Ceutorhynchus obstrictus (cabbage seed pod weevil)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Natural enemies
- Notes on Natural Enemies
- Plant Trade
- Impact Summary
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Ceutorhynchus obstrictus (Marsham, 1802)
Preferred Common Name
- cabbage seed pod weevil
Other Scientific Names
- Ceuthorrhynchus assimilis sensu auctt. non (Paykull, 1792)
International Common Names
- English: cabbage seed weevil; cabbage seedpod weevil; cabbage shoot weevil; turnip seed weevil
- Spanish: ceutorrinco de la colza; ceutorrinco de los nabos; gorgojo de las selicuas de la colza
- French: ceutorrhynque des siliques; charançon de la graine du chou; charançon de la graine du chou; charançon des siliques du colza
- Portuguese: gorgulho das síliquas da colza
Local Common Names
- Denmark: skulpesnudebille; skulpnudebille
- Estonia: kõdra-peitkärsakas
- Finland: rapsikärsäkäs; rapsikärsäkäs
- Germany: Kohlschotenrüssler; Rapsrüssler; Ruessler, Kohlschoten-; Ruessler, Raps-
- Italy: ceutorrinco delle rape; punteruolo del ravizzone; punteruolo della colza; punteruolo delle silique delle crucifere
- Netherlands: koolsnuitkevertje; koolzaadsnuitkever
- Norway: skulpesnutebille
- Sweden: blygra rapsvivel; blygrå rapsvivel
- CEUTAS (Ceutorhynchus assimilis)
Summary of InvasivenessTop of page
C. obstrictus is native to Europe, where it is a major pest of both swede rape (Brassica napus) and turnip rape (Brassica rapa). New generation adults can also cause damage by feeding on other brassica crops such as broccoli and cabbage before hibernating.
C. obstrictus was first reported in North America (British Columbia) in 1931 and from there spread to much of continental USA and eastern Canada. It was first reported in southern Alberta in 1995 and spread north and east at a rate of ca 60 km per year. By 1999 had become an important pest of oilseed rape crops in Canada and is predicted to spread to infest the whole of the rape-growing area of western Canada.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Uniramia
- Class: Insecta
- Order: Coleoptera
- Family: Curculionidae
- Genus: Ceutorhynchus
- Species: Ceutorhynchus obstrictus
Notes on Taxonomy and NomenclatureTop of page
The name Ceutorhynchus assimilis had been used consistently for this species since the nineteenth century even though it was a junior primary homonym (Pope, 1977) and therefore invalid according to the International Code of Zoological Nomenclature. However, Opinion 1529 of the International Commission for Zoological Nomenclature, published in 1989, conserved Paykull's use of the name Curculio assimilis. Unfortunately, Colonnelli (1993) later showed that Paykull's original species was not the current one, but the species now known as Ceutorhynchus pleurostigma, which it predates. Colonnelli (1993) used the name Ceutorhynchus obstrictus in place of C. assimilis. C. obstrictus is widely used in current literature and is the current preferred name. Other spellings of Ceutorhynchus are common in the older literature. Ceuthorhynchus is an amended name and Ceuthorrhynchus is a misspelling.
DescriptionTop of page
The eggs are creamy white, smooth, cylindrical with rounded ends, and about 0.6 mm long by 0.4 mm wide. They are often covered with a mucus-like material.
The larvae are legless, with a creamy white body and a yellow to brown head capsule, and grow to a length of 3-5 mm and a width of 1-2 mm. The body is normally slightly curved ventrally. The larvae are described by Heymons (1922) and Hoffmann (1951). The fully grown larva also has the following features: epicranial suture half the length of the head; mandible longer than wide and bidentate at the apex; maxillary palps two-segmented; labial palps two-segmented, the basal segment extremely short; abdominal segments each with four transverse folds dorsally. There are three larval instars.
Pupae are about 2 mm long and occur in earthen cells in the soil. They are initially white, but then turn yellow. The pupae are described by Heymons (1922) and Hoffmann (1951). The pupa is exarate with projecting legs, rostrum and elytra. There are nine tergites and 5 sternites visible ventrally. The elytra are smooth with five visible fine grooves.
