Manitoba Insect and Disease Update: June 3, 2014

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Summary: Flea beetles in canola and cutworms are the insects of most concern currently. In some of the earlier seeded canola fields the effectiveness of the seed treatments has been reduced, and insecticides have been applied. Most of the reports of flea beetle damage and control have been from Central and Eastern Manitoba.

 

 

When does the canola become more resistant to flea beetles: In a study in Manitoba, yield was reduced most when plants were damaged during the seedling to second true leaf stage, but was not reduced  when they were damaged after reaching the third to fourth true leaf stage (Bracken and Bucher. 1986. The Canadian Entomologist. 118: 319-324). So as a general guideline, once canola reaches the 3 to 4 leaf stage it can usually compensate well for feeding by flea beetles.

 

Estimating % defoliation. The threshold suggested for controlling flea beetles that are feeding on canola is 25% defoliation and flea beetles still actively feeding. Estimating % defoliation can be tricky, as our eyes are drawn to the damage and it is easy to overestimate  how much feeding has occurred, Having a photo key as a guide can be useful for learning to assess defoliation levels. Work done at Agriculture and Agi-Food Canada in Saskatoon produced the photos in the following factsheet, which can be used to help assess levels of defoliation. Note the minor levels of feeding, such as 10% are normal and can happen even when seed treatments are still working effectively. Flea beetles need to feed to acquire insecticide in the seed treatment. These low defoliation levels will have no significant impact on the yield of canola.

 

 

High levels of feeding by cutworms has been noted in sunflowers in the Carman and Sanford areas, and in corn in the Starbuck area. In many fields the dingy cutworm seems to be the dominant species this year. Dingy cutworm overwinters as a partially grown larva, so is fairly large at a time when some of our other common species are still quite small. So one possible reason we may be seeing more dingy cutworms currently is that they are fairly large, making them easier to find, and doing more damage,

 

Some have been noting feeding belowground on stems in fields where they were finding cutworms, and wondering if this was cutworm or wireworm feeding. Some species of cutworms will feed belowground. Either cutworms or wireworms can cause feeding to the belowground part of stems.

 

Insecticide application tips for cutworms: 

 

Diamondback moth: Diamondback moth counts have increased in some locations in eastern Manitoba and the Swan River Valley. Currently, the highest cumulative counts, from trapping in May and early-June are 98 (Stead), 55 (Morris), 46 (Morris), and 42 (St. Adolphe). Yellow on the map below indicates area where the counts are starting to increase. In these areas, make sure to look for larvae of diamondback moth while scouting fields. However, these increasing counts are of male moths, and do not necessarily mean many larvae will be present, as many factors regulate populations. 

 

 

Bertha armyworm: The map below indicates at what stage the pupae of the bertha armyworm are in their development. Currently, some may have accumulated enough heat to be about 60-75% through their pupal development. Initial emergence can occur when the average pupal development is about 80%. So for those involved in putting out pheromone-baited traps for bertha armyworm, next week is a good time to get the traps out.