Cabbage seedpod weevil – Survey and maps

Overview
Survey and maps

Note that this is not a forecast. It is a summary of the situation in Alberta in the summer of 2023.

Methodology

The 2023 survey covered the canola growing areas of southern and central Alberta. 222 fields in 50 municipalities were sampled in late June to early July when canola was at 20 to 25% flower. Surveyors sample by taking twenty-five 180 degree sweeps through the crop starting at the field edge and walking toward the centre of the field.

Survey findings

2023 Cabbage seedpod weevil survey map

The survey found cabbage seedpod weevil below economic levels in southern Alberta with the exception of a couple of hotspots. There could have been fields at or near threshold, but the nature of the survey means they were not sampled. In southern Alberta, it will still be important to scout canola when in early flower, especially those first to flower fields, to make control decisions.

In 2023 we found weevils in Ponoka County, counts were well below threshold. In the past, we have seen economic threshold levels as far north as a line between Red Deer and Consort Alberta

Agriculture and Agri-Food Canada at Beaverlodge conducts the canola survey in the Peace River region where as of yet no cabbage seedpod weevils have been found.

Cabbage seedpod weevil overwinters as an adult so the risk of infestation is further indicated by the adult population of the preceding fall. High numbers of weevil adults in fall will likely mean significant infestation levels in the following spring. This map does not adjust for the emergence of the new generation in the fall or overwintering conditions, although cooler temperatures and rainfall in August favours the development of the new generation of weevils and may lead to higher numbers in the following year. The cabbage seedpod weevil takes roughly eight weeks to develop from egg to adult. Development time will vary somewhat depending on weather conditions, especially temperature. There is one generation per year. Follow this link for further information about the life cycle.

Crop damage from cabbage seedpod weevil can occur from:

larval feeding within developing pods (larva consumes five to eight seeds, this is the major source of losses)
premature shattering of damaged pods
new generation adults that emerge in the fall feeding on nearly developed seeds (usually concentrated only on very late crops).
bud-blasting (potentially reducing yield in dry years though not experienced in Alberta)

The larval feeding can result in yield losses of 15 to 20% in each pod infested.

Cabbage seedpod weevil adult abundance is best monitored by using sweep net samples. Sampling should begin when the crop first enters the bud stage and continue through the flowering period. Select ten locations within each field, and at each location count the number of weevils from ten 180 degree sweeps. Sampling locations should include both the perimeter and interior of the field to obtain a representative estimate of weevil numbers throughout the field. Proper Sweepnet Technique.

This monitoring procedure will also give an indication of the number of lygus bugs and diamondback moth larvae present and may serve as an early warning for those insects, provided that the same fields are monitored into the early pod stage.

The 2023 survey was completed by Agriculture and Irrigation, Plant and Bee Health Surveillance Section staff. Thank you Adriana Van Tryp for your dedication to the field sampling program. Your contribution to the program was significant.

Thank you Jon Williams, Agriculture and Agri-Food Canada, Saskatoon for preparing the map.

Canola Council of Canada

Survey maps

Resources

Life cycle information

Proper Sweepnet Technique

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