Manitoba Insect & Disease Update

Issue 6:  June 28, 2017

 
Summary
 
Insects: 
Alfalfa weevil continues to be reported at high levels in some alfalfa fields.
Cereal leaf beetle and thrips are noticeable in some fields of small grain cereals, but at below economical levels. Initial tests of levels of parasitism of cereal leaf beetle larvae show quite high levels in the Central region, with samples from other regions soon to be tested. Thanks to those agronomists who have provided field locations, or collected and sent in samples of cereal leaf beetle larvae to be tested for parasitism. See last weeks update for details on collecting and submitting cereal leaf beetle samples to determine level of parasitism.
Low levels of thistle caterpillars have been noted. Although these will feed on sunflowers, the level of feeding is not normally economical, and they can do a lot of feeding to thistles.
 
Pathology:
Bacterial blight in oats has been detected in the Red River Valley. As temperatures increase plants will typically grow out of the symptoms.
Loose smut has been detected in a wheat field near Niverville.
Phytophthora root rot (PRR) in soybean has been reported in the Red River Valley. This disease thrives under wet conditions and can cause yield loss depending on incidence. Typically PRR affected plants will prematurely senesce and not set seed.
 
Bacterial Blight in Oats
 
Bacterial blight in oats has been reported in various areas of the Red River Valley. This disease thrives under cool and moist conditions. Typical symptoms symptoms include water soaked areas that eventually turn necrotic. The affected areas often form longitudinal blotches or stripes. As temperatures warm up the plants are able to grow out of the symptoms. Yield loss is not generally a concern with this disease.
 
 Photo credit of Brunel Sabourin, Antara Ag
 
Loose Smut in Wheat
 
There has been a report of smut in wheat from the Niverville area. This disease is also favoured by cool humid/wet conditions. Smut is typically managed through using resistant varieties and seed treatments. The variety where symptoms were observed is Carberry. Symptoms of smut typically show up at the same time as head emergence. In the affected field, incidence was low overall with scattered plants showing up across the field.
 
 Photo credit of Dillon Thoms, Prairie Sky Crop Solutions
 
Phytophthora Root Rot in Soybean
 
There have been multiple reports of Phytophthora root rot (PRR) in soybean in the Red River Valley. In at least one case, the incidence is reported as up to 30% of plants affected. PRR is caused by an oomycete and thrives under water logged conditions. Symptoms can show up in patches in the field (low spots) or as scattered plants. Severely infected plants will completely senesce, but the leaves will stay attached. PRR is common in areas of the province that have had a history of soybean production and in years where there are water-logged conditions.

Photos credit of Indra Ariyaratne, MB Ag & Brunel Sabourin, Antara Ag
 
Scouting for thrips in barley
 
Barley thrips can at times get to economic levels in barley, but populations can be quite variable between and within fields. What should be monitored is levels when the flag leaves are first visible until the heads are completely emerged from the boot. 
Economic thresholds have been established;
the threshold (thrips/stem) =   (Cost of control / expected dollar value per bushel)/ 0.4. 
This often works out to around 4 adults per stem, but because the value of barley can vary greatly depending on use and market, it is good to use this simple formula to determine the economics of potential management. Insecticides are only going to provide value if applied before heading is complete, if levels of thrips are high. Also note that barley thrips may be more numerous near protected field margins than other areas of the field.   
 
Insect Monitoring Programs
 
Bertha Armyworm: Cumulative counts from traps for bertha armyworm are currently quite low. Highest counts so far are:
 
Table 1. Highest cumulative trap counts for bertha armyworm adults over the trapping period June 4 to June 28, 2017.
 
Location Region Count
Napinka Southwest 107
Baldur Central 37
Glenboro Central 26
Tilston Southwest 26
 
A more detailed update of bertha armyworm counts in Manitoba is available on the Manitoba Agriculture website at: https://www.gov.mb.ca/agriculture/crops/insects/bertha-armyworm-forecast.html
 
Diamondback Moth: Monitoring adults of diamondback moth with pheromone baited traps has been underway since the beginning of May, and is now finishing. Overall counts have not been too high. There were 2 weeks, June 4 to 17, when higher counts did occur in some traps. These counts can not predict larvae levels in fields with the traps, and the purpose of the traps is to assess regionally areas where higher priority should be given to monitoring for larvae. Highest counts have been in the Northwest and Interlake, and the lowest counts in the Southwest. Table 2 below summarizes the highest cumulative counts in Manitoba.
 
Table 2. Highest cumulative trap counts for diamondback moth adults over the trapping period May 1 to June 28, 2017.
 
Location Count
Minitonas 180
Teulon 107
The Pas 93
St. Adolphe 83
Whitemouth 75
The Pas 69
 
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.
Larvae of diamondback moth are at noticeable (although not economical) levels in some fields in Manitoba, and some pupae have been observed as well.
 
A more detailed update of diamondback moth counts in Manitoba is available on the Manitoba Agriculture website at: https://www.gov.mb.ca/agriculture/crops/insects/diamondback-moth-forecast.html
 
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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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