Cabbage Seedpod Weevil

Table of contents

    Important Tips for Best Management

    • Adult cabbage seedpod weevil (CSPW) populations from the preceding summer are used to generate predictive CSPW population maps to forecast the risk of infestation for the following summer.
    • Early flowering increases the risk of infestation, as adult CSPW are attracted to the first yellow canola flowers available, which indicate a suitable food source and egg laying location.
    • Control alternative host plants in all crops prior to the bud stage of your canola crops, including brassicaceous weeds and volunteer canola (especially if flowering early).
    • Although adults feed on flower buds, most of the economic crop damage occurs when larvae feed within pods and destroy developing seeds.
    • Scout for CSPW as the crop begins to flower. If the CSPW numbers are near threshold at this point, continue to scout through the early flowering period of the field and nearby fields.
    • Select four locations within each field: two paired locations, with each pair of locations in a distinctly different portion of the field and each location (within a pair) separated by at least 50 m. At each location count the number of weevils from ten 180° sweeps with a sweep net.
    • For most accurate results, sweep between 11 am and 5 pm (when temperatures are above 20°C), when the insects will be most active.
    • The action threshold for applying insecticides is about 20 weevils in 10 sweeps during canola flowering, varying slightly with canola price and cost of control.
    • Insecticide application targets adults when crops are in 10 to 20% flower to avoid eggs being laid in newly formed pods. This is the stage when 70% of plants in the field have at least 3 to 10 open flowers.
    • Spray in the morning or evening when temperatures are between 15°C to 24°C for best results and to protect pollinators. Avoid spraying under a strong temperature inversion or when temps exceed 25°C.
    • Total parasitism by natural enemies in western Canada has usually been less than 15% but is still important for limiting weevil populations and the frequency of outbreaks.
    • Consider trap cropping on large fields to reduce insecticide costs and minimize exposure of natural predators.


    About the cabbage seedpod weevil

    Background

    The cabbage seedpod weevil, Ceutorhynchus obstrictus (Marsham) (Coleoptera: Curculionidae), is native to Europe and was first detected on prairie cropland in canola crops near Lethbridge, Alberta in 1995[4], it is now a major pest of Canadian canola (Brassica napus L. and Brassica rapa L.).

    The first North American observation of the weevil was in southwestern British Columbia [1] and has since spread to encompass most of continental U.S.A. [2], the interior of B.C. [3], as well as the prairies of western Canada.

    On the prairies, the insect was initially found in southern Alberta, and for several years remained primarily south of the TransCanada highway in Alberta, but now continues to expand north and east into the Saskatchewan river valleys and the western edge of Manitoba. By 2016, there were potential economic infestations identified as far north as Drumheller, Alberta and Kindersley, Saskatchewan.

    See cabbage seedpod weevil distribution maps from Saskatchewan here and from Alberta here.

    At least one other introduction of the cabbage seedpod weevil appears to have occurred in Québec [5], and the Ontario Ministry of Agriculture, Food and Rural Affairs also lists it as a pest, primarily of winter and very early spring seeded canola.

    The cabbage seedpod weevil completes its larval development only in certain species of Brassicaceae, although adults may feed on nectar and pollen of a wide range of brassicaceous species [6],[7],[8].

    Host plants include:

    • canola, 
    • brown mustard, 
    • cole crops (such as cabbage, broccoli and cauliflower) and 
    • cruciferous weeds (such as wild mustard, flixweed and stinkweed). 


    Host plants are either true hosts or food hosts. Both hosts can provide food, especially pollen, for adult feeding, but only those with large seedpods that can sustain larval development are true hosts. Examples of true hosts are canola, brown mustard and wild mustard while examples of food hosts are flixweed, stinkweed and hoary cress.

    Life Cycle

    Cabbage Seedpod Weevil Life Cycle

    Adult cabbage seedpod weevils overwinter in the soil beneath leaf litter in tree shelterbelts, roadside ditches and woodlots. Late in the season (September to early November) they select overwintering sites and burrow beneath the soil surface where they are protected from low temperatures.

