggs on Liberty than on Q2 or Conquest. Development of females from larva to prepupa was faster on Liberty and Conquest than on Q2 (Sarfraz et al., 2007).
Host-plant resistance work also revolves around incorporating one or more novel pesticide genes into oilseed rape and Brassica vegetables. Transgenic canola carrying the cry1Ac gene was developed and tested for P. xylostella control in field and glasshouse trials in the USA (Ramachandran et al., 1998) but no such transgenic crops are registered yet. There is increasing interest in getting this type of resistance registered in Australia (Canola Council of Canada, 2014). Bt-cabbage and Bt-cauliflower plants were also developed and tested against P. xylostella, but due to regulatory and liability issues the transgenic vegetables were not field released and the project ceased in 2010 (Russell et al., 2011).
A sex pheromone consisting of three chemical components: (Z)-11-hexadecenal, (Z)-11-hexadecenyl acetate, and (Z)-11-hexadecenyl alcohol is now available commercially. This pheromone attracts male adults and suitable traps are used to kill the moths attracted to the pheromone. Extensive studies have already determined the optimal proportion and leading of the pheromone components, effective distance and longevity (Chisholm et al., 1983, Chow et al., 1978, Lee et al., 1995) in order to use the pheromone more effectively. This pheromone has been used for monitoring P. xylostella populations in the field (Baker et al., 1982). During the past 5 years, Japanese scientists have succeeded in achieving mating disruption in cabbage fields using high concentrations of the pheromone (Ohno et al., 1992). A 1:1 mixture of (Z)-11-hexadecenal and (Z)-11-hexadecenyl acetate known as 'Konaga-Con' is now commercially available in Japan. Collaborative multilocation studies in Japan have shown promising results (Ohbayashi et al., 1992), but 'Konaga-Con' use is still not cost effective. Experiments to evaluate the efficacy of a blend of pheromones to disrupt mating of diamondback moth and cabbage looper (Trichoplusia ni) when dispensed simultaneously from Yoto-con-S R 'rope' dispensers showed some promise in suppressing numbers of P. xylostella larvae to below predetermined threshold levels (Mitchell et al., 1997).
This involves both classical biological control and the conservation of endemic natural enemies. In general, the former is emphasised because P. xylostella is an introduced pest in most countries. Introduction of exotic parasites to control pest insects has been practised for decades. This approach has considerable promise for the control of P. xylostella; however, it has been practised only sporadically over the past 50 years. Widespread and often indiscriminate use of insecticides has frustrated recent efforts and delayed the establishment of parasites and their beneficial effects.
In one of the earliest parasite introductions, Diadegma semiclausum and Diadromus collaris were introduced in New Zealand from England (Hardy, 1938, Thomas and Ferguson, 1989). These introductions continue to suppress P. xylostella populations until now, and the challenge today is to incorporate this natural control into a commercial IPM.
In Australia, prior to the introduction of effective exotic parasites, P. xylostella caused serious damage (Wilson, 1960). Among the introduced parasitoids, D. semiclausum became established throughout Australia, including Tasmania. Diadromus collaris was established principally in Queensland, New South Wales, Victoria and Tasmania and Cotesia plutellae in Australian Capital Territory, New South Wales and Queensland. These introductions resulted in heavy parasitism of C. plutellae (72-90%) and marked reduction in damage to crucifers (Wilson, 1960; Goodwin, 1979; Hamilton, 1979).
In the early 1950s, D. semiclausum was introduced from New Zealand into Indonesia's crucifer-growing areas in the highlands of Java (Vos, 1953) where it became established. However, because of over-use of insecticides, the beneficial effects of this parasite in the control of P. xylostella in the field were not realized until the mid-1980s (Sastrosiswojo and Sastrodihardjo, 1986). With substitution of chemical pesticides by Bacillus thuringiensis in the early 1980s, the parasite proliferated. This parasite has now been introduced from Java to the highlands of other islands in Indonesia.
In the Cameron Highlands of Malaysia, where crucifers are grown throughout the year, P. xylostella was a serious pest. However in 1977-78, Malaysian entomologists introduced D. semiclausum, and D. collaris. Although these parasites became established soon after introduction it was not until the late 1980s when chemical insecticides were substituted by B. thuringiensis that the impact of these parasitoids was fully realized. The combined parasitism has drastically reduced the need for insecticide applications and since then areas of cabbage production are increasing (Ooi, 1992).
