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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect Update

July 21, 2008                   

Compiled by: John Gavloski, Entomologist, Manitoba Agriculture, Food and Rural Initiatives,
Crops Knowledge Centre, Phone: 204-745-5668; Fax: 204-745-5690

To report observations of insect activity or control that may be of interest or importance to others in Manitoba, please send messages to the above contact addresses.

To be placed on an e-mail list to be notified when new Manitoba Insect Updates are posted, please contact John Gavloski at the address or numbers listed above.


Summary

Alfalfa weevils continue to be a concern on alfalfa. Grasshoppers are a concern in localized areas and some controls continue to be applied. Wheat midge continue to emerge; some insecticides have been applied in the western part of Manitoba, but generally many fields are too far advanced for wheat midge to be able to damage the crop. Some higher populations of pea aphids in peas have been controlled in the western part of Manitoba. Bertha armyworm trap counts are at moderate levels in the western part of the province.  Larvae of sunflower beetles and sunflower bud moth are quite abundant in some sunflower fields in the Eastern and Central parts of the province.

Recent Insect Concerns and Observations

Aphids on peas: Some economic levels of aphids in peas and control have been reported from the southwest part of the province. One of the big questions this week is how and at what stages aphids injure peas? Aphid feeding on peas in the flowering and early pod stage can result in lower yields due to less seed formation and smaller seed size. Protein content and other quality issues do not appear to be affected.

The following table relates the yield loss in peas for average aphid counts from 1 to 8 per 20-cm tip of a field pea stem when about 25 % of the crop has begun to flower.

Aphids per tip

% yield loss

1

3.4

2

4.9

3

6.1

4

7.1

5

8.0

6

8.8

7

9.6

8

10.3

Most of the damage that aphids do to peas is to the pods before they start to fill. If you think that most of the pods have already started to fill, spraying would be too late and would not be economical.

Why will wheat midge not feed on wheat that has flowered? In an earlier update, we looked at flowering in wheat and it was stressed  that once wheat flowers, it is resistant to wheat midge and insecticides are not needed (and detrimental to long-term wheat midge management), even if high levels of wheat midge are present in the field after flowering. The reason for this is that when wheat flowers it changes chemically, and a substance called ferulic acid is produced. Ferulic acid makes the wheat an unsuitable host for wheat midge and larvae on the plant after ferulic acid is produced will starve to death. Wheat midge can still be flying at night in large numbers in wheat fields that are at or past flowering. And they may still lay eggs on the heads. But larvae from these eggs will not be able to feed on the wheat and will starve. So there would be no reduction in wheat midge feeding by applying insecticide to a wheat field that has flowered, even if you are still seeing a lot of wheat midge in the field. There is also a parasite, Macroglenes penetrans, that is very effective at reducing wheat midge numbers. Data from Saskatchewan indicated that on average 30 to 40% of the wheat midge population is killed by this parasite in a given year. Insecticides applied for wheat midge will kill this parasite. So applying insecticides after flowering not only does not reduce wheat midge feeding, but kills off the parasite that hopefully will be reducing the population going into next year. So more harm than good is accomplished. Insecticides can be a valuable tool in wheat midge management, but proper consideration of economic thresholds and crop staging is very important.

Alfalfa weevil update: More reports of alfalfa fields with extensive damage by alfalfa weevil are coming in. The damage appears to be quite widespread across the province. Once most of the larvae turn to pupae, the significant defoliation for this year will be done. The period of time with larvae present has been very long this year. While taking sweep net samples in alfalfa in the Carman area late last week, we were still able to find various stages of larvae, although pupae and adults were also present. Those growing alfalfa for hay need to be aware of the potential damage alfalfa weevil can do, and check their fields regularly so if needed they can adjust their cutting date to minimize the damage and reduce the population of alfalfa weevil larvae.

Leaf Diseases in Corn: Dr. Phil Northover, plant pathologist with MAFRI, provides the following update regarding leaf diseases in corn.

Northern Corn Leaf Spot ( Bipolaris zeicola) and Northern Corn Leaf Blight (Exserohilum turcicum) , have been observed and confirmed in three corn fields in the past week on corn. While this does not appear to be a widespread outbreak, and three fields is by no means an epidemic, this is a disease that could pose some producers problems if wet and cloudy conditions continue for the summer. Northern corn leaf spot is generally restricted to lower leaves and is often of little concern. Northern Corn Leaf Blight however can be quite damaging especially when leaves above the ear are heavily spotted right after pollination.

Recent temperatures and weather conditions have been very conducive to spread of Northern Corn Leaf Blight with rainfall and temperatures between 18°Cand 27°C, being the most conducive for infection and the highest degree of damage.  Northern corn leaf blight tends to occur in areas associated with humid conditions. The fungus can survive in crop leaf debris, so planting back into corn should be avoided for at least one season. 

Most field corn hybrids have adequate resistance that fungicides are not warranted, unless disease pressure is high. Sweet corn hybrids are often at much greater risk, and fungicide application can be economical. For more information on these leaf diseases in corn, see: http://www.mwvr.info/corn(sweet)

 

Surveys and Forecasts

Bertha Armyworm: Highest trap counts continue to be in the western part of Manitoba. Highest trap counts so far are 883 (Virden), 681 (Minitonas), 411 (Brookdale), and 352 (Neepawa). Counts in this range put these regions in a risk category labeled “Uncertain”; where infestations may not be widespread, but fields that were particularly attractive to egg-laying females could be infested. Data from the bertha armyworm adult monitoring program are available at:

https://www.gov.mb.ca/agriculture/crops/insects/bertha/index.html

Crop Scouting Reminders

Prior to harvest is a good time to be cleaning out bins and equipment in anticipation of the coming harvest. Insects that feed on stored grain can survive on stored grain in farm equipment, as well as grain in bins. So in addition to cleaning out bins, it is also a good idea to clean out any old grain or grain dust that has accumulated in combines, truck beds, grain wagons, augers, and other equipment used to move grain. Otherwise the grain could be going into the bins already containing stored grain insects.

Insect Identification Quiz

                                    

Question: While scouting sunflowers you notice that on some plants there are these piles of black material (Figure 1) in clusters on either the buds or leaf axils of the plants. Upon splitting the bud or stem near these black piles you notice white larvae (see Figure 2) inside the bud or stem. What are these larvae.

Answer: These piles of black frass are left behind by larvae of the sunflower bud moth. They are quite noticeable in some sunflower fields in the eastern part of Manitoba currently. When they get into the buds they can cause the heads to be distorted. There are no controls for them, and they have always been considered to not be highly economical. But in recent years there has been a lot of damage to buds in some fields, and some would contend that the damage can be significant. However, there is currently no methods or products developed for managing this insect.