    ACTION: Scout fields in the fall to assess the size of the adult population. Adult populations the preceding fall can be used to predict the risk of infestation the following year.

    Adults emerge from their overwintering sites in spring with peak emergence occurring when soil temperatures reach 15°C [9]. Following their emergence, adults fly to patches of early-flowering brassicaceous weeds, and are especially attracted to stands of wild mustard, Sinapis arvensis L., and volunteer canola [8].

    Before canola crops enter the bud stage, adults can be found on wild mustard, flixweed, hoary cress, stinkweed and volunteer canola. When disturbed, the adults often drop to the ground and play dead. After several seconds they resume activity.

    ACTION: Control brassicaceous weeds including volunteer canola.

    Adults invade canola crops in June, in the bud to early flowering stages and feed on canola pollen, nectar, buds, and racemes. Significant clustering of adults along field edges in early stages of invasion is followed by more uniform distributions as canola development proceeds to flowering and pod enlargement [12].

    Mating occurs on canola plants and ovariole development requires that females feed on raceme tissue of the crop or volunteers [13]. Most eggs are laid soon after flowering, when pods are still developing. During egg laying a female makes an opening in the pod wall with her mouthparts, and then turns to deposit an egg into the opening [14]. The female then brushes her abdomen over the area, releasing a pheromone that deters other females from laying eggs in the same pod [15].

    ACTION: Insecticide application targets adults when crops are in 10 to 20% flowerto avoid egg laying in newly formed pods. Applying insecticides prior to full bloom ensures that fewer pollinators will be around the plants when they are sprayed. The presence of beneficial insects and bio control agents for other insect pests should encourage producers to use the higher range of the economic threshold (ex. use close to 40 weevils rather than 25 weevils in 10 sweeps during canola flowering, when beneficial insects and bio control agents are present). Eggs are very small, oval and opaque white. Usually only a single egg is deposited per pod. However, two or more eggs can be laid per pod during outbreaks. Eggs hatch in about six or seven days. Females continue to lay eggs until they die later in the season.

    Larvae develop rapidly in spring canola, progressing through three larval instars in approximately 50 days in southern Alberta [22],[8]. During this time, each larva consumes five to six canola seeds [23]. When mature, the larva chews an opening in the pod wall, drops to the soil, burrows in, and pupates in an earthen cell. Adults emerge about 14 days later [8].

    There is one generation per year.

    Influence of Environment

    Overwintering survival significantly declines as temperature moves from +5°C to -5°C and the supercooling point where the insect can no longer stop from freezing is -7°C. Longer overwintering periods have much less effect on mortality [10].

    Peak emergence of the adults in the spring is when soil temperatures reach 15°C [9].

    Flight dispersal by weevil adults is dependent on temperature and relative humidity [11]. The mean temperature required for a flight height of 1 m is 12°C, and flight height increases with temperature. On the other hand, increases in relative humidity are associated with reduced flight heights and dispersal distances [11].

    Cold, wet springs = lower cabbage seedpod weevil infestations.

    The distribution and abundance of the cabbage seedpod weevil have been monitored yearly in western Canada since 1997. Predictive models based largely on climate data indicate that this pest will eventually disperse to all regions of canola production in western Canada, including the Peace River region.

    To help monitor the spread of cabbage seedpod weevil, distribution maps are available on the websites of provincial departments of agriculture, such as the Prairie Pest Monitoring website.


    Identify cabbage seedpod weevil

    Cabbage Seedpod Weevil

    Photo by Phil Thomas

    Adult weevils are ash-grey and approximately 3 to 4 mm (0.1 to 0.2") long. They have a prominent curved snout that is typical of most weevils.

    Scouting techniques

    Adult populations the preceding fall can be used to predict the risk of infestation. High numbers of weevil adults in the fall will likely mean significant infestation levels in the following spring, although a severely cold winter with little snow cover could reduce the survival of overwintering adults. 

