Topics for the Week

Insect update: diamondback moths

Each spring, adult populations of diamondback moths ride wind currents northward from infested regions in the southern or western U.S.A. Once in the Canadian prairies, the moths can cause economic levels of feeding damage throughout the season, with much of their damage occurring when they feed on the exterior of developing pods.

To protect beneficial insects and make the best economic decisions, apply insecticide only if diamondback reach the economic threshold of 200-300 larvae/m2 (20-30/ft2, or approximately 2-3 larvae per plant) at late flowering or podding. For easier counting, pull up all plants from one square foot and beat them on flat surface, such as a sheet or truck hood, to dislodge them. Because diamondback cycle rapidly (they can achieve three or four generations within a growing season), accurate and frequent scouting is critical. (Prairie Pest Monitoring Update: Week 11) (Diamondback moth economic threshold)

Insect update: lygus bugs

Crop damage from lygus bugs depends on which matures faster: the crop or the pests. Start looking for lygus bugs as canola comes out of flower, and then again one week later. If lygus counts exceed the economic threshold (20-30 insects per 10 sweeps in late flower to early pod), plan to spray insecticide at early- to mid-pod before pods get leathery. That said, the right time to spray is when insects are big enough (third instar stage with wings starting to develop) to feed on seeds. If they aren’t developed enough, wait: a hard rain can push down first and second instars and kill them naturally. In some hot and dry areas, two generations of lygus may be present at the same time. Lygus bug populations in canola appear to be independent of populations in nearby alfalfa fields. (Ensure proper growth stage when lygus bug scouting) (Lygus bug scouting, timing and thresholds) (Proper sweep net technique)

Time to start looking at harvest management options

As canola completes flowering, producers are starting to get a fairly good understanding of their crop’s yield potential. Now is a good time to consider harvest management options. Deciding whether to swath or straight cut is about more than a cultivar’s pod shatter tolerance rating: individually consider each field’s crop stage variability, height, and plant stand density. It will be difficult to time a preharvest glyphosate application on a field that has inconsistent growth stages. Lodged canola will likely require swathing. (We’ve heard some reports of lodging as early as flowering due to heavy winds blowing across canola rooted in saturated soil). (Scenarios: swath or straight combine) (Preparing to straight cut your canola)

Combine College happened yesterday in Dauphin, MB and will run again tomorrow (July 28) in Portage la Prairie, MB. It’s not too late to register for tomorrow’s sessions.

Consider ROI before spraying for sclerotinia…

• … a second time:

When a strong crop faces high sclerotinia risk, especially high moisture and long flowering, there can sometimes be economic justification for a second pass. That said, a study by Rude (1989) found that, if conditions are favourable throughout the risk period, infection level and yield loss potential were notably higher when canola plants were infected at early bloom than at late bloom. While late season infections don’t typically contribute as much to yield loss, an application will help reduce inoculum going back into the field.

• … on reflowering canola:

Deciding whether to spray for sclerotinia when a crop reflowers after hail depends on the crop’s yield potential at reflowering and the odds of bringing that yield to harvest. While the economics look different at $20/bu canola than at $12/bu, the crop still needs to be 30-35 bu/ac to justify the application. Keep in mind too that, once the canopy is closed, the majority of petals from reflowering canola will stay close to the plant, decreasing the risk of sclerotinia spreading.
(Factors in the sclerotinia spray decision) (Sclerotinia stem rot symptoms)