Canola has made it past one of its largest insect threats – flea beetles – but that doesn’t mean it’s insect-free for the rest of the growing season.
Here are a few other insects to keep a watch for.
Cabbage seedpod weevil #
Adult cabbage seed pod weevils (CSPW) do a little bit of feeding on canola buds and stems, but it’s their larvae that cause the most damage. Larvae feed in the pods and destroy developing seeds, then exit the pod through a tiny hole. The entire pod is then typically lost due to fungal pathogens moving into the exit hole left behind by the larvae.
The adult CSPWs are an ash-grey colour and around 4mm long with a prominent curved snout. It’s best to scout for adults during bolting into flowering to assess levels. An insecticide application minimizes the eggs laid, and the economic threshold for applying insecticides is between 25 to 40 weevils in 10 sweeps during canola flowering.
Bertha armyworm #
Provincial governments across Canada run monitoring programs to capture bertha armyworm moths to determine if outbreaks will occur. Start scouting for larvae feeding on the crop after peak flowering or about two weeks after peak trap catches of adult moths. It’s the feeding larvae that can defoliate a crop quickly, and once the leaves are gone, they move up and start feeding on the pods. Because of that, Bertha armyworms are considered climbing cutworms.
Economic thresholds for applying an insecticide exist and are determined by the average number of larvae per square metre, but canola prices and spray costs are also used to make the decision to spray.
Diamondback moth #
The diamondback moth is also surveyed in the spring to assess adult numbers, but once again, it’s the feeding larvae that creates damage in canola. The larvae are tiny and green, much smaller than Bertha armyworm larvae, and dance around in the canopy.
The economic threshold for an insecticide application depends on the crop stage, amount of defoliation, and larvae populations. The threshold during vegetative and flowering stages is 10 to 15 larvae per square foot, and 20 to 30 larvae per square foot during late flowering to podding stages. Diamondback moths can have several generations in a season, but thankfully some natural enemies do exist to help provide some biological control of the insect pest.
Lygus bugs #
Lygus bugs can also have several generations in a growing season. They are small, oval-shaped insects that feed on the sap of buds, flowers, and pods of canola. Both the adult and nymph stages can cause damage to canola, leaving behind dark circular patches on the pods, which become susceptible to pathogens moving in. Once pods become mature and leathery, the lygus bugs lose interest in feeding on them.
Recently updated economic thresholds for applying an insecticide are now two to three lygus bugs per sweep during the end of flowering and early pod ripening stages.
A few others to watch #
Grasshoppers are not a typical canola pest but drought conditions across the prairies have increased their numbers, and they typically move into canola once their preferred feed sources have been grazed down. Action thresholds depend on size, feeding damage, and the forecast. It’s best to scout for them along field edges.
Midges, both swede midge and canola flower midge, are ones to keep an eye out for. The adult midge lays eggs in buds and once they larvae hatch, they eat away at the forming buds and flowers.
On guard #
An economic threshold is the level of insect feeding damage at which lost yield is expected to exceed the cost of insecticide control. They are used to help growers decide if a spray application is warranted to avoid economic losses. It is important to utilize these thresholds as the canola crop is resilient and can handle lots of feeding before yield is lost. Sometimes, though, different insect species are creating damage at the same time and lower thresholds should be considered.
Limit insecticide applications to help keep beneficial insect populations high and minimize the risk of developing resistant insect pest populations. Be actively scouting throughout the growing season as you just never know which insect pest might be thriving.