    Research has determined that when more than 25% of pods are damaged by cabbage seedpod weevil larvae (when scouting at the appropriate time), noticeable yield reduction has occurred and insecticidal control was warranted [28],[29]. If less than one in four pods show exit holes (at plant maturity) cabbage seedpod weevil numbers likely remained below thresholds or insecticide control was successful in reducing numbers to below damaging levels.

    When: In the spring when the crop first enters the bud stage and continue through theflowering period. Sweep between 11 am and 5 pm (when temps are above 20°C) and when winds are calm. The insects will be most active and sweep counts will bemore accurate.

    Where: Sample both the perimeter and interior of the field to obtain an accurateestimate of weevil numbers throughout the field.

    How: Sweep at four locations within each field, where selected as locations within each pair located 50 m or more apart and each set of paired locations should be in distinctly separate parts of the field. At each location count the number of weevils from ten 180° sweeps with a sweep net. Previously 10 locations within each field were recommended; however recent data (Cárcamo et al. 2019) suggests four locations as described provide a similar assessment of insect levels in more than 90% of fields recently studied.

    Economic Threshold: 25 to 40 weevils in 10 sweeps (allows for a range of pricing for canola and insecticide applications).

    A few other weevil species may also be found in canola but do not require control measures. The most common of these is a closely related species, Ceutorhynchus neglectus, about one-half the size of the cabbage seedpod weevil that will feed on canola but prefers flixweed.

    (Complete Guide to Crop Protection - 2019)

     

    Type of Crop Damage

    Canola in the pre-bolting to early flowering stage can compensate for adult weevil feeding on buds and stems. The majority of crop damage occurs when larvae feed within pods and destroy developing seeds. Each larva consumes about five seeds during its development. Although this amount represents only 15 to 20% of the total yield of a particular pod, these pods are also predisposed to premature shattering. 

    Canola pods harbouring cabbage seedpod weevil larvae may appear distorted. When larvae consume some seeds within pods, the undamaged seeds enlarge and mature, often leaving misshapen pods.

    Larvae emerge from pods via exit holes. Pods with exit holes shatter before harvest more frequently than non-infested pods, and exit holes may serve as entry points for fungal spores that can germinate to further reduce yield [25].

    The exit hole of a CSPW larva

    Over-wintered adults feeding on developing canola buds can also cause bud-blasting and reduced yield potential in dry years when the ability of plants to compensate is limited.

    In addition, when the new generation of adults emerges in late summer, weevils can feed through the pod walls in late-maturing fields to consume canola seeds, reducing yield and quality [24],[25].

    Dual infestations: In southern Alberta and Saskatchewan, the cabbage seedpod weevil often co-occurs with lygus bugs (Lygus spp.) (Hemiptera: Miridae), which are intermittent pests of canola.

    A 4-year research study published in 2019 assessed the impact of cabbage seedpod weevil control on lygus bugs. Data collected found that spraying for weevils at the early flower stage (earlier than the optimum stage for lygus), could reduce lygus abundance. Fields flowering early enough to require control for cabbage seedpod weevil however, were not fields that later experienced high lygus infestation. Growers should continue to scout and manage cabbage seedpod weevil and lygus bug separately (Cárcamo et al. 2019).

    Minimize infestations

    Natural enemies and biological control

    Extensive sampling of canola fields in southern Alberta and Saskatchewan from 2001 to 2006 revealed a total of 15 parasitoid species attacking the weevil, representing five families of Hymenoptera [54],[53],[12],[55].

    The dominant natural enemy fauna of cabbage seedpod weevil in the prairies is comprised of 14 species of larval ectoparasitoids with more than 90% of all parasitism inflicted by the pteromalids (parasitic wasps) Trichomalus lucidus (Walker), Chlorocytus sp., and Pteromalus sp., and the eulophid Necremnus tidius (Walker) 53,12,55.

    Parasitism of cabbage seedpod weevil larvae increased from 0.1% in 2002 to 5.0% in 2004 [12], and increased further from 2004 to 2005, but total parasitism was usually less than 15% [56],[55]. These parasitism levels are considerably lower than those common in Europe, where rates can reach 50 to 90% [57],[58],[59] and reflect that the parasitoid fauna did not coevolve with the weevil, but instead is comprised of native species that expanded their host ranges to exploit abundant cabbage seedpod weevil larvae in canola fields [12],[55].

