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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect Update

June 15, 2009                   

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives, Crops Knowledge Centre,
Phone: 204-745-5668; Fax: 204-745-5690

To report observations of insect activity or control that may be of interest or importance to others in Manitoba, please send messages to the above contact addresses.

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Summary

Cutworms continue to be a problem in localized areas, and flea beetle feeding on young canola seedlings is a concern and may intensify as we get into some warmer weather.

Recent Insect Concerns and Observations

Cutworms: Cutworms continue to be a concern, mainly in the central Region of the province. Hot spots seem to be in the Treherne-Holland area, MacGregor area, and near Sanford. Sunflowers and canola are the crops where economical populations have been reported from so far. Samples taken from heavily infested sunflower fields in the Sanford area showed dingy cutworm to be the main species in that region. Larvae were still in their middle instars, so not near pupating yet. Dingy cutworms are primarily leaf feeders. So there is less likely to be the extensive cutting of stems as some other species of cutworms can do. However, when feeding on seedling, defoliation can be severe when dingy cutworm populations are high.

The photo below is of dingy cutworms from the Sanford area. Note that down the very middle of the back there is a thin light line. And on either side of this thin line there is a broader series of somewhat diagonal markings that look like tire tracks, or to form “V’s” on the back. They also have 4 equal-sized black dots on the back surface of each abdominal segment.

dingy cutworm  
Dingy cutworm  

Flea beetles on Canola: Some canola fields required foliar sprays to protect from flea beetle damage over the past week. This seems to be more of a common occurrence across the province, rather than isolated regions as with the cutworms. It is likely that the insecticide in the seed treatment of any canola seeded in early to mid-May is reduced to levels that will no longer provide sufficient control of flea beetles. Any of these fields that still do not have 3 or 4 true leaves need to be scouted carefully for flea beetle feeding, as well as potential cutworm feeding. Flea beetle feeding will intensify as we get into warmer weather. It’s hard to predict when the current populations of adult flea beetles will start to die off, but results from some of our traps show populations may still be quite high.

 

Surveys and Forecasts

Diamondback Moth Forecasting: Counts of diamondback moth continue to be low. Data and interpretations can be viewed at:
https://www.gov.mb.ca/agriculture/crops/insects/db/index.html

Bertha Armyworm Forecasting: Traps can now be set up to monitor bertha armyworm moths.
Crop Scouting Reminders and Tips

A few of the questions that some have had this week about crop scouting include:

When assessing flea beetle feeding, is it just the holes in the cotyledons and leaves we are counting?
Flea beetles feed on the cotyledons and stems of canola seedlings, and continue to feed on the leaves as the plant develops. Feeding by flea beetles typically consists of small holes or pits in the surface of leaves. Although the initial feeding does not penetrate the leaf completely, tissues below the injury eventually dry up and break or fall out giving a shot-hole appearance. You will often see a lot of brown scarring, but not necessarily holes, where flea beetles have fed. This leaf tissue is no longer photosynthetically active, and needs to be included (along with the holes) as flea beetle damage when doing assessments.

When the canola plants have 1 or 2 true leaves expanded, what do you rate to determine the level of flea beetle damage to the plants; just the true leaves, or true leaves and cotyledons?
You are rating average damage to the plant as a whole, including the cotyledons. Not just the new leaves.
Your goal is to have an estimate of the average flea beetle feeding in the field. Some prefer to look at several plants in an area and estimate what seems to be the average damage.

One possible source of error is that it can be very difficult to provide an average damage rating to a plant, especially when we have to focus on more than 1 leaf, and the leaves and cotyledons may have very different levels of feeding damage. Photo guides can often be helpful in making more accurate assessments. It is natural for our eyes to be drawn to the damage and to overestimate the actual level of damage (it is sometimes hard to appreciate the amount of healthy material still left when we are focused on how much has been damaged). Some photo guides are available to assist in estimating % defoliation. In addition, on page 343 of the Guide to Crop Protection there are a series of photos showing different levels of defoliation. Although this key uses sunflower leaves, it can be helpful to those scouting other crops such as canola, as it demonstrates how much defoliation must occur before we hot 25%. Some minor pitting of canola leaves in normal, even when seed treatments are effectively working. So don’t get too nervous over minor levels of pitting, this is normal. But do keep an eye on canola fields that are still seedlings, especially as the weather heats up.