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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

July 25, 2011                   

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives
Phone: (204) 745-5668; Fax: (204) 745-5690
Holly Derksen, Plant Pathologist, Manitoba Agriculture, Food and Rural Initiatives
Phone: (204) 750-4248; Fax: (204) 745-5690

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 contact address.

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.


 

Recent Insect and Plant Pathogen Activity

Cereals

Barley Yellow Dwarf: There has been a confirmed case of Barley Yellow Dwarf Virus in a barley field in the Gladstone area. This particular field had a late seeding date; therefore, the presence of BYD may have a more serious outcome. When this disease shows up at early stages of the crop it can have a significant effect on yield. Crops at later stages (flag leaf and beyond) may also develop symptoms of BYD but these symptoms are less likely to result in significant yield loss.

Aphids: High levels of aphids continue to be reported from some fields of cereals in the Central and Interlake regions of Manitoba. Aphid thresholds are generally set at 12-15 / stem prior to the soft dough stage. In any cereal crops that are not yet in the flag leaf stage and the oat-birdcherry aphid is one of the species present, a lower threshold may be appropriate due to the risk of the aphids vectoring barley yellow dwarf virus.

Armyworms: Armyworm populations may be subsiding as they start to become pupae. Also, hot temperatures can result in armyworm mortality. In a study looking at the effects of temperature on armyworms, survival was still good at 29ºC, but larvae had trouble surviving to the pupa stage at a constant temperature of 31ºC (The Canadian Entomologist. 101: 1320-1327). Insects vary in their response to high temperatures, but armyworms do not do as well under continuous high temperatures.

Canola

Diamondback moth: Levels of diamondback moth larvae have been reported around the threshold level of 20-30 / square foot, and some insecticide applications have occurred in the Beausejour area. Although larvae can be found in other areas as well, generally levels are lower and the Beusejour area is the only area currently reporting economic populations. Recall that the pheromone-based monitoring for adults of diamondback moth earlier in the year indicated 2 areas of higher trap counts, the Beausejour area and an area south of Morris.

Shaking the larvae off plants in a foot square area is the best way to access if you are near or over the economic threshold for diamondback moth. Finding the larvae while using a sweep net can tell you of their relative abundance in a field, but this data cannot be used to determine if a population is high enough to be economical.

Diamondback moth larva and pupa
Figure 1. Diamondback moth larva (right) and pupa (left)

Sclerotinia: Sclerotinia pressure in canola has been relatively low for most of the province due to the lack of precipitation the last couple of weeks. Some areas of the province received rain over the weekend and canola in this area that is in early flower it should be scouted for the presence of apothecia. It usually takes 7 days of soil moisture at or above field capacity for apothecia to release Sclerotinia spores, so be sure to scout your field for the week following the precipitation event.

 

Surveys and Forecasts

Bertha armyworm monitoring: Counts so far are generally in the low risk category, which is a cumulative count of less than 300 moths, although a few traps in the Swan River valley have reached cumulative counts in the uncertain risk category.

Table 1. Highest cumulative trap counts for bertha armyworm in Manitoba as of July 25, 2011

Location

Cumulative
Trap count

Durban

771

Minitonas

326

Bowsman

303

Neepawa

291

Data from the bertha armyworm monitoring program and a chart on how to interpret the data are available at: https://www.gov.mb.ca/agriculture/crops/insects/bertha/index.html

Banded Sunflower Moth: Pheromone-baited traps show that adult banded sunflower moths are now flying. As sunflower plants start moving into the early bloom stage, it is important to monitor for insects such as Lygus bugs and banded sunflower moth.