    Small numbers of adult weevils were parasitized by Microctonus melanopus (Ruthe), a braconid wasp found in southern Alberta [54].

    Researchers investigated a classical biological control program for cabbage seedpod weevil control. In B.C., three European parasitoids [60],[61] were released in 1949 but recent surveys found only one species established, Stenomalina gracilis (Walker) [60],[61], and unfortunately this species is non-specific in the range of weevil hosts it attacks [62].

    Researchers must also consider compatibility between weed and insect pest biocontrol [63]. A number of weevil species in the subfamily Ceutorhynchinae have been released in North America for the biological control of various weed species, or are under evaluation for release. Parasitoids selected for release to control cabbage seedpod weevil should not also attack these beneficial weevils. Further work on biological control of cabbage seedpod weevil is currently underway through the Canola AgriScience Cluster.

     

    Field Management (cultural control)

    Trap Cropping: At present, trap cropping is the most promising cultural strategy for controlling the cabbage seedpod weevil. This approach takes advantage of the weevil concentration that often occurs at field edges when weevils first invade a canola field.

    Trap crops were investigated for control of cabbage seedpod weevil in southern Alberta from 2001 to 2004 using large commercial fields with a main crop of B. napus and the perimeter planted at the same time to a B. rapa cultivar [37]. B. rapa flowered about one week earlier than the main crop of B. napus and effectively concentrated weevils where they could be controlled with a pyrethroid insecticide to prevent their movement into the main crop.

    Strategy: Concentrate weevils in perimeter of field.

    How: Plant perimeter with B. rapa and the main crop with B. napus. Alternatively, astrip of the same variety planted 7 to 10 days before the rest of the field can also serveas a trap for adult weevils.

    Limitation: Worked well in large square fields that were 1.6 by 1.6 km even whenweevil populations were high. However, this approach was not as effective when fieldswere smaller and narrow, especially when weevil densities far exceeded the economicthreshold.

    The advantages of trap cropping can include substantial savings in costs of insecticide and fuel from reduced spraying applications, and the method has the potential to conserve populations of natural enemies and other beneficial insects in the main crop area, which may comprise up to 90% of the field. A requirement for successful implementation of a trap crop strategy is to carefully monitor the trap crop so that insecticide can be applied before beetles disperse in large numbers to the main crop

    Seeding date: Seeding later in spring, at recommended seeding rates, can help reduce weevil infestations and damage [38]. Early seeding of canola (in late April in the southern prairies) can put canola crops at risk of higher infestation levels of the weevil than for crops seeded later (in early May) [38] because early-seeded crops can be especially attractive to invading weevils. Cabbage seedpod weevils are more likely to infest the earlier flowering crops. First flowering canola crops in an area should be monitored most carefully - it is characteristic of cabbage seedpod weevil to identify the yellow flowers as marking for a food source and egg laying location and to migrate there (and once in flowering canola they may have no need to move, other than to disperse more evenly within a field over time).

    Early seeding = higher risk of cabbage seedpod weevil infestation

    Delaying seeding, though, must be balanced with the agronomic advantages of early seeding, such as higher levels of residual soil moisture from snow melt, and earlier maturity that avoids summer heat waves and early spring frost.

    Soil fertility: Weevils tend to cluster in fields where sulfur content is high in plant tissue because females prefer to feed and lay eggs on plants with high levels of sulfur and low levels of nitrogen [16]. Larval development time increases with increasing nitrogen levels [17].

    Use balanced nitrogen/sulphur fertility.

    Sulfur fertilization can result in increases in sulfur-containing glucosinolate compounds in plant tissue [18],[19], and the breakdown products of these compounds are attractive to adult weevils [20],[21].

    Resistance breeding

    Not all species of Brassicaceae are equally susceptible to infestation by the cabbage seedpod weevil. For instance, B. rapa is more susceptible than B. napus and susceptibility of Brassica juncea (L.) Czern. (brown mustard) is approximately equivalent to B. napus [39],[40]. Sinapis alba is resistant to the weevil [41],[40], and this resistance has provided the foundation for research to develop weevil-resistant canola. 

