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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect Update

June 30, 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 are a concern on alfalfa in the Interlake region. Diamondback moth larvae are showing up in canola fields in the southeast, but not at levels of economic concern. Wheat midge emergence has been delayed this year; emergence has not yet started.

Recent Insect Concerns and Observations

Alfalfa weevil

Defoliation from Alfalfa weevil has become a concern in many alfalfa fields in the Interlake and in the Lac du Bonnet areas. In alfalfa hay fields where they are a concern, many are cutting the alfalfa early to control the alfalfa weevil. In seed fields where alfalfa weevil is a concern, cutting is not an option, and some spraying has occurred. Insecticides applied to control plant bugs prior to putting out leafcutter bees should also kill alfalfa weevil. If using dimethoate (Cygon or Lagon), note that the labels indicate “reduction only”. Some have commented in the past that they were only getting partial control of alfalfa weevil using dimethoate.

Wheat Midge

As wheat approaches or moves into the heading stage, many are wondering about the status of wheat midge. Because of the cool spring, wheat midge are behind normal in their development. So emergence may be a bit later than normal. Normally adult wheat midge begins to emerge during the last few days of June or early July, with populations peaking and potentially at levels of concern during the second and third week in July. Then the population of adult midge begins to decline. I have not seen or heard of emergence of adult wheat midge yet this year. Wheat is susceptible to wheat midge from the time the head is visible until the wheat has flowered (anthers are visible). So, if your wheat will have anthers by the second week in July this year, you will not likely have economic damage by wheat midge, even in regions that may have higher wheat midge levels. It is the wheat that will have headed out but not yet flowered during the second and third weeks in July that is most likely to be at risk of damage by wheat midge. A reminder that if wheat is past the susceptible stage to wheat midge (anthers are present), insecticides targeting wheat midge not only will not provide economic benefit, but can causes harm by decreasing the populations of parasites and predators that keep wheat midge and other potential pests in check. Over the next few weeks I’ll try to post information on when wheat midge are emerging and where “hot-spots” will be. Agronomists and farmers who may have information regarding wheat midge levels that they could contribute are encouraged to send in such information so this can be noted in the updates and others in the region can be made aware of this.

Wheat Midge Emergence Map

The Canadian Wheat Board is producing wheat midge emergence maps based on growing degree day accumulation. The map is available at:

http://www.cwb.ca/public/en/farmers/weather/midge/popups/forecast.jsp

This can be a valuable tool for helping to predict when emergence starts and the % of wheat midge that should have emerged. Maps will be updated daily, Monday to Friday, throughout July. Note that for Manitoba the map indicates that wheat midge should not be emerging in Manitoba yet.

 

Surveys and Forecasts

Diamondback mothDiamondback moth traps have been up for about 8 weeks in many areas, and can now be removed. Highest counts over this period were from traps near Stonewall (56), Steinbach (47), and Emerson (46). These are not alarming numbers, although it is still advised that agronomists and farmers should pay attention for diamondback moth larvae or feeding damage while scouting canola, particularly in the south Interlake and Southeast. Diamondback moth larvae have been noted feeding on canola in the southeast, but not at economical levels. Data from the diamondback moth trapping program and an interpretation of the data are available at: https://www.gov.mb.ca/agriculture/crops/insects/db/index.html

At this point in the season, information on the levels of diamondback moth is best gained by tapping plants over something where larvae can be counted, or by taking sweepnet samples when doing a general assessment of insects in the fields. Taping or shaking diamondback moth larvae off plants to do proper economic threshold assessments is necessary if levels in the net appear high.

Bertha Armyworm: All trap counts reported are very low so far. Bertha armyworm adults are very slow in emerging this year. Data from the bertha armyworm adult monitoring program are available at:

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

 

Publication Update

The economic threshold tables for bertha armyworm and lygus bugs have been updated to account for higher commodity prices than when the original tables were written. These revised threshold tables can be found on the MAFRI website at:

https://www.gov.mb.ca/agriculture/crops/insects/fad12s00.html for lygus bugs

https://www.gov.mb.ca/agriculture/crops/insects/fad03s01.html for bertha armyworm

Also – the Oilseed chapter for the WCCP Guide to the Integrated Control of Plant Pests has recently been revised and includes these updated thresholds. It can be found at:

http://www.westernforum.org/wccp_guidelines.htm

 

Insect Identification Quiz

Alfalfa Weevil Larvae

Question: The insects in this picture are alfalfa weevil larvae. They are the larvae of a beetle, and thus not true “caterpillars”, which are larvae of Lepidoptera (butterflies and moths). What is an easy way to tell alfalfa weevil larvae from caterpillars like alfalfa looper, alfalfa caterpillar and green cloverworm, all of which are green and also occur on alfalfa?

Answer: Look for legs, or the absence of. Alfalfa weevil have no legs, the true caterpillars all have 3 pairs of legs at the front, and 2 to 5 pairs of fleshy legs on the abdomen known as prolegs.

You will also notice the alfalfa weevils are usually green, with a white stripe down the back and a dark head. However, as we saw for the cutworms last week, the colour can vary. Young alfalfa weevil larvae are more of a light yellow, and they usually change to green as they get older. Although the white stripe down the back is usually quite visible, sometimes you do have to look harder to see this. Note that one of the alfalfa weevil larvae in the photo is curled up. When you disturb them they will often curl up and fall from the plant.