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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect Update

July 7, 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 continues to be a concern on alfalfa. Grasshoppers are a concern in localized areas and some controls have been applied, mainly to the area around and at the edges of fields. Wheat midge are just starting to emerge. Diamondback moth from adults that blew into Manitoba are now starting to pupate; numbers from the first generation have been low. 

Recent Insect Concerns and Observations

Alfalfa weevil

Alfalfa weevil continues to be a concern in alfalfa fields. Highest populations seem to be in the Interlake, however heavy populations have been reported from fields right across Manitoba.

The larvae are the most damaging stage of alfalfa weevil (see last weeks update for a photo of the larvae). They get to be about 1 cm long when fully grown. So in many fields they should be starting to pupate soon. Adults will be present again later in the summer, but once they pupate the more damaging larval stages will be done for this year. For hay fields it is the first cut and the regrowth of the second cut that are most vulnerable to alfalfa weevil. The cutting process can result in significant kill of alfalfa weevil larvae, however it is still recommended to monitor the regrowth.    

Wheat Midge

Emergence: Wheat midge are just starting to emerge but have not reached high levels in any areas yet. Reports  received from field scouts so far are that only very low levels have been seen.

The Canadian Wheat Board wheat midge emergence maps are another tool that can be used to predict what % of wheat midge may have emerged in a general region. It can be viewed at:

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

Note that this map is based on growing degree day accumulation, and does not factor in soil moisture and other factors that may influence emergence. So as with most forecasts, it is meant to suggest when more intensive monitoring is advised, and not a tool to try to base control decisions on.

When is wheat susceptible to wheat midge? Wheat is susceptible to feeding by wheat midge from as soon as the heads are visible (even if they are not yet above the flag leaf), until anthers appear (the wheat is in flower). Once the wheat flowers, chemical changes have occurred in the grain which make it resistant to wheat midge. So wheat fields should not be sprayed with insecticides to control wheat midge once anthers are present. Doing so is redundant, because the wheat is already resistant to wheat midge, and can also kill natural enemies of wheat midge and other insects, leading to increase problems in the future. In Figure 1 below, the wheat head on the left has the anthers emerged, the wheat head on the right does not.

Wheat Heads (anthers emerged on left head)

Identifying Wheat Midge: Make sure you know how to tell the difference between wheat midge and another type of fly called lauxanid fly when scouting wheat for wheat midge. The following factsheet has a picture (Figure 3) that will help you tell the difference, and may even assist you in passing this weeks insects identification quiz (those who fail may risk paying higher input costs than needed):  https://www.gov.mb.ca/agriculture/crops/insects/fad47s00.html

 

Surveys and Forecasts

Diamondback moth:  Diamondback moth pupae are being found on canola plants. This first generation of diamondback moths has not been at levels approaching economic thresholds. From this point on in the season there will likely be all stages (eggs, larvae, pupae, adults) present in canola fields. Areas that need to be watched most carefully for the second generation are the southeast and southern Interlake portions of Maniotoba. Overall, diamondback moth levels have been low.

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: Trap counts are starting to increase, but numbers still do not indicate threatening populations anywhere. Highest trap counts so far are 220 (Minitonas), and 215 (Virden). Data from the bertha armyworm adult monitoring program are available at:

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

 

Insect Identification Quiz

Lauxanid fly

Question: Your wheat has just started to head out, and you are out scouting to see how abundant wheat midge are in your field. It seems that about every second plant has one of the flies in the photo above on it. What do you do?

Answer: The fly in this figure is not wheat midge, and belongs to a group of flies known as lauxanid flies. These are not pests of wheat or other crops. Lauxanids breed in decaying vegetation, and will be present in many fields, often in high numbers.