Adults are matt grey, 2-3.5 mm long, with a distinctive, long, narrow, downward-curved rostrum (snout) on the front of the head. The rostrum is more than 5 times as long in front of the eyes as it is wide just in front of the eyes. The prothorax has a notch in the middle of the underside front edge where the rostrum can rest. There are 7 segments in the antennal funiculus. The elytra are black, but the elytral interstices have fine hairs and greyish white scales (about 60 µm long) all over, which results in an overall grey appearance. Near the mid-line of the elytra, the interstices have 1-3 irregular rows of scales along their length. There is no tooth on the hind femora and all tarsi are black to dark brown, similar in colour to the femora and tibiae. The tarsal claws are simple, not toothed.
DistributionTop of page
C. obstrictus is found across much of Europe, where it is considered native. It was first recorded in Canada in 1931 and has been established in British Columbia since then (Dolinski, 1979), but was only recorded in Alberta in 1996 (Butts and Byers, 1996). The first record for the USA was in Washington in 1936 and it is now found across much of the USA (Harmon and McCaffrey, 1997a; Brodeur et al., 2001).
In older literature, this pest was called Ceutorhynchus assimilis. The list of countries for cabbage seedpod weevil therefore includes records of both C. assimilis and C. obstrictus.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 25 Feb 2021
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Federal Republic of Yugoslavia||Present|
|United Kingdom||Present, Widespread|
|Canada||Present||Present based on regional distribution.|
|-Quebec||Present||Original citation: Brodeur et al. (2001)|
|United States||Present||Present based on regional distribution.|
|-Maryland||Present||Original citation: Anon (1977)|
History of Introduction and SpreadTop of page
C. obstrictus is native to Europe. It was first reported in North America in the lower mainland of British Columbia in 1931 (McLeod, 1962b). From there it spread south and east to much of continental USA (McCaffrey, 1992; Buntin et al., 1995) and eastern Canada (Brodeur et al., 2001). It was first reported in southern Alberta in 1995. It spread north and east at a rate of ca 60 km per year, and by 1999 had become an important pest of oilseed rape crops in Canada (Dosdall et al., 2001). Dosdall et al. (2002) predict that it will spread to infest the whole of the rape-growing area of western Canada.
Risk of IntroductionTop of page
Hosts/Species AffectedTop of page
C. obstrictus lays its eggs, usually singly, in the pods (siliquae) of a wide range of species in the family Brassicaceae. It damages crops in this family that are normally harvested for seed. This is mainly oilseed rape, but also mustards, although white mustard (Sinapis alba) appears to be resistant (Gratwick, 1992; Brown et al., 1996; McCaffrey et al., 1999). Other members of the family that are normally harvested before setting seed, such as cabbages, are only affected when grown as seed crops. In the wild, charlock (Sinapis arvensis) is a frequent host in the UK and is also used by adult females as a source of food for ovary maturation before moving to crops when they flower (Williams and Free, 1978). Kalischuk and Dosdall (2004) evaluated seven species of Brassicaceae for susceptibility to seed weevil; Brassica rapa was the most susceptible, Brassica napus, Brassica napus x S. alba, Brassica tournefortii and B. juncea were intermediate and S. alba, B. nigra and Crambe abyssinica were least susceptible.
Volatile glucosinolate catabolites, such as isothiocyanates, are important cues to the orientation of C. obstrictus (Bartlet et al., 1997, 1999; Klukowski et al., 1998). Oviposition has been found to be highest in populations of Brassica oleracea producing high amounts of 3-butenylglucosinolate (Moyes and Raybould, 2001).
Host Plants and Other Plants AffectedTop of page
|Brassica juncea var. juncea (Indian mustard)||Brassicaceae||Other|
|Brassica napus var. napus (rape)||Brassicaceae||Main|
|Brassica nigra (black mustard)||Brassicaceae||Other|
|Brassica oleracea var. botrytis (cauliflower)||Brassicaceae||Other|
|Brassica oleracea var. capitata (cabbage)||Brassicaceae||Other|
|Brassica oleracea var. gemmifera (Brussels sprouts)||Brassicaceae||Other|
|Brassica oleracea var. italica (broccoli)||Brassicaceae||Main|
|Brassica rapa subsp. oleifera (turnip rape)||Brassicaceae||Main|
|Brassica tournefortii (African mustard)||Brassicaceae||Other|
|Brassicaceae (cruciferous crops)||Brassicaceae||Other|
|Sinapis alba (white mustard)||Brassicaceae||Other|
|Sinapis arvensis (wild mustard)||Brassicaceae||Wild host|
Growth StagesTop of page
SymptomsTop of page
Pods (siliquae) show lightened flecks on the outside, where larvae have eaten seeds inside the pod. In mature pods, small pinhead-sized emergence holes are visible in the discoloured areas of the pod walls. Inside infested pods, 2-5 of the seeds are partially or entirely eaten and frass is visible nearby (Krüger, 1984).
