Manitoba Insect & Disease Update
Issue 2: June 6, 2018
Summary
Diseases: No disease incidents reported this week.
Insects: Flea beetles and cutworms continue to be the main insect concerns. Flea beetles are a concern in many canola fields and foliar insecticide applications are occurring in many areas. There has also been some reseeding of canola because of flea beetle feeding.
Now that cutworms are getting larger and their feeding more prominent, some economic populations have been detected.
Flea Beetles
Foliar insecticide applications to control flea beetles on canola have been occurring in all agricultural regions, and there are reports of some second insecticide applications for flea beetles. There are also more reports of reseeding of canola because of feeding by flea beetles, particularly in the Southwest and Central regions.
How long does canola remain susceptible to flea beetles? A general guideline is that once the canola has 3 or 4 true leaves expanded, it can usually compensate well for feeding by flea beetles. Research in Manitoba found that 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.
Cutworms
Cutworm feeding has intensified over the past week as cutworms get larger and their feeding more noticeable. There are reports of insecticide use for cutworm in oats, wheat, canola, corn, sunflowers and dry beans. Some of the heaviest feeding seems to be in the Central region and southwest. There are a couple of reports of patches in oat fields in the Central region being reseeded because of cutworm feeding. Redbacked cutworm seems to be the dominant species in some of these fields, although some dingy cutworms are being found as well.
Damaged black bean plant and redbacked cutworm Photo by Jason Voogt - Field2Field Agronomy
How long will cutworms feed: If there are still a lot of cutworms under an inch long, then there is still ample time for higher populations to do economic damage. As a general guideline, once they get over an inch long they will be pupating soon and applying controls may be more expensive then the damage you are preventing.
Blackleg in Canola
Early season infection of blackleg in canola can occur, especially when the crop has been wounded by hail, wind, frost, or insect feeding. The question on growers mind is whether or not to use a fungicide at the herbicide timing to protect their crop from blackleg infection. Studies have shown that an early fungicide application (strobilurin fungicides applied at the 2 to 4 leaf stage) can result in disease reduction and also a yield increase. However, the yield increase in this trial was only observed when a susceptible cultivar was grown (i.e. Westar) and the disease pressure was extremely high (continuous canola rotation). The corresponding infection level in the untreated treatment was 90% incidence with an average severity of 3 (i.e. 51-75% of the stem cross section discoloured from blackleg infection). This is an extremely high infection level compared with our provincial averages, which are typically around 15 to 20% incidence with an average severity of 1.5.
Insect feeding, from flea beetles or other insects, can definitely make your plants more susceptible to infection, but there are other factors to consider as well:
• What is your rotation? A field with tight rotation (2-year or less) is also higher risk for blackleg infection.
• Have you seen yield loss from blackleg in the past?
• Has the environment been conducive for infection? The release of ascospores from overwintering bodies (see image) is favoured following a rainfall and when temperatures range from 16 to 20°C.
• Is your crop worth it? How bad is the insect damage, do you want to be investing more money in this particular field?
• What is your rotation? A field with tight rotation (2-year or less) is also higher risk for blackleg infection.
• Have you seen yield loss from blackleg in the past?
• Has the environment been conducive for infection? The release of ascospores from overwintering bodies (see image) is favoured following a rainfall and when temperatures range from 16 to 20°C.
• Is your crop worth it? How bad is the insect damage, do you want to be investing more money in this particular field?
Finally, it will always come down the level of risk an individual grower is willing to take. There is no one right answer for everyone when it comes to this issue.
Overwintering bodies (pseudothecia) of blackleg on canola stubble
Overwintering bodies (pseudothecia) of blackleg on canola stubble
Pea Leaf Weevil
Pea leaf weevil has never been found in Manitoba, but has been located in Saskatchewan not far from our borders. So if you are scouting peas, have a look for notches in the leaves. The adult weevils will make these notches. It is their larvae, which feed on the Rhizobium nodules, that can be damaging to the plant. If you think you see feeding from the adult weevils on pea leaves, please contact John Gavloski (see contact number at the bottom of the update) so we can evaluate this further. The following link is to a factsheet from Alberta on peas leaf weevil and shows the leaf notching to look for: https://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/prm11287
Insect Monitoring Programs
Bertha Armyworm: A reminder for those setting up traps for bertha armyworm that this week (June 4 to 8) is the ideal timing for bertha armyworm traps to be set up.
Diamondback Moth: Monitoring adults of diamondback moth with pheromone baited traps has been underway since the beginning of May. Overall counts have been low. Out of 78 traps only 8 have had cumulative counts greater than 10. Highest counts have been in the Central region and Interlake, but overall levels are low . Table 1 summarizes the highest cumulative counts in Manitoba.
Table 1. Highest cumulative trap counts for diamondback moth adults over the trapping period April 29 to June 6, 2018.
| Location | Count |
|---|---|
| Oak Bluff | 78 |
| Warren | 62 |
| Balmoral | 31 |
| Seven Sisters | 17 |
| Teulon | 15 |
| Morris | 14 |
Larvae of diamondback moth have not yet been observed or reported.
A more detailed update of diamondback moth counts in Manitoba is available on the Manitoba Agriculture website at: http://www.gov.mb.ca/agriculture/crops/insects/diamondback-moth-forecast.html
Insect Identification Quiz
While search under debris in the field for cutworms you disturb this beetle that quickly runs along the ground for cover. What is this beetle?
Answer: This is a ground beetle. In Manitoba we have about 350 species of ground beetles, and some are quite common. They are mainly predators, and cutworms are one of the insects many species of ground beetles like to eat. Some species of ground beetles will also eat weed seeds. A field study in Alberta found 7 species of ground beetles feeding on redbacked cutworm larvae or pupae.
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Compiled by:
John Gavloski, Entomologist Holly Derksen, Field Crop Pathologist
Manitoba Agriculture Manitoba Agriculture
Phone: (204) 750-0594 Phone: (204) 750-4248
To report observations on insects or plant pathogens that may be of interest or importance to farmers and agronomists in Manitoba, please send messages to the above contacts.
To be placed on an E-mail list so you will be notified immediately when new Manitoba Insect and Disease Updates are posted, please contact John Gavloski at the address or numbers listed above.