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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

August 2, 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

Wheat Streak Mosaic Virus: There is a confirmed case of Wheat Streak Mosaic Virus(WSMV) in the La Riviere area. Symptoms of this disease include light green and yellow streaks on the leaves. In severely diseased crops, where infection is present early, stunting of the plants may also be observed. This virus is transmitted via the wheat curl mite. Both the virus and the vector overwinter on winter wheat crops and wheat volunteers. In the spring the vectors are transferred via the wind from the infected winter wheat and volunteers to neighbouring spring wheat fields. This is what is known as the "green bridge" mode of transmission, in other words as long as there is green wheat tissue available to the vector the virus will continue to spread. Growers in affected areas need to be vigilant in scouting for this disease and ensuring control of wheat volunteers in both fall and spring. Environmental symptoms may resemble WSMV so if you are not sure what you are seeing contact your local GO representative to have a sample sent in to the Crop Diagnostic Lab.

Armyworms: Some high levels of armyworm larvae were reported over the past week in the Swan River valley. Some higher levels were also reported from the Beausejour area, but most were over an inch long and it appeared the population was starting to turn to pupae. Some armyworms collected from the Winkler area that I have been rearing are all now pupae, so we should notice the larval populations drop dramatically. They can produce a second generation in Manitoba, but this is often too late to do much economic damage. For those scouting cereals in the Swan River valley, keep an eye on the size of the larvae. When they get over an inch long they will be pupating soon.

Canola

Diamondback moth: Levels of diamondback moth continue to be high, particularly in the Beausejour area, although some high populations have also been reported recently in the Niverville and Dugald area, the northern region of the Swan River valley, and the northern Interlake. Over the past week many of the larvae have turned into pupae. One of the more common questions last week was how long it will take until we see larvae again. The table below shows how long diamondback moth spends in each stage.

Table 1. Duration of development (days) for various stages of diamondback moth at 20 and 25ºC

Duration of development (days)

 

            Stage (instar) of larva

 

Temp (ºC)

Egg

1

2

3

4

Egg & larva

Pupa

Total

20

5.5

3.8

2.6

2.2

3.3

16.9

6.7

23.6

25

3.0

2.0

1.9

1.6

2.9

11.1

5.1

16.2

Some of the earlier seeded canola may be beyond where diamondback moth is a concern by the time the next generation of larvae appear, but in some of the later-seeded canola diamondback moth larvae may be something to continue to monitor.

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

Lygus bugs: Some economic levels of Lygus bugs have been reported from the Niverville area. This is an insect that you need to use the sweep net for to determine levels in canola. The following link includes tables to help determine economical levels of Lygus bugs in canola:
https://www.gov.mb.ca/agriculture/crops/insects/fad12s00.html

Pulse Crops

Soybean aphids: Soybean aphid levels have been increasing in central Manitoba. There still seems to be some confusion over when it is economical to control soybean aphids. The economic threshold (where control is advised to prevent economic damage) is not 250 aphids per plant. The economic threshold is when there are on average at least 250 aphids per plant, and the population is increasing, and the plants are in the R1 (beginning bloom) to R5 (beginning seed) growth stages. Studies at multiple location have found that the economic injury level (level of insects that cause damage equal to the cost of preventing the damage) for soybean aphids on soybeans is about 670 aphids per plants. So you don't want to exceed this. In every field the amount of natural enemies and other factors regulating the population of soybean aphids is going to differ, but if these regulating factors are low the soybean aphids can potentially increase from 250 to 670 in about 7 days. This is why the economic threshold has been set at 250 aphids per plant on average and the population still increasing. If it is not likely to get to 670 per plant on average, then you would spend more controlling the aphids than they will cost you in yield. Every field will be different; some may build to about 250 aphids per plant on average and stay at that level, which would not be economical to control, while populations in other fields may keep advancing beyond the 250 per plant average, in which case your odds of the control being economical are greater. Also note the threshold allows for 7 days for aphids to increase from the economic threshold to the economic injury level. One question that sometimes comes up is if the value of soybeans increases or is high, is it reasonable to lower the economic threshold. Lowering the economic threshold is not the best approach when soybean prices are high. The economic threshold has already been set well below what the economic injury level is. When soybean prices are high, effort should be made to get the control applied quicker than 7 days. The economic injury level is influenced by the value of the crop, and would be lower when the crop value increases. Considering the large gap between the economic injury level and economic threshold, and the fairly long amount of time estimated for aphid populations to get from economic threshold to economic injury level, just make sure to get the insecticide applied shortly after the economic threshold is reached when the price anticipated for the crop is good.

