Manitoba Insect & Disease Update

Issue 4:  June 14, 2017

 
Summary
 
Pathology: Recent hail events in the province have left producers wondering whether or not they should be using a fungicide at the herbicide timing. 
 
 Insects: Flea beetles in canola, and cutworms continue to be the main insects of concern, although for both concern is diminishing as canola advances to stages less susceptible to feeding from flea beetles, and some cutworms larvae turn to pupae.  Alfalfa weevil larvae are noticeable in some alfalfa fields. 
 
Post-Hail Fungicides
 
For any of the spring crops, a fungicide applied at this time of year would be hitting the early herbicide-timing. For spring cereals, fungicides at this time can be used to help manage leaf diseases, such as tan spot and Septoria leaf blotch. Spraying a fungicide at the early timing will only protect the leaf tissue that has already emerged. When there is disease pressure, a reduction in disease severity may be realized, however yield bumps are more inconsistent.
For canola, fungicides can be used at the herbicide timing to help manage blackleg. Timing is key when it comes to this practice, as the most consistent results are when you get in early, around the 2 to 4 leaf stage. Hail (and wounding in general) makes plants more susceptible to blackleg infection, so if you are planning a post-hail fungicide application it is also important to get out there as soon as possible. Protecting the plants while the wounds are still open will provide the best results. In the absence of hail or excessive wounding, strobilurin fungicides have been shown to be more effective than triazoles at reducing disease. However, yield increases were only observed with strobilurin fungicides used at the 2 to 4 leaf stage in susceptible cultivars. Yield was not significantly affected in moderately resistant or resistant cultivars.
Overall, it is important to assess whether a fungicide application is worth it. What is the yield potential of your crop and how much of that yield has been compromised due to the hail event? A fungicide will not rescue your crop from hail damage, but can help protect your crop from the disease issues it is now more susceptible to.
 
Alfalfa Weevil
 
Alfalfa weevil have been hatching, and high and above threshold levels of larvae have been noted in some alfalfa fields in the Interlake. 
In alfalfa fields for hay, if alfalfa has reached the bud or early bloom stage, immediate cutting may kill many alfalfa weevil larvae.
 
                                          
                                                        Figure 1. Larva of Alfalfa Weevil.
Alfalfa weevil monitoring tips: A suggested monitoring strategy in alfalfa hay fields is; collect 30 stems while walking in a "w" shaped pattern in the field, and place them into a white pail. Beat the stems against the side of the pail to dislodge the larvae. The economic thresholds for chemical control of alfalfa weevil are:
• less than 30 cm crop height - one larvae/stem
• 30 to 40 cm crop height - two larvae/stem
• Three larvae/stem requires immediate action regardless of the height of the crop.

In alfalfa seed fields, a sweep net can be used or the percent of foliage tips showing damage can be assessed to make decisions based on the following nominal thresholds:
Foliage assessment: 35 to 50 per cent of foliage tips show feeding damage.
Larvae in sweep net: 20-30 3rd/4th instar larvae per 180 degree sweep of insect sweep net.
Insect Monitoring Programs
 
Bertha Armyworm: A reminder for those setting up traps to monitor adults of bertha armyworm that traps should be set up as soon as possible if they are not set up already.
Data is just starting to come in, but at this early stage is quite low, as expected. The highest count so far is 11 from a trap near Emerson.
 
Diamondback Moth: Diamondback moth monitoring with the pheromone baited traps has been underway since the beginning of May. Very low levels of moths were caught in traps in May and early-June. The highest cumulative trap count on June 6 was 19.  Some higher levels of moths were caught in traps the following week, however. By June 13th 5 traps had cumulative counts greater than 50, and the highest cumulative count was 125. Table 1 below summarizes the highest cumulative counts in Manitoba.
 
Table 1. Highest cumulative trap counts for diamondback moth adults over the trapping period May 1 to June 13, 2017.
 
Location Count
Minitonas 125
Teulon 71
Whitemouth 62
St. Adolphe 61
Ste. Agathe 60
The Pas 40
 
The levels are not out of the norm of what we usually see, and how large populations of larvae get depends on several factors including weather, and some key natural enemies, some of which also move into Manitoba on winds from the south. For example, Diadegma insulare, a parasitoid that lays eggs into diamondback moth larvae,  is not known to overwinter in Canada, and is believed to migrate northwards along with diamondback moth.
By early this week larvae of diamondback moth were at noticeable (although not economical) levels in many fields in Central Manitoba, and some pupae were detected as well.
 
A more detailed update of diamondback moth counts in Manitoba is available in the Manitoba Agriculture website at: https://www.gov.mb.ca/agriculture/crops/insects/diamondback-moth-forecast.html
 

  Larva of diamondback moth              Pupa of diamondback moth           
  Photos by Jason Voogt - Field 2 Field Agronomy - showing some of the stages of diamondback moth seen this week.
 
Insect Identification Quiz
 
The insect in the photo below was found on alfalfa near the edge of a field. What is the insect, and are they a concern on alfalfa or any other field crop? Hint - there were defoliated trees nearby.
                                   
                           Photo by Ingrid Kristjanson - Manitoba Agriculture
 
This is forest tent caterpillars (Malacosoma disstria). Forest tent caterpillars will sometimes be found in crops when populations are very high, but this is usually because the trees they were on were heavily defoliated, and they are searching for other trees to feed on. They are not a pest of any of our field crops. They have been noted to feed on crop plants, but their preference is deciduous trees.
So odds are any crop feeding would be along field edges that have lots of deciduous trees and will be quite minor.  They should start turning to pupae soon.
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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.
 
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