Damage from other pests linked to cabbage seed weevil may also be used as symptoms when these other pests are present. Feeding and oviposition holes in the pod wall caused by C. obstrictus allow the brassica pod midge (Dasineura brassicae) to lay eggs through the pod wall, which leads to 'bladder pod' symptoms with swollen yellow pods and premature shedding of seed. The damage to the pod wall can also allow entry of disease, for example canker (Leptosphaeria maculans), leading to dark-edged spots on the pods (Newman, 1984).
List of Symptoms/SignsTop of page
|Fruit / discoloration|
|Fruit / internal feeding|
|Seeds / external feeding|
|Seeds / frass visible|
Biology and EcologyTop of page
The life cycle has been well studied in France (Bonnemaison, 1957), Poland (Dmoch, 1965) and the UK (Williams, 1978). There is one generation per year. Adults spend the winter in diapause, usually away from crops, in dry soil, leaf litter or underneath scrub. In April or May, when the air temperature exceeds 15°C (Bonnemaison, 1957; Kjaerpedersen, 1992), they fly to flowers and feed on plant tissue, which the females need to mature their ovaries (Ni et al., 1990). Before crop infestation, wild brassicaceous plants and weeds or volunteer oilseed rape can be important food sources. The adults migrate to oilseed rape or other brassicaceous seed crops as they start to flower, probably attracted by plant volatiles, particularly the isothiocyanates (Free and Williams, 1978b; Bartlet et al., 1993; Bartlet et al., 1997; Smart et al., 1997; Klukowski et al., 1998; Evans and Allen-Williams, 1998). Males respond to the odour of overwintered virgin females that have spent some time on flowering rape (Nazzi et al., 2001). Numbers on the crop peak during flowering and decline as the pods develop. Crop edges are usually more infested at the start of crop invasion, but this effect declines during the season (Free and Williams, 1979a) and later spatial distribution on crops can be complex and patchy (Murchie et al., 1999; Ferguson et al., 2000; Ferguson et al., 2003).
The life cycle is better synchronised with winter crops than spring crops. Adults feed for 3-4 weeks on buds, flowers, pods and stem tips before mating (Williams and Free, 1978), so there is a delay before laying eggs on winter rape, but they start laying at once on spring rape.
Females bore a hole through the pod wall with their rostrum and then use the ovipositor to insert a single egg into the pod, which is then marked with an oviposition deterrent pheromone, by brushing the abdomen along the pod (Kozlowski et al., 1983; Mudd et al., 1997; Ferguson et al., 1999a,c). Pods of medium length, about 20-40 mm long, are preferred for oviposition. Adults lay a total of 25-240 eggs during their life.
The larvae hatch after 6-10 days, but can take as long as 30 days at low temperatures. Larvae are found mainly during June and feed within the pods for 14-21 days, but can take as long as 40 days if the weather is cold. Most infested pods contain only one larva, although two or even three sometimes occur. Each larva consumes about five seeds each. There can be heavy larval mortality from parasitoid wasps (Williams, 2003). When fully grown, larvae bore an exit hole through the pod wall and drop to the soil. After burrowing down to a soil depth of 10-70 mm, they produce a cocoon and pupate. This usually occurs before a crop is swathed or harvested. The old adults die off during June. The new adults emerge after 15-19 days and feed on any remaining crop pods or on wild hosts during July and August before finding a site to overwinter.
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Metastenus pura||Parasite||Canada||seed crops|
Notes on Natural EnemiesTop of page
The parasitoids that attack C. obstrictus have been reviewed by Lerin (1987), Murchie (1996) and Williams (2003). The literature contains much synonymy and misidentification. In addition, records are often simply of parasitic wasps emerging from hosts and it is then uncertain whether the wasp is a parasitoid or a hyperparasitoid.
