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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

August 8, 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

Canola

Bertha Armyworm: Some higher populations of bertha armyworm have been reported in an area of the Central Region from Pilot Mound to Glenboro to Treherne. High populations have also been reported near Austin and from canola fields in the Interlake near Teulon and Arborg.

One of the questions that some are asking is how long it takes the bertha armyworm to go through its larval stages. While some factsheets suggest it takes 6 weeks, this is only likely to be the case under very cool conditions. The following table shows how long it takes the bertha armyworm to go through the egg and larval stages at different temperatures.

Mean duration in days of the egg and larval stages of bertha armyworm at constant temperatures

 

Stage

                                                                  Temperature (ºC)

16

20

24

28

 

 

 

 

 

Egg

10.5

7.1

4.8

3.4

 

 

 

 

 

Larval Instar 1

7.6

4.9

3.2

4.1

              Instar 2

4.8

3.5

2.3

1.6

              Instar 3

5.0

3.4

2.3

1.8

              Instar 4      

5.0

3.7

2.3

2.0

              Instar 5

5.4

3.9

3.0

2.5

              Instar 6

15.1

10.4

7.9

6.0

 

 

 

 

 

Total Larval stage

42.9

29.8

21.0

18.0

From: Bailey, C.G. 1976. The Canadian Entomologist. 108: 1339-1344.

When temperatures are averaging in the low 20's, 3 to 4 weeks is a more realistic length of larval development that can be expected. Also note that although the last larval stage (instar 6) can be quite long, for a few days at the end the larvae will not be feeding as they get ready to pupate.

When they are fully grown, larvae of bertha armyworm are about 4 cm (1.5 inches) long. One of the things that can make scouting confusing is that there can be 3 main colour forms of bertha armyworm; black, brown or green. The figures below show the black and brown colour forms of bertha armyworm.

betha armyworm 1 bertha armyworm 2
Figure 1. Bertha armyworm Figure 2. Bertha armyworm

Lygus bugs: Some economic levels of Lygus bugs continue to be found in canola fields in the Niverville, Altona and Beausejour areas. 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
A study by entomologists at Agriculture and Agri-Food Canada in Winnipeg found that samples taken along the edge of commercial fields and at various distances into the field all gave similar estimates of Lygus bug densities (The Canadian Entomologist. 1998: 837-851). Thus when canola stands get more difficult to walk through, this is one insect that you can potentially get away with sampling near the edge of the field.

Diamondback moth: In some areas diamondback moth seems to be less of a concern due to most of the population being in the pupa stage, but there are still some areas where numbers of larvae are high and of concern. Some fields in the Dauphin/Grandview area were reported to be above threshold, and some canola fields in the Teulon, Arborg and Riverton areas were sprayed for diamondback moth. Some high levels of larvae were also reported from the Niverville area.

A reminder that if applying insecticide for any insect in canola, the shortest preharvest intervals are 7 days to swathing or direct-combining. Once a canola crop is within 7 days of swathing, insecticides can not be applied for any insect.

Protecting Honey bees: A reminder that if insecticides will be applied and crops are still flowering to take precaution to protect honey bees and other pollinators. This will be of obvious benefit to beekeepers, but can also help farmers maximize the yield and value of their crop. Even crops that don't rely entirely on insect pollinators can have a yield increase when pollinators are present in the field. For example, the yield of canola can be increased 5-10% by the presence of honey bees. So if we are expecting about 30 bushels per acre, and $10 per bushel, if we use the lower estimate and assume a 5% increase in yield due to the bees, bees are likely increasing profit by about $15.00/acre.

If an insect pest is above an economic threshold, delaying applying an insecticide until after flowering is complete will ensure these yield advantages are achieved and pollinators protected. But if it is decided that insecticides should be applied during flowering, there are a few things that can be done to reduce harm to the bees

• Spray as late in the day as you can. Honey bees go back to their hives at night, and reenter the field the next day. So spraying in the evening will minimize contacting the bees.

• Use an insecticide that is less harmful to bees. A table on "Field Hazards of Insecticides to Bees" on page 388 of the 2011 Guide to Crop Protection can help with this.

• If you know of beekeepers that may have hives near your field, contact them at least 48 hours prior to applying insecticides to the field if possible. There are steps they can take also to minimize harm to the bees.

Pulse Crops

Soybean Insect Update: Soybean aphids remain the top insect concern in soybeans. Although some populations above economic threshold have been found, in some fields levels have been approaching but have not exceeded the economic threshold of 250 per plant on average and the population still increasing. Natural enemies can get to levels where the soybean aphid population will plateau and stop increasing. But every field will be different, so soybean aphids are something to keep checking. Once the soybeans reach the R6 stage the damage the aphids can do becomes less.

Both spider mites and green cloverworm have been found in some soybean fields, but at levels that appear to not be of economical concern so far.

 

Insect Identification Quiz

Over the past week there have been several calls and Emails to help with the identification of insects that turned out to be natural enemies. So this week we will reintroduce our Insect Identification Quiz to the updates with a focus on predaceous insects.

The 3 insects pictured below are all predators of other insects. See if you can identify them.

lady beetle hover fly damsel bug
Figure 3. Figure 4. Figure 5.

Figure 3 is the larval stage of a lady beetle, this species is the sevenspotted lady beetle.
Figure 4 is the larva of a hover fly. There are many species, as mentioned in last week's update, and green and brown are the 2 most common colours for the larvae here in Manitoba.
Figure 5 is a damsel bug. They are quite common this year and their diet includes such insects as aphids, small caterpillars, mites, and even Lygus bug nymphs.