Cabbage seedpod weevils are a perennial problem for canola and mustard growers in southern Alberta. Their range has expanded up to the northern and south central parts of the province in the last couple of years and, says Scott Meers, Alberta Agriculture and Forestry insect management specialist.
“Saskatchewan has economic infestations as far as east Regina and Indian Head, north towards Outlook, but the most intense ones are still southwest, says Scott Hartley, Saskatchewan’s provincial specialist for insect pest management.”
Cabbage seedpod weevils are somewhat sensitive to cold winter weather, says Meers, but since “we never really had cold temperatures,” we can assume that adults have survived overwintering in fairly good shape, and that farmers in affected areas should be sweeping and monitoring early for these bugs.
The first thing you’ll want to do is scout your fields. It’s “the first in-flower fields” that are at greater risk from higher numbers of cabbage seedpod weevil, says Meers, so you’ll want to be sweeping your fields early, and at several different field locations. These plants are typically blooming at the end of June or early July depending on the weather and conditions in your region. That early time period is the key time to assess your crops.
In those early flowering fields, the field edges will likely have higher numbers of adult cabbage seedpod weevils, if they’ve found their way to your land, says Meers. Once the fields fill out, the weevils can spread out and the numbers can even out throughout the entire field. Since the adult weevils don’t start laying eggs “until the pods have developed to somewhere around three-quarters of an inch long,” says Meers, you need to catch them in that window before they start laying eggs — that’s the crucial time period. When the pods get longer than the three-quarter inch mark, that’s the time to worry. “If the eggs hatch, then the larvae eat the seeds.”
Meers advises farmers to take their sweep nets out “and do some counts and that’s how you see whether or not you’re getting over threshold.”
The current economic threshold is 2.5 weevils per sweep. If you scout multiple locations in the same field, average your findings. If your count is lower than 2.5 per sweep, don’t worry about doing anything more.
Over the threshold
If your fields are above the economic threshold for cabbage seedpod weevil, “we’re recommending a full-year application of insecticide,” says Meers. “Generally people are using Decis or Matador.” (These products are sold by Bayer and Syngenta, and have deltamethrin and lambda cyhatothrin as active ingredients.)
“Be aware of beekeepers in your area and work with them to avoid any damage to their hives and their bee populations,” Meers advises. “Phone your nearby beekeepers and let them know when you’re going to spray. Most damage to bees can be avoided if the beekeepers knows what’s going on.” With the proper information, beekeepers can manage their hives and make sure they are protected.
Historically the first in-flower crops in places like southern Alberta where the pest has been established for years, “almost always get sprayed,” says Meers, and “the last in-flower seldom get sprayed because the numbers are just not high enough.”
Where high numbers of cabbage seedpod weevils are left uncontrolled, yield losses can head into the 15 to 25 per cent range. “If a pod is missing six to 10 seeds, that really adds up,” Meers says.