C. obstrictus is attacked by parasitoids at all stages of its life cycle, although mostly in its larval stage, by ectoparasitoid wasps of the Chalcidoidea and Braconidae. The most frequent species is usually Trichomalus perfectus, with Mesopolobus morys and Stenomalina muscarum also being common. All three are in the family Pteromalidae. Levels of parasitism by ectoparasitoid wasps are usually high, for example 31-73% in the UK (Murchie, 1996), 12-65% in France (Jourdheuil, 1960), 11-43% in Germany (Laborius, 1972) and 20-68% in Poland (Dmoch, 1975). Herrström (1964) recorded 100% parasitism at one site in Sweden.
Murchie (1996) found that host feeding by T. perfectus may also contribute to larval mortality. There is growing evidence that some post-flowering insecticide treatments for C. obstrictus also kill the parasitoids and so reduce natural biological control (Alford et al., 1996; Murchie et al., 1997).
T. perfectus has been found to be edge-distributed on rape crops only during the early phase of its immigration in May which occurs later than immigration of its host (Murchie et al., 1999). The spatio-temporal distributions of T. perfectus in relation to those of its host have been found to be complex and aggregated (Murchie et al., 1999; Ferguson et al., 1999b). Spatial distributions of C. obstrictus and T. perfectus larvae have been found to be strongly associated (Ferguson et al., 1999b). A density-dependent relationship of parasitism of C. obstrictus by T. perfectus has not been detected (Ulber and Vidal, 1998).
The commonest parasitoid of adults is Microctonus melanopus (Braconidae), but levels of parasitism are variable and its importance for natural biological control of C. obstrictus is not clear (Bonnemaison, 1957; Harmon and McCaffrey, 1997a; Fox et al., 2004).
Several parasitoid species were introduced to Canada in 1949 in an attempt at biological control (Hill, 1987).
Carabids have been shown to cause mortality of C. obstrictus larvae in the soil (Büchs and Nuss, 2000).
The entomopathogenic nematode Steinernema feltiae has been shown to be able to reproduce in the larvae of C. obstrictus (Nielsen and Philipsen, 2004).
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Flowers/Inflorescences/Cones/Calyx||arthropods/adults||Yes||Pest or symptoms usually visible to the naked eye|
|Fruits (inc. pods)||arthropods/larvae||Yes||Pest or symptoms usually visible to the naked eye|
|Growing medium accompanying plants||arthropods/pupae||Yes||Pest or symptoms usually visible to the naked eye|
|True seeds (inc. grain)||arthropods/larvae||Yes||Yes||Pest or symptoms usually invisible|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page
C. obstrictus is a widespread and common pest. However, its economic impact is often not as great as its abundance would suggest. It is usually considered a major pest of oilseed rape and other brassicaceous seed crops across Europe and much of the USA and Canada (Gratwick, 1992; Harmon and McCaffrey, 1997a; Dosdall and McFarlane, 2004). It is the most economically important pest infesting oilseed rape during flowering (Walters and Lane, 1994b).
The economic impact is complicated by three major factors. The first is the considerable ability of oilseed rape, the main crop host, to compensate for damage; as many as 60% of the pods can be removed without loss of yield (Williams and Free, 1979), although late compensation may give immature pods at harvest. Ahman (1993) also reported that symptoms related to attack by C. obstrictus were not correlated with seed yield. Tatchell (1983) found that artificial injury to pods of infestation by C. obstrictus did not result in a significant reduction in seed yield. These situations were compensated for partly by a slight increase in the number of axillary racemes and by slight increases in the number of pods on each axillary raceme and in the 1000-seed weight. These resulted mainly from the diversion of nutrients to other yield-bearing organs.
The second factor is that damage by the brassica pod midge (Dasineura brassicae) is dependent on prior damage by C. obstrictus. The midge lays its eggs through holes in the pod wall produced by the feeding, oviposition or larval emergence of the weevil and there is very little midge damage in the absence of the weevil (Sylvén and Svenson, 1975; Stechmann and Schutte, 1978; Skrocki, 1979). In fields of summer turnip rape in Sweden, Sylvén and Svenson (1975) used field and laboratory counts to show that there was a sharp increase in attack by D. brassicae with an increase in density of C. obstrictus. The average loss in yield varied from 25 to 70% depending on the number of weevils introduced.
