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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

July 18, 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

Armyworms: High populations of armyworms continue to be reported, mainly in the Morden and Winkler, and Elma / Whitemouth areas. Older larvae of armyworms likes to hide under debris on the soil during the day, so if seeing feeding on cereal leaves, look on the soil to see if armyworms may be the cause.

Stage of larvae is something to consider when determining the potential economic impact of an armyworm population. If most larvae are greater than about 2.5 cm, they will be finishing feeding and pupating soon, so control in these situations is generally not recommended.

The length of time that armyworms spend in each of their stages depends on temperature, but they normally spend about 30 days as larvae, passing through six stages of growth. The following table is from a study in Ontario where the length of time in each larval instar was determined.

Table 1. Length of various stages of armyworm larvae

Stage of larva

Length (days)

   First instar

4.8

   Second instar

3.3

   Third instar

3.3

   Fourth instar

3.8

   Fifth instar

4.4

   Sixth instar

10.3

Total time as larvae

29.9

When larvae are in their sixth instar, they will cease feeding about four days before they turn to pupae.

Armyworm larvae
Figure 1. Armyworm larvae

Aphids: High levels of aphids have been reported from several fields of cereals in the Eastern, Central and Interlake regions of Manitoba. Aphid thresholds are generally set at 12-15 / stem prior to the soft dough stage. However, if the oat-birdcherry aphid (Rhopalosiphum padi) is the dominant species and they are present in late seeded fields that are still quite young, a lower threshold should be used. The photo below is of oat-birdcherry aphids on wheat plants. Note the brown patch at the end of the abdomen on this species of aphid, and that this is a darker aphid than the English grain aphid, which is the other species of aphid common on cereal crops this year.

Oat-birdcherry aphid
Figure 2. Oat-birdcherry aphid

Thrips on cereals: Thrips and feeding by thrips are being noticed on cereal crops in the Eastern and Central parts of Manitoba. Wheat is one of the crops where this is most frequently being reported this year. Feeding appears as small white streaks can be found on the leaves of the plants. Thrips are not always easy to detect unless you look very carefully since they are quite small insects. Thrips are under 2 mm when fully grown, so you may have to look hard to see the thrips and know that they are the cause of the white streaks on the plants. Thrips can be more abundant around the edges of a field , so make sure to compare levels from different areas of the field if monitoring the extent of thrips injury. Also note that if levels of thrips are high, insecticide treatments are only effective before heading is complete.

Rust: There have been reports of leaf rust in spring wheat as well as oat crown rust in the Elm Creek area. A separate report of rust (species not identified) in oats came last week from the Beausejour area. Rust spores have arrived in Manitoba; be sure to keep in touch with local agronomists to know what the incidence of rust is in your area.

Canola

Sclerotinia: Although much of the province has been dry for the last week the threat for Sclerotinia infection remains. Generally speaking, conditions are optimal for Sclerotinia spore release when there is one to two inches of rain within one to two weeks of bloom and when temperatures are below 30°C. However, heavy dews can often provide enough moisture for spore release especially in thick canopies where the morning dew takes longer to dry up. The forecast for the upcoming week is again for hot temperatures but with the threat of precipitation across most of the province. Most canola fields are still blooming and therefore still at risk for Sclerotinia infection.

Pulse Crops

Soybean aphid: Low numbers of soybean aphids continue to be reported from soybean fields in the Central region of Manitoba. At this point it is something to monitor, but populations are remaining below levels that would be economical to control.

Green Cloverworms: Some green cloverworms are starting to be noticed in dry beans and soybeans in the central region of Manitoba. So far only low populations and levels of feeding have been noticed, so this is also something to monitor but currently not an economic threat.

Green cloverworm
Figure 3. Green cloverworm

 

Surveys and Forecasts

Bertha armyworm monitoring: Counts from the bertha armyworm traps are starting to rise, but most areas of the province are still in the low risk category, which is a cumulative count of less than 300 moths. The only count over 300 so far is from a trap near Durban, which has a cumulative count of 528.

Table 2. Highest cumulative trap counts for bertha armyworm in Manitoba as of July 18, 2011

Location

Cumulative
Trap count

Durban

528

Bowsman

242

Glenboro

221

Minitonas

212

Neepawa

209

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