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The weevils are coming

The cabbage seedpod weevil is edging closer to Maniboba’s southwestern border. However, the insect, which is now a chronic pest in the southern parts of Alberta and Saskatchewan, has not moved into more northern areas of these provinces, and may not fare well in the longer, colder winter conditions of southern Manitoba.

The cabbage seedpod weevil overwinters in leaf litter in southern Alberta and Saskatchewan. Since it was found in Alberta in 1995, it has proliferated quickly.

“In our field surveys in 1997 when we took a sweep in the most heavily infested fields, we would get, on average, 0.5 weevils per sweep,” says Lloyd Dosdall, professor of Agricultural Entomology at the University of Alberta. “By 1999 we were getting 10 weevils per sweep and by 2000 we were getting 110 weevils per sweep, so the population was growing astronomically.”

Over-reaction to the pests in the early years, which may have caused some farmers to spray insecticides when not really necessary, was reducing numbers of beneficial insects along with the weevils.

Insect predators present in the soil, like beetles and spiders, are feeding on the cabbage seedpod weevils during their pre-pupal stage and parasitoids, like wasps, eat the emerging weevil larvae inside the canola seedpods, helping to reduce numbers.

Life cycle

There is just one generation of cabbage seedpod weevils per year. They take about eight weeks to develop from egg to adult and favour cool, damp conditions.

Adult cabbage seedpod weevils over-winter in deep leaf litter along tree lines or road margins, where they can survive on fat reserves as long as the temperature doesn’t fall below -7 C, when they will freeze and die. They emerge in the spring once temperatures reach 12 to 15 C.

Once canola plants begin to bolt and start flowering, weevils are attracted. Female weevils lay one egg in the seedpod. They prefer to lay eggs in pods about an inch to one-and-a-half inches long. After laying the egg, females brush the pod with a chemical that tells other females that an egg is in that pod.

The larva will complete its life cycle inside the pod and consume around 20 per cent of the seeds. If weather conditions are such that the canola pods mature very quickly, this can prevent the larva from completing its life cycle — it will either die in the pod or when trying to exit.

The larva exits by chewing a hole in the pod, then drops to the ground where it pupates. If conditions are extremely hot and dry it will become trapped in the soil and won’t finish its pupation.

Once it finishes the pupal stage it emerges as an adult and looks for any plant in the cabbage family to feed upon to build up fat reserves prior to overwintering.

Scouting and spraying

The adult cabbage seedpod weevil is a small, ash-grey beetle, three to four millimetres long. It is a hard-bodied insect with mouthparts that include a long snout. The larvae are white and grub-like and about two mm long. They feed within the seedpod of the host plants.

Begin scouting once the canola or mustard crop starts to flower in June, using sweep nets to catch the adult beetles as they are attracted to the emerging flowers. The sweeps should be done at 10 different locations in each field, with 10 180° sweeps at each location. The weevils will generally be present in higher numbers along the edges of the field. Spraying should be done at late bud to early flowering. “It’s important to time the insecticide properly because once the larva is inside the pod, it’s protected and there is nothing you can do to prevent damage,” says Héctor Cárcamo, an insect pest management research scientist at AAFC’s Lethbridge Research Centre. “The ideal crop stage to spray is around 10 to 20 per cent flowering. You don’t want to wait until the crop is in full flower, because the female weevils will have already laid their eggs.”

Other non-target insects that may also be affected by insecticide application include important pollinators like bees.

Planting a “trap crop” that matures earlier than the main crop around the edges of the field, for example Polish canola, or planting a strip of the main crop a couple of weeks earlier at the field edges, may cause most of the weevils to aggregate in that area.

Varieties with some resistance to cabbage seedpod weevil may soon be available to farmers in major canola-growing areas of Western Canada, and may also mean that Manitoba farmers can breathe a little easier. †

About the author

Contributor

Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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