The third and final factor is that pod damage by C. obstrictus increases the likelihood of canker caused by Leptosphaeria maculans (Newman, 1984). In canker-resistant and -susceptible cultivars sown in field trials in England, half were treated with insecticides and half were untreated. Considerably less insect damage (including that caused by Ceutorhynchus) was found in treated than untreated plots. The pattern of canker infection reflected that of insect damage; nearly all upper stem cankers were associated with insect damage (Newman, 1984).
Damage is more likely on winter crops, particularly late-flowering ones, than on spring crops, because of the time of emergence of the adults (Alford and Gould, 1976; Free and Williams, 1978a). The proportion of pods infested with C. obstrictus has been recorded as twice that in spring rape plants. In addition, in lightly infested fields, weevils tended to be concentrated at the edges, but the opposite occurred as infestation levels increased. Infestation of winter rape pods by C. obstrictus also increased with the number of years rape had been grown on the farm (Free and Williams, 1978a).
Direct damage by C. obstrictus on seed crops is caused only by consumption of seeds. Larval infestation reduces the yield of infested pods by about 18%, but this loss is reduced by compensation (Williams and Free, 1979). In winter rape in the UK, an infestation level of one adult per plant gives a 4% loss of crop yield, which is about half the cost of applying an insecticide. The spraying threshold in the UK is therefore two adults per plant, unless there is a high infestation of D. brassicae or the insecticide is tank-mixed with a fungicide, in which case the threshold is one adult per plant (Lane and Walters, 1994). The threshold for spring rape in the UK is also two adults per plant. Alford et al. (1996) consider that insecticide treatment for C. obstrictus is rarely justified in England and Wales. Cage experiments in France have shown that yield loss is not significant when fewer than 25% of pods are infested (Lerin, 1984), and infestations are not usually more than this (Williams and Free, 1979). They also showed that adults did not affect the numbers of pods or seeds per pod, while larvae damaged about 5.5 seeds per pod. The results established a relationship between percentage loss in infested plants relative to uninfested plants on the basis of yield means adjusted for a constant number of pods (Lerin, 1984). Coutin et al. (1974) report that both feeding by adult insects on buds and young pods and by larvae on seeds cause damage. In a trial, control of C. obstrictus increased yields from 5.07 to 5.39 g seed per plant.
In Finland, an infestation rate of 0.25 weevils per plant caused a yield loss of about 5-10% (80-160 kg/ha), which is about the cost of insecticide treatment, but since densities are rarely so high in the field in Finland, treatment for the weevil alone is rarely necessary (Tulisalo et al., 1976). When initial populations were 0.5-10 adult weevils per plant, the loss averaged 19-80%. The oil content and 1000-seed weight also decreased in heavy infestations. Very high populations induced slight compensatory growth. In Poland, larvae of C. obstrictus reduced seed yield by 21.2%, but only by 13.3% when larvae were parasitized by Hymenoptera (Palosz, 1980). It was also calculated that C. obstrictus reduced seed yield by 207 kg/ha and 195 kg/ha in 1965 and 1966, respectively.
The potential for yield loss in northern Idaho, USA, is 15-35% in untreated winter rape and 3-6 adults per sweep can cause economic losses (McCaffrey et al., 1986). In Georgia, one, two and three larvae per pod reduced seed weight per pod by 20.2, 38.1 and 52.2%, respectively. Larval injury did not consistently affect kernel weight or grain oil content. Yield loss increased linearly by ca 1.7% for each 1% increase in percentage of infested pods when larval infestation of pods exceeded 23% infested pods (Buntin, 1999). Pod infestations have reached 60-70% in trials in northern Georgia, USA, suggesting the weevil will be an important pest in this area (Buntin and Raymer, 1994).
Occasionally adults migrating from oilseed rape can cause feeding damage to cauliflower and calabrese (Wheatley and Finch, 1984).