The photo below show soybean aphids on a soybean leaf. The 2 slug-like insects on the leaf are both larvae of hover flies, although different species. There are about 500 species of hover flies in Canada (these are the bee-like flies you often see hovering in front of flowers) and one of the favorite foods of the larvae are aphids.

Soybean aphids and hover fly larvae
Figure 2. Soybean aphids and hover fly larvae

For those scouting soybean fields, please note that it is impossible to accurately count soybean aphids when numbers are high, and would be extremely time consuming. All you can realistically do is try to estimate roughly how many are on a plant. Also note when you are doing your scouting to select plants to do counts on randomly. If you look for plants with lots of aphids on them to do your estimates on, your estimates will not reflect the average population in the field. In many fields there are currently pockets where levels are over 250 per plant, but many plants around these pockets that have very little or no soybean aphids on them. In many of these situations the fields are not above 250 per plant on average. So don't panic if you find a few pockets of higher levels in the field, but do keep watching the overall levels and how well things seem to be regulated.

Spider mites on soybeans: A sample of soybeans from the Oak Bluff area, with leaves with brown and yellow speckling, was received at the diagnostic lab last week. The leaves contained spider mites. Spider mite populations can potentially build to noticeable levels in drier years. The species we sometimes get on the soybeans in Manitoba is the twospotted spider mite (Tetranychus urticae). In past years when they have been more abundant, we have seen some fairly dramatic edge effects with spider mites, where some of the plants near a field edge had lots of spider mites and feeding, and the interior of the field had very little. At this point this is something else to look for when scouting soybean fields. If you see leaves that have some speckling and webbing, tap them over a tray or solid surface (I often take an old camping wash-pan into the fields with me to tap plants over) and look for tiny specks that will be moving around. Although the photo below makes them look big, spider mites are only about 0.3 to 0.4 mm long when fully grown, so you are looking for very tiny specks that are moving.

Twospotted spider mite on soybeans
Figure 3. Twospotted spider mite on soybeans

Also note, there is another group of mites that can occur on soybeans called phytoseiid mites, which are predaceous and will eat the spider mites. They will be orange or white but will not have the black spots on either side of the body that the twospotted spider mites have.  

Surveys and Forecasts

Grasshopper Survey: Manitoba, Saskatchewan and Alberta have for many years surveyed grasshopper populations in August to predict the regional risk from grasshoppers the following year. The data is mapped, and this forecast is used by farmers, agronomists, and agricultural retailers to plan for the following season.

A reminder to farm production advisors, that counts are done during August, when the majority of grasshoppers are in the adult stage. Agronomists and farmers who would also be interested in estimating grasshopper numbers in the fields they are in and have this information included in the survey are encouraged to see the survey protocol for more details of the survey and where to send data. Estimates of grasshopper levels can be collected during regular farm visits. The grasshopper survey protocol is located at: https://www.gov.mb.ca/agriculture/crops/insects/fad95s00.html

Please note that the forms still say 2010 and because of the publication ban I am not able to repost the protocol with the 2011 dates on the form. Use these forms and I'll make the date adjustment when the forms are sent in.

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 and Neepawa areas have reached a cumulative count in the uncertain risk category. A trap near Durban has counts in the moderate risk category, having a cumulative count of 1,051. At most sites the moth counts peaked around mid- July and are now declining. The only report so far of levels of larvae of concern was from the Glenboro area. Agronomists and farmers in the Swan River valley, Neepawa area, and Glenboro area should make sure to scout fields for bertha armyworm. Note that for bertha armyworm or any of the insect pests of canola, no insecticides can be applied within 7 days of swathing. So make sure to scout early enough that if problems are encountered there is time to do something about them.

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

Location

Cumulative
Trap count

Durban

1,051

Minitonas

365

Bowsman

330

Neepawa

320


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

The trapping period for adult bertha armyworms is now complete, and traps can be pulled.