Cabbage Seedpod Weevil

By Faith Hillsden, Survey Tech Summer Student and James Tansey PhD, AAg, Provincial Insect and Vertebrate Pest Management Specialist

July 2024

Cabbage seedpod weevil is an occasionally major agricultural pest in Saskatchewan that is showing up in relatively high numbers in the southwest in 2024. These weevils feed on canola, brown mustard and wild mustard, significantly reducing yields when population numbers are high. As canola fields begin flowering throughout Saskatchewan it raises the importance of scouting and control measures to reduce the feeding damage done by adult weevils and the likelihood of egg laying in developing pods.

Cabbage seedpod weevil overwinters as an adult in the soil beneath leaf litter in tree shelterbelts, roadside ditches and woodlots. Emergence occurs in spring when soil temperatures reach 15 degrees. Adult weevils begin feeding on early flowering brassicaceous weeds and move into canola fields during early flowering. Adult weevils can consume a broad number of brassicaceous plants as food hosts but have a much smaller list of plants that can support larval growth.

Mating occurs on canola plants. Eggs are laid soon after flowering, when the pods are still developing. Females use their mouthparts to chew an opening in the pod and deposit an egg inside. They then turn around and deposit on oviposition-deterring pheromone to dissuade other females from laying an egg in that pod. Pods are a valuable and very limited resource, and larvae will cannibalize each other. This means that most pods contain only one egg, but in very high populations two or more eggs can be found within one pod. Larvae develop within pods, consuming seeds as they grow. Once they complete three larvae instars, or developmental phases, they chew an opening in the pod and the larvae drop into the soil to pupate. The new generation of adults emerges about 14 days later.

Feeding damage occurs in both larval and adult stages, with most economic damage done by larval feeding within pods. Adults begin feeding on canola in July, during the early flowering stage, where they feed on pollen, nectar, buds and racemes. This feeding activity can contribute to bud-blast and reduced yield in developing canola. The larvae will feed on five to six seeds and feeding leaves the pods predisposed to shatter. After maturation, larvae chew an opening through the pod, leaving the pod susceptible to fungal spores in wet conditions. The new generation of adults, which emerge in late summer, can also damage pods by feeding on seeds through pod walls.

Cabbage seedpod weevil adults can be found in canola, brown mustard, cole crops (cabbage, broccoli, and cauliflower), and cruciferous weeds (wild mustard, flixweed, and stinkweed). Canola, brown mustard and wild mustard are the true hosts, as these crops have large seedpods that sustain larval development. Although yellow mustard pods are nearly big enough to support larval growth, chemical defences make them unsuitable as good larval hosts and this crop is considered resistant.

Scouting for Cabbage seedpod weevils is important for determining population numbers and the need for control measurements. Monitoring should start when the canola crop first enters the bud stage and should continue through the flowering stage. A minimum of four sites should be sampled within each field, with at least 50 metres between each site, making sure that both the perimeter and interior of the field are being sampled. Generally, a loop of samples is done starting at the field edge and moving into the field and out again. At each site the number of weevils should be counted per 10, 180 degrees sweeps. When sweeping, make sure to sweep the top of the canopy, as these weevils are found on the buds and flowers of host crops. Cabbage seedpod weevil can be concentrated on field margins or flowering areas, so sweeping various areas within the field gives a more accurate representation of population density. The economic threshold for applying insecticides is 25-40 weevils in 10 sweeps (2.5 - 4 per sweep) of flowering canola. Use the lower threshold for plants under stress and the 4 per sweep for a crop growing under good conditions.

Insecticides are used to control Cabbage seedpod weevil populations that have exceeded economic threshold to reduce further feeding damage. Apply insecticides when crops are in the 10-20% flowering stage, to reduce the number of eggs being laid in pods. Here is a list of registered insecticides for control of Cabbage seedpod weevil, current at the time of publication:

• Deltamethrin - Decis 5 EC/Poleci 2.5 EC/FBM Deltamethrin – group 3A
• Lambda-cyhalothrin - Matador 130 ED/Silencer 120 EC/Labamba/Zivata – group 3A*
• Lambda-cyhalothrin and chlorantraniliprole - Voliam Xpress – groups 3A and 28*

*Do not use lambda-cyhalothrin products if the crop is intended for animal feed. This includes the meal.

Cabbage seedpod weevil has some natural enemies that can influence populations, including the larval ectoparasites, Trichomalus lucidus, Chlorocytus sp., Pteromalus sp., and Necremnus tidius. Parasitism of weevil larvae increased in Alberta from 0.1% in 2002 to 5.0% in 2004 and further increased in 2005. The effect that these parasitoids are having on weevil populations is currently unknown.

Cabbage seedpod weevil is a potentially economically damaging pest when control is not adequately used, which raises the importance of monitoring populations. Sampling canola fields can give an accurate representation of population densities, which indicates the need to control. The Cabbage seedpod weevil survey, led by Agriculture and Agri-Food Canada in collaboration with the Ministry of Agriculture and other partners is now underway throughout Saskatchewan to determine population densities of these weevils.