The cabbage seedpod weevil (Ceutorhynchus obstrictus) (Figure 1.) was introduced to North America from Europe about 70 years ago.
Host plants of the cabbage seedpod weevil all belong to the mustard family (Brassicaceae), and include canola, brown mustard, cole crops (e.g. cabbage, broccoli, cauliflower) and cruciferous weeds (e.g. wild mustard, flixweed, 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
- Brown mustard
- Wild mustard
Examples of food hosts
- Hoary cress
Crop damage from cabbage seedpod weevil can occur in several ways. Adults feed on the developing flower buds, causing bud-blasting and reduced yield potential in dry years, when the ability of plants to compensate is limited. Larvae feed within developing pods with each larva consuming about five seeds during its development. Although this amount represents only 15 to 20 per cent of the total yield of a particular pod, these pods are predisposed to premature shattering.
Larvae emerge from pods via exit holes. In humid weather, these exit holes provide an entry point for fungal infections, and additional seeds can be damaged. When new generation adults emerge in late summer, they can invade nearby fields and damage the immature pods of late-seeded canola by feeding directly on the seeds through the pod walls (Figure 2).
Because the adult is the over-wintering stage of the cabbage seedpod weevil, the risk of infestation can be predicted based on the adult population of the preceding fall. High numbers of weevil adults in 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 over-wintering adults.
Adult Cabbage Seedpod Weevils
Adult weevils are:
- Approximately 3 to 4 mm (1/6 in.) long; and
- Prominent curved proboscis (typical of most 'snout' weevils).
The adults over-winter beneath leaf litter in tree shelterbelts, roadside ditches, and woodlots, sometimes in very high numbers. Late in the season (September to early November), they select over-wintering sites and burrow beneath the soil surface where they are protected from low temperatures. In spring, they emerge from these sites over a period of several weeks and seek out host plants.
Adults occur most commonly on the buds and flowers of host plants, but during windy days, they move to sheltered areas within the plant canopy. 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.
Mating occurs from spring to early pod development, usually on a host plant. When small pods develop, the females can deposit an egg through the pod wall onto, or adjacent to, a developing seed.
Eggs are very small, oval, and opaque white. Most often, only a single egg is deposited per pod; however, two or more eggs can be laid per pod during outbreaks. larvae are competitive and typically only one survives within the pod.
Eggs hatch in about six or seven days, and females continue to lay eggs until they die later in the season.
Larvae are white and grub-like, without legs or eyes (Figure 4). Soon after hatching, the larvae begin feeding within the pods on developing seeds. Larval development takes approximately six weeks, and during this time, a single larva consumes about five canola seeds. There are three larval stages (instars).
Mature larvae chew small, circular exit holes in the pod walls (Figure 5), drop to the ground, burrow in, and pupate within earthen cells (Figure 6). New generation adults emerge about 10 days later and feed on immature canola or other green cruciferous plants until late in the season when they enter over-wintering sites.
The distribution and abundance of the cabbage seedpod weevil have been monitored 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. In 2000, the cabbage seedpod weevil extended its range from Alberta into southwest Saskatchewan. the northern and eastern reaches of its range continues to expand.
Integrated Pest Management
Cabbage seedpod weevil adult abundance can be monitored by taking sweep net samples. Sampling should begin when the crop first enters the bud stage and continue through the flowering period. Select 10 locations within each field, and at each location count the number of weevils from 10 180 degree sweeps. Sampling locations should include both the perimeter and interior of the field, to obtain an accurate estimate of weevil numbers throughout the field.
This monitoring procedure will also give an indication of the number of lygus bugs present and may serve as an early warning for lygus damage, provided that the same fields are monitored into the early pod stage.
A few other weevil species may also be found occasionally in canola, but these 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.
Canola pods harbouring cabbage seedpod weevil larvae often appear distorted (Figure 7). When larvae consume some seeds within pods, the undamaged seeds enlarge and mature, often leaving misshapen pods.
At present, trap cropping is the most promising cultural strategy for controlling the cabbage seedpod weevil. This approach takes advantage of the concentration of weevils that often occurs at the field edges when they first invade a canola field. By planting a trap border of early flowering Brassica rapa on the perimeter of the main crop of B. napus, cabbage seedpod weevils may be controlled with an insecticide applied to the perimeter before they spread throughout the field. Alternatively, a strip of the same variety planted seven to 10 days before the rest of the field, can serve as a trap for adult weevils.
Other potential cultural control strategies being investigated by researchers include the effect of altering seeding rates, row spacings and fertility regimes.
Although B. napus, B. rapa, and B. juncea (brown mustard) are susceptible to infestation by cabbage seedpod weevil, yellow mustard (Sinapis alba) is resistant. Yellow mustard crops will not require monitoring or control measures.
There are insecticides registered for control of cabbage seedpod weevil in canola, mustard and rapeseed. Refer to The Guide to Crop Protection for registered products.
Chemical control is recommended when an average of 25 to 40 weevils are collected in ten sweeps sweep, following the monitoring guidelines above. When the value of the canola exceeds $8 per bushel an economic threshold of two adult weevils per sweep ( average of 20 in ten sweeps) is more applicable. If control is required, the best time to spray is when crops are in 10 to 20 per cent flower, to avoid egg laying in newly formed pods. This is the stage when 70 per cent of plants in the field have at least three to 10 open flowers. Spraying should be done late in the day to minimize harmful effects to beneficial insects in the crop, especially bees. Always consult the product label for appropriate rates and application guidelines.
Predators, parasites, and diseases are important in regulating insect pest populations. To date, the impact of biological control agents on cabbage seedpod weevil populations in Western Canada has been limited. However, in Europe and the United States, parasitic wasps (Figure 8)are effective for reducing both adult and larval weevil populations. The most important species are Microctonus melanopus, a wasp that parasitizes adult weevils and Trichomalis perfectus, a wasp that attacks weevil larvae within the pods.
Research in 1998 and 1999 found that weevil populations in southern Alberta were non-parasitized, but some evidence of parasitism was found in 2000. A major focus of work in the future will be to enhance the abundance and dispersal of these parasites, thereby reducing the need for insecticide use.
History of the Cabbage Seedpod Weevil
The weevil was discovered in British Columbia in 1931, and from there it dispersed south and eastward, so it now occurs throughout most of the United States.
It was first found infesting canola in southern Alberta in 1995, and since then, the weevil has spread to central Alberta and southwestern Saskatchewan. In 2000, it was found in Québec for the first time.