Summary

Pea aphids and wheat midge have reached economic threshold levels in some fields in the southwest. High counts of bertha armyworm moths are occurring in quite a few traps, suggesting a larger area of the province could have problems with the larvae this year than was the case last year.

Recent Insect Concerns and Observations

Timing of Bertha Armyworm Control

Twice last week I had calls from northwest Manitoba regarding farmers wanting to tank mix in an insecticide with their fungicide for sclerotinia management to try to kill adult moths of bertha armyworm so they wouldn’t have to control the larvae later. The rationale was that an application to the field was already being made, and the insecticide was cheap, so it can’t hurt to try this, right?

Actually it can do a lot more harm than good trying to manage insect populations that are below economic threshold. And in this case it is also strongly not advised because the application is trying to target a stage where there is no data to show that the application can repeatedly result in a significant reduction in damage; my gut feeling is that any control achieved would be spotty and not repeatable. And, if there is a high population of adult moths in the area, odds are there are already some eggs that have been laid on the plants which will not be killed by the insecticide. So spraying for larvae may be necessary later anyway.

Ultimately, one of the factors that has a large role in ending an outbreak cycle of bertha armyworm is natural enemies. There is a hairy fly, known as a tachinid fly, and a large orange ichneuminid wasp that over the long term can be very effective at reducing bertha armyworm populations. Spraying insecticides when we are not targeting an economic population, can cause harm because it kills these parasites.

Even if the insecticide were free, it would not be advised to be spraying to try to see what kill of adult bertha armyworm moths could be achieved.

Agronomists who are aware of this practice occurring in there region; please try to get this information out.

Wheat Midge

There has been some insecticide applied to wheat in the Hamiota area to control wheat midge.

A lot of wheat is advancing quite quickly. When wheat flowers (this is when you see the yellow anthers on the heads) there are chemical changes that occur that make the wheat naturally resistant to wheat midge. Thus insecticides targeting wheat midge should not be applied to fields that are already flowering. This can do more harm than good since not only is the wheat no longer susceptible to wheat midge, but you will also be killing the parasites of wheat midge and other natural enemies of potential pests in the field. The critical stages to scout for wheat midge, and apply controls if economic thresholds are surpassed, is from as soon as heads are visible until about 75% of the heads have anthers.

A factsheet with more detailed information on wheat midge is available on the MAFRI website at:

• http://www.gov.mb.ca/agriculture/crops/insects/fad22s00.html

Wheat Stem Maggots

White heads of wheat are noticeable in some fields that should otherwise still have green heads. One possible cause of this is wheat stem maggot. There are diseases and environmental factors that can also cause white heads in wheat, so if is not always immediately obvious what the cause is. If wheat stem maggot is the cause, most of the heads will easily slide out of the stem when you pull on them. And if you slit the stems you may be able to see the pale green maggots inside. They also tend to leave frass in the stem that looks like sawdust. In some of the fields around Carman the white heads are quite noticeable, and they have also been reported to be quite noticeable in the Treherne/Holland, Neepawa, and many fields in southwest Manitoba areas.

This is another example of something that can be very noticeable, because of the way the white heads stand out, but is rarely of economic significance. If even 1 in a few hundred plants have a white head, it often looks bad to some when scouting the field because the white heads are so noticeable everywhere you look. But the reality is that this will have very little impact on overall yield. And there are no insecticides registered for wheat stem maggot anyway. So it is worth checking out what is the cause of the white heads, but for wheat stem maggot it is not something to get too alarmed over.

There are crop production practices that can help minimize future populations of wheat stem maggot. These may need to be considered if growers or agronomists are concerned over the levels seen in fields. More information on wheat stem maggot can be found on the MAFRI website at:

• http://www.gov.mb.ca/agriculture/crops/insects/fad19s00.html

Surveys and Forecasts

Bertha Armyworm Trap Results: Results from monitoring adults of bertha armyworm continue to climb. A trap near Bradwardine has now collected over 1200 moths, so this region of the southwest is at high risk, and a trap near Swan Lake is now at 1162. There are several traps in the 500–1000 range.

As mentioned in earlier updates, the early indication is that potentially economical populations could be found a lot further east in the province than last year. This data should be used as a guide to help prioritize field scouting for the larvae. In many areas of the province it will be critical that canola fields are scouted frequently from about mid-July to mid-August to know what the bertha armyworm levels are.

Also note that populations of larvae can vary greatly between fields that are close together. Crop staging at the time of egg laying is one of the factors that can influence this. So it is important to check all fields and not make assumptions based on levels in nearby fields.

Trap data, a risk map, as well as a table indicating how to interpret the trap data can be found at:

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