Detection and InspectionTop of page
Sampling, trapping and rearing techniques for oilseed rape pests, including C. obstrictus, have been reviewed by Williams et al. (2003). Adults are found on flowers and developing pods and can be sampled by beating the top of flowering plants over a tray, which should be reasonably large (for example, 30 cm by 25 cm) and preferably white to contrast well with the dark adults. C. obstrictus remains still for a while after landing on the tray, whereas pollen beetles (Meligethes spp.) run or fly off. For casual inspection, a hand can be used instead of a tray. Alternatively, plants can be shaken over a large funnel with a collecting bottle beneath. Sweeping can also be used; a standard sweep is a 180 degree swing of a 38 cm diameter net through the upper canopy in warm weather. When the adult is common, many can be observed on buds, in flowers or on young pods without the need to beat or sweep the plants.
Crops of oilseed rape can be inspected during flowering by beating the tops of the main racemes over a white tray for 20 plants spread out over a transect across the field, not just at the crop edge (Walters and Lane, 1994a). The highest count from three such transects should be used. The population size can then be calculated in terms of adults per plant. Inspections should be done on warm (at least 15°C), dry days with little wind. On cool days, the adults move down the plants and fewer are caught. Inspections when the temperature is below 11°C are inaccurate, but at 11-14°C, multiplying the count by 2 gives a reasonable estimate for comparison with economic thresholds (Walters and Lane, 1994b).
Adults can also be trapped in yellow water traps, particularly if baited with isothiocyanates or other components of the odour of Brassica plants, and it may be possible to use this to monitor migration into a crop in spring and colonization of a crop in summer (Smart and Blight, 1997; Smart et al., 1997). To detect larvae, it is necessary to collect developing or mature pods and cut them open. Larvae are found in between the seeds. Damage can also be assessed by collecting mature pods and inspecting them for the exit holes made by emerging adults.
Similarities to Other Species/ConditionsTop of page
Larvae are similar to several other beetle larvae. The size and absence of legs separates them from most other beetle larvae. Other mining larvae of Ceutorhynchus species, such as the cabbage stem weevil (C. pallidactylus) are virtually identical and are best distinguished by where they are found; no similar larvae are found in pods. Kirk (1992) provides a simple key to insect larvae found on oilseed rape and cabbages in the UK.
Adults are very similar to several other Ceutorhynchus spp. that are common on the host plants. They can be distinguished from cabbage stem weevil (C. pallidactylus) and rape winter stem weevil (C. picitarsis) by their black to dark-brown, rather than reddish-yellow, tarsi. They can be distinguished from C. rapae by the absence of a tooth on the hind femora. An advanced key to adult Curculionidae in Europe is provided by Freude et al. (1983). A simple key is available for adult beetles found on oilseed rape and cabbages in the UK (Kirk, 1992).
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
There is some scope for reducing damage by selecting flowering time. Damage is more likely on winter crops, particularly late-flowering ones, than on spring crops, because of the time of emergence of the adults (Alford and Gould, 1976; Free and Williams, 1978a; Free and Williams 1979b).
Breeding for resistance to C. obstrictus has not been the main priority for plant breeders. Pest incidence on commercial lines has not been found to differ consistently or substantially (Williams, 1989) but some lines of Brassica spp. show resistance (Harmon and McCaffrey, 1997b). White mustard (Sinapis alba) appears to be immune (Gratwick, 1992; Brown et al., 1996). Bartlet et al. (1999) suggested two ways for improving the glucosinolate content of oilseed rape for pest resistance. The first involves rape lines with low constitutive but high induced glucosinolate levels. The second involves rape lines with a high proportion of glucosinolate types that do not catabolize to isothiocyanates, particularly the higher alkenyl isothiocyanates. Transgenic oilseed rape lines expressing the serine proteinase inhibitor CII or the cysteine proteinase inhibitor OC-I have been tested on C. obstrictus larvae with no deleterious effects although various reactions were observed on gut enzymes (Bottino et al., 1998; Girard et al., 1998).
There is growing interest in enhancing conservation biological control of the pests in oilseed rape crops, including C. obstrictus (Williams, 2004; Williams et al., 2005). C. obstrictus suffers heavy mortality from ectoparasitoid wasps, particularly the pteromalid Trichomalus perfectus. The importance of this natural biological control should be considered when applying pesticides. There is growing evidence that some post-flowering insecticide treatments for C. obstrictus have killed the parasitoids and so reduced considerable natural biological control (Alford et al., 1996; Murchie et al., 1997). The spatio-temporal distributions of T. perfectus in relation to those of its host have been described (Murchie et al., 1999); distributions of larval C. obstrictus have been found to be strongly associated with those of T. perfectus (Ferguson et al., 1999b). Several parasitoid species were introduced to Canada in 1949 in an attempt at biological control. Although parasitoids were imported from England, UK, it was found that T. perfectus was already present and had, presumably, been accidentally introduced with its host (McLeod, 1962a; Hill, 1987). Carabids have been shown to cause mortality of C. obstrictus larvae in the soil (Büchs and Nuss, 2000).
