OMAFRA Report: White mould, cabbage seedpod weevil management in canola

A weekly report prepared by the staff of the Ministry of Agriculture, Food and Rural Affairs (OMAFRA).  If you require further information, regarding this report, call the Elora Resource Centre at 519-846-0941. For technical information call the Agricultural Information Contact Centre at 1-877-424-1300 or visit

Flowering has begun in many winter canola fields. Two key pests in winter canola at this growth stage are cabbage seedpod weevil (CSW) and white mould (a.k.a. Sclerotinia stem rot). These are also both pests of spring canola, and the management recommendations are the same for both types of canola.

Cabbage seedpod weevil

CSW may begin to appear just prior to bolting and are found on flower buds until pods begin to form. Monitor from the first appearance of flower buds through to the end of flowering. They emerge from overwintering sites when temperatures are above 15°C.

Adults are a conspicuous, dark grey weevil with a prominent curved snout and measure 3-4 mm in length. If present, they are usually easily observed on flower buds and are most abundant at field edges.

Adult weevils will feed on flower buds causing bud abortion, but the adult feeding damage rarely causes economic yield losses. Yield reduction is caused by the CSW larvae, which feed on seeds within developing pods and create exit holes leading to shatter losses.

Scout fields using a sweep net; conduct 10 sweeps (180˚ from shoulder to shoulder) across the top of the plants in different locations within the field and count the weevils. Sweeps should be conducted in the interior and perimeter.

An average of 2-4 per sweep (greater than 20 per 10 sweeps) is the threshold for spraying insecticide. Insecticides should be applied at 10% to 20% bloom, when plants have approximately 3 to 10 open flowers, to protect developing pods. Insecticides can be added to fungicide in a one-pass system if CSW reach threshold during appropriate timing for control of white mould (20% to 50% bloom). Registered insecticides include Decis, Matador and Voliam Xpress.

The key to protecting yield is to prevent adult CSW from laying eggs in newly formed pods. Adults do not lay eggs until pods are approximately 2 cm long. If insecticides are applied too early, new flushes of CSW may invade the crop and lay eggs in developing pods leading to yield loss.

Spray late in the evening to reduce the risk to pollinators that will be foraging the crop. Note that there are often a lot of different harmless insects in winter canola fields because they are some of the first flowers on the landscape.

White mould

Canola fields have a thick, tall canopy. It is almost always moist and humid in the canopy unless the stand is very thin and plants have not branched due to pests or abiotic stresses. The moisture in the canopy and the moderate temperatures that commonly occur during flowering stages of winter canola provide ideal conditions for mould development where a source of inoculum is present. Ideal canola-growing weather is also ideal for white mould (optimum temperature for mould is 15-25°C) so there is typically a high risk of white mould in healthy fields. A preventive fungicide application has good return on investment.

Application should occur at 20-50% bloom. Check growth stage of plants across the field, not just at the field edge. Count flowers on the main stem. Generally, it takes a crop two to four days to move from first flower to 10% bloom and flowing can last up to three weeks. The goal is to coat as many flower petals with fungicide as possible prior to significant petal drop; deteriorating flower petals are very susceptible to infection. Spores colonize flower tissue, and the disease enters other healthy plant tissues when infected flower petals land on leaves or stems. Infection can then rapidly spread across neighbouring plants in the field.

Meghan Moran is a canola and edible bean specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs.

Meghan Moran