In Taiwan, P. xylostella has been a serious pest since 1960s. Cotesia plutellae, reported to parasitize P. xylostella since 1972, could not give adequate control, so D. semiclausum was imported from Indonesia. This parasite failed to get established in lowlands but in highlands it was established within the same season (AVRDC, 1988). This cool-temperature parasite now occurs throughout the highland areas of Central Taiwan and provides substantial savings in P. xylostella control. Studies indicated a temperature range of 20-30°C is optimum for parasitization by C. plutellae and 15-25°C for D. semiclausum (Talekar and Yang, 1991). Parasitism by D. semiclausum drops rapidly at temperatures approaching 30°C.
In the Philippines, a single release of D. semiclausum in 1989 at the beginning of the season resulted in 64% parasitisation of P. xylostella, and an 80-90% drop in pesticide use (Poelking, 1992). Ofelia (1997) reported that the establishment of D. semiclausum in cabbage achieved economical effective control of P. xylostella providing substantial savings for farmers per hectare in a cropping season. In discussing natural enemies, the important role of predators in the management of P. xylostella should also be emphasised as has been suggested in various studies based on population dynamics and the indirect effects of insecticides (Nemato, 1985; Sivapragasam et al., 1988).
There has been significant recent interest in the use of insect pathogens such as Beauveria bassiana (Ma Jun et al., 1999; Yoon et al., 1999; Shelton et al., 1998), Metarhizium anisopliae (Amiri et al., 1999), Zoopthora radicans (Furlong and Pell, 1997), baculoviruses (Kadir et al., 1999; Kariuki and McIntosh, 1999) and entomopathogenic nematodes (Yang et al., 1999; Baur et al., 1998) as potential biological control agents of P. xylostella. One of the major limiting factors in the use of entomopathogens in the management of P. xylostella is the efficiency of their delivery systems and studies have been undertaken to try and improve this aspect to enable fuller exploitation of these natural enemies (Asokan, 1999; Ebert et al., 1999; Mason et al., 1999; Wright and Mason, 1997). Environmental factors are also important for the effectiveness of these pathogens (e.g. Z. radicans, Furlong and Pell, 1997a), as is their integration with other control measures such as sex pheromones (Furlong and Pell, 1997b). Scientists at IITA, Benin, have successfully developed and field tested the use of a biopesticide based on B. bassiana in cabbage farms in West Africa (IITA, 2009).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Throughout the world P. xylostella is considered the main insect pest of cruciferous vegetables (e.g. cabbages, broccoli and cauliflowers) and oilseed crops (e.g. canola and mustard) (Furlong et al., 2013). The estimated cost for controlling P. xylostella was US $1 billion annually in the early 1990s (Javier, 1992) but it is unclear how this was calculated. Between 1993 and 2009 the global area of cruciferous vegetable and oilseed crops increased by 39 and 59%, respectively. In 2009, an estimated 3.4 million hectares of cruciferous vegetables and over 31 million hectares of oilseed rape were cultivated worldwide (FAOSTAT, 2012). Such increased production of cruciferous crops has increased the pest status of P. xylostella, now costing the world economy an estimated US $4-5 billion annually (Zalucki et al., 2012).
Members of the plant family Brassicaceae occur in temperate and tropical climates throughout the world and P. xylostella occurs wherever crucifers are grown (Talekar and Shelton, 1993). The economic impact of P. xylostella is difficult to assess because it occurs in diverse small-scale and large-scale agricultural production areas, but a request (dated July 2000) sent to members of the Diamondback Moth Working Group was answered with some indications of its importance to specific regions.
The economic impact of P. xylostella can be evaluated by several methods. If one looks simply at the value of the crop, and states that a 'normal' population of P. xylostella would render each plant unmarketable, then one can calculate simply the value of the crop minus the cost of the applications and present that as the economic impact. However, there does not appear to be any reliable data on the total worldwide value of crucifers nor the losses incurred by P. xylostella. The lack of data on control costs is due to the large number of insecticides used against P. xylostella, their variable costs, the variable number of applications and their efficaciousness. This method would also be unreliable as some of these applications on crucifers may be targeted against other insect pests such as aphids or other Lepidoptera including the cabbage looper, Trichoplusia ni, or the imported cabbageworm, Pieris rapae. However, the data in this section give some indication of the importance of P. xylostella in various regions of the world.
China has the largest human population in the world and cruciferous vegetables make up an important part of the Chinese diet. The acreage of cabbages and cauliflower in 1999 in China was 1.2 million ha (FAO, 2000). P. xylostella is widespread in most provinces in China. There are five or six generations in Jilin Province in Northeastern China, and up to 20 generations in Guangdong Province in Southern China. It has been the most important insect pest of cruciferous vegetables, especially in Southern China and the Changjiang River Valley, in the past 20 years. If no sprays were applied for control of P. xylostella, the crop losses of the summer crop of cabbage in Jiangsu were 99% in 1992 and 80% in 1994, compared with the plots treated by insecticides (21-23 tons/ha.) (Zhao et al.,1996). The estimated control costs are ca US$100/ha for each crop for the peak periods (in April/May and September/October).