Control is mainly by insecticide. Efficacy evaluation of insecticides against C. obstrictus is published by Buntin (1999) and OEPP (2003). It is not usually necessary to spray for the weevil alone, but an insecticide is often applied against cabbage seed weevil and brassica pod midge together. Some farmers apply tank-mix insecticide as a prophylactic when applying fungicide (Walters and Lane, 1994b). The applications are often unnecessary as treatment thresholds are rarely exceeded.
Insecticide treatment has changed over recent decades. Until recently, the organophosphate triazophos was used widely and was applied post-flowering. However, it is now known to have been particularly detrimental to parasitoid wasps, such as Trichomalus perfectus, which are important natural biological control agents (Alford et al., 1996; Murchie et al., 1997). It was also harmful to honeybees and caused many colony losses (Stevenson et al., 1980; Williams, 1980). Resistance to organophosphates has been found (Lakocy and Grabarkiewicz, 1974), and it has now been banned from use.
Pyrethroids, such as alphacypermethrin, are cheaper and are now used instead. They are applied during flowering, so they are not used when the parasitoids are active, and are safe for bees (Debray and Tipton, 1984).
Crop edges are usually more infested at the start of crop invasion, but this effect declines during the season (Free and Williams, 1979a; Ferguson et al., 2000), so any benefit of border treatment declines as infestation increases.
The oviposition deterring pheromone (ODP), which is produced by adult females and brushed onto pods, deters oviposition and so has potential for use in pheromonal control (Ferguson and Williams, 1991; Mudd et al., 1997; Ferguson et al., 1999a,c).
Early Warning Systems
Computer-based farmer advisory services or decision support systems (DSS) for the management of pests of oilseed rape, including C. obstrictus, are being developed in the UK and in Germany. In the UK, this will be part of the UK decision support system for arable crops (DESSAC). It aims to prevent excessive use of pesticides and to improve spray timing by forecasting pest populations and calculating the economics of control (Mann et al., 1986). The German system ProPlant is already on the market (Johnen and Meier, 2000) and phenological models for parasitoids are being developed for integration into this system (Williams et al., 2005). Systems for forecasting damage have also been developed in France (Lechapt, 1980) and Germany (Riedel, 1989).
Field Monitoring and Economic Thresholds
Adults are sampled by knocking and shaking the top of flowering plants over a tray (see: Detection and Inspection Methods; Economic Impact).
There is growing interest currently in developing IPM strategies for the oilseed rape crop that minimise insecticide use and optimise biological control by enhancing the activity of naturally occurring parasitoids and predators (Williams, 2004; Williams et al., 2005). There are considerable advantages in employing IPM because of the complicated interaction between pests such as cabbage seed pod weevil and brassica pod midge and the need to ensure that insecticides do not reduce parasitoid populations (Regnault et al., 1980; Evans and Scarisbrick, 1994). However, it is also difficult because of the variety of pests and diseases attacking the crop. Monitoring of cabbage seed pod weevil with semiochemical-baited traps may allow insecticides to be targeted more efficiently (Smart and Blight, 1997; Smart et al., 1997). Successful integrated pest management will need multidisciplinary approaches and the use of expert systems (Evans and Scarisbrick, 1994; Morgan et al., 1998; Johnen and Meier, 2000). Spatial within-field distributions of C. obstrictus have been found to be complex and patchy with potential for spatial targeting of insecticides (Ferguson et al., 2000, 2003). Turnip rape and early flowering oilseed rape have been found to have potential to protect oilseed rape from infestations of C. obstrictus (Buntin, 1998; Nerad and Vasak, 2000; Cook et al., 2004).
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Tulisalo U, Korpela S, Pohto A, 1976. The yield loss caused by the seedpod weevil Ceuthorrhynchus assimilis Payk. (Col., Curculionidae) on summer turnip rape in cage experiments. Annales Entomologici Fennici. 42 (2), 98-102.
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