    Susceptibility: B. rapa > B. napus = brown mustard. Yellow mustard is resistant.

    Researchers have developed resistant germplasm based on Sinapis alba L. (yellow mustard) crosses with B. napus [42]. The resistant germplasm expressed both antibiotic and antixenotic resistance to the weevil: 

    • Weevils preferred not to feed or oviposit on the resistant lines, and when they did, larval development times were longer and larval biomass was less [42],[43]
    • Overwintered females that fed upon the resistant germplasm had reduced egg development [43].


    The rights to the resistant germplasm have been sold to a seed company who is incorporating the resistance into new Canadian canola hybrids under development. European breeders are also interested in the germplasm, as some areas, especially Germany, have weevil populations resistant to a number of pyrethroid insecticides.

    Reduce economic losses

    Economic Threshold

    The economic threshold for applying insecticides is currently 25 to 40 weevils in 10 sweeps (Cárcamo et al. 2019) during canola flowering, to allow for significant changes in canola price or insecticide application costs. [25],[31].

    Economic Threshold: 25 to 40 weevils in 10 sweeps. (Complete Guide to Crop Protection - 2019)

    Chemical control options

    Do not spray too soon, as the weevil will typically continue to invade for at least a week to 10 days beyond the opening of the first flowers. 

    Timing: When crops are in 10 to 20% flower to avoid egg laying in newly formedpods.This is the stage when 70% of plants in the field have at least 3 to 10 open flowers, and the first pods are formed at 25 mm (3/4”) allowing egg laying to begin.

    Spraying at this stage will allow the weevils more time to move into the field, but at the same time it is still early enough to keep them from laying a significant number of eggs in newly forming pods. Spraying after this stage when populations are at or above threshold may result in yield loss and will also impact beneficial insects, including pollinators, which have moved into the field.

    Keep in mind that canola can compensate for weevil feeding on buds and stems. There is good moisture in many areas typically infested by cabbage seedpod weevil, and the crops are growing vigorously with the heat. This will allow the plants to compensate for bud feeding that may take place prior to flowering.

    Spray in the morning or evening at temperatures of 15 to 24°C for best results and to protect pollinators. Do not spray under a strong temperature inversion or when temperatures exceed 25°C.

    Insecticide products for control of cabbage seedpod weevil. Check your provincial crop protection guide for up to date listings of registered control products.

    Product Chemical Group
    Name
    Chemical Group
    Number
    Active
    Ingredient
    Preharvest
    Interval
    Application2 LD50
    (mammalian toxicity1)
    Matador; Silencer Pyrethroids 3A Lamda-cyhalothrin 7 days Aerial or ground 64 - 110
    Decis 5 EC / Poleci 2.5 EC Pyrethroids 3A Deltamethrin 7 days Aerial or ground 395
    Matador 120 EC / Silencer 120 EC Pyrethroids 3 Lamda-cyhalothrin 7 days Aerial or ground 278
    Voliam Xpress Diamide and Pyrethroids 3, 28 Chlorantraniliprole & Lambda-cyhalothrin 7 days Aerial or ground 98.11

    1 LD50 values represent the relative toxicity of a pesticide. They represent the dose (in mg/kg body weight) that will kill 50% of the test animals. Thus the lower the number the greater the toxicity. Values given are for oral LD50.

    2Provincial crop protection guides restricts growers from more than one application for cabbage seedpod weevil per season by either ground or aerial application.

    Source: Alberta Crop Protection Guide

    Additional data sources: Saskatchewan Guide to Crop Protection and Manitoba Guide to Crop Protection

     

    References

    Cárcamo, H., Meers, S., & Herle, C. (2019). Managing cabbage seedpod weevils (Coleoptera: Curculionidae) in canola (Brassicaceae) – are Lygus (Hemiptera: Miridae) affected? The Canadian Entomologist, 151(1): 85-93. Available online.

    Date modified: July 8, 2019