Kazuo Hirai (National Institute of Agrobiological Resources, Tsukuba, Japan, personal communication) notes that in Japan, P. xylostella is only one of the pests which growers have to treat for, the others being Mamestra brassicae, Pieris rapae and Plusia nigrisigna [Autographa nigrisigna]. Damage by these pests can be very serious, especially in the summer. When these crops are harvested before June or after November, a good yield is possible with less damage.
In the USA, the importance of P. xylostella is variable. In Texas, TX Liu (Texas Agricultural Experiment Station, Weslaco, USA, personal communication) has suggested that 100% of cabbage and at least 20% of broccoli in Texas would be unmarketable. This translates to $40 million to $70 million for Texas cabbage and about $400,000 for broccoli based on the latest data from the Texas Agricultural Statistical Service (TASS, 2000). A similar situation also occurs in Florida, USA, where P. xylostella is a main pest of crucifers. In the more northerly latitudes of the USA, the situation is very different. Cathy Eastman (Entomology Department, University of Illinois, USA, personal communication) notes that cruciferous crops (cole crops, cruciferous greens and cruciferous root crops such as horseradish) are grown on about 30,000 acres in the Midwest (US Dept. Commerce, 1998). In her experience in Illinois, she estimates that >80% of the acreage will need to be treated at least once for the P. xylostella and P. rapae complex each season. It is difficult to separate out the importance of each species because they occur simultaneously. Depending on the season, most growers may treat 2-3 times for this complex. This is the same situation noted by AM Shelton (Department of Entomology, Cornell University, New York, USA, personal communication) in New York, although during hot, dry years he notes that P. xylostella will be a much more difficult problem and suggests that if no treatments were applied the approximately $50 million cabbage crop would be unmarketable. California is a main USA producer of broccoli where it was grown on 49,815 ha and had a farm gate value of ca $450 million in 1997. A severe infestation by P. xylostella in 1997 resulted in crop losses estimated at >$6 million (Shelton et al., 2000).
Mexico is a major producer of broccoli and related crucifers used for processing and export to the USA. Most production is located in the El Bajio region where >30,000 ha of broccoli are produced with a total farm gate value of >$63 million. The most abundant lepidopteran pest of cruciferous plants in Mexico is P. xylostella. It greatly reduces the yield and quality of the crop and accounts for the majority of insecticide use in crucifer production (Diaz-Gomez et al., 2000). If no sprays were applied for control of P. xylostella, it is reasonable to conclude that all plants would be unmarketable.
In Australia, Greg Baker (Entomology Unit, SARDI, Adelaide, Australia, personal communication) notes that P. xylostella attacks the 136,000 hectares of major Brassica vegetable crops and is considered the chief insect pest. Crop loss due to P. xylostella damage in an average year is estimated to be ca $AS 8 million and control costs $12 million. Another important crop attacked by P. xylostella is rape and in Australia this is grown on ca 1 million ha. The crop loss in rape due to P. xylostella is estimated to be ca $AS 3 million and the control cost ca $AS 6 million.
In Germany, Martin Hommes (Institute for Plant Protection in Horticulture, Braunschweig, Germany, personal communication) notes that P. xylostella attacks cruciferous vegetables and field crops such as rape on a regular basis. These crops are grown throughout Germany with high concentrations of cabbage and rape in the northern parts of Germany. Cruciferous vegetables amount to one-third of the total field vegetable growing area in Germany. There are no comprehensive data on yield losses due to P. xylostella attack. Although P. xylostella is one of the three main lepidopterous pests in Germany and the larvae could be found in nearly every field, the damage in general will be low. In most years, the attack level by P. xylostella will be below an injury level and the pest will be controlled by spraying against the other two main lepidopterous pests, P. rapae and M. brassicae. In some years, particularly during hot, dry weather conditions, heavy attack and corresponding high yield losses can be observed. This situation is probably very similar in the Netherlands as well, where 8500 ha of cabbages and cauliflower are grown.
Other large producers of cabbages and cauliflower are India (530,000 ha), the Russian Federation (162,700 ha), South America (7000 ha) and the combined area of Indonesia, Thailand and Vietnam which have a total of 78,655 ha (FAO, 2000). Other cruciferous crops are also attacked by P. xylostella but the value of these crops is unknown. Losses by P. xylostella in all these areas, especially in South-East Asia, can be very severe as P. xylostella has developed resistance to many insecticides (Furlong et al., 2013).