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Manitoba Agriculture, Food and Rural Initiatives

Manitoba Insect and Disease Update

August 22, 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.

Note: This will be the last regular Manitoba Insect and Disease update for 2011. Further updates may be provided on an as needed basis.


Recent Insect and Plant Pathogen Activity


Lygus bugs: Some high levels of Lygus bugs continue to be reported in canola fields, mainly in the Eastern and Central regions of Manitoba. Canola remains vulnerable to feeding injury from Lygus bugs from late-flowering to mid pod, when the seeds are still soft when you roll them in your fingers. You will still find Lygus bugs in the field in the later-podding stages, however the injury they cause will be less.

The affect of Lygus bugs on yield of canola is about 0.12 bu/acre per plant bug per 10 sweeps at the end of flowering (growth stage 4.4 – flowering is complete, seeds enlarging in lower pods) or the beginning of pod formation (growth stage 5.1 - when seeds in the lower pods are full size but translucent). At growth stage 5.2 (when seeds in the lower pods are green) the affect of Lygus bugs on yield of canola is about 0.088 bu/acre per plant bug per 10 sweeps. Economic injury levels have not been developed for Lygus bugs in canola at growth stages 5.3 (seeds in lower pods mottled green-brown) and beyond, as feeding is less likely to be economical.

Figure 1 Lygus bug adult on canola

Late-summer populations of flea beetles:  Over the past couple of weeks, the fall population of flea beetles has become very noticeable in some canola fields. Some have been enquiring whether such populations can be economical. No economic thresholds are currently available for fall populations of flea beetles in canola. Research on this has recently been completed at AAFC in Saskatoon. Except in situations where there are extremely high populations (100’s per plant on average) feeding on plants in the very early podding stages, these populations are not likely to be economical. The biggest concern with the feeding on plants in the early-podding stage is potential increases in pod shattering and loss of seed.

Spraying for flea beetles in the fall for the purpose of reducing the risk the following spring is not recommended. Flea beetles are mobile enough that you could still have problems in the spring, as they move in from neighboring areas. In addition, there can be big differences between fall and spring populations. So while noting where flea beetle populations are high in the fall can help make decisions regarding seed treatments the next spring, a strong correlation between fall and spring populations has never been established.

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.

Canola storage: A reminder that Malathion CANNOT be used to treat bins where canola will be stored or to treat canola as it goes in to storage. These applications can result in residues in the canola that are unacceptable in some of Canada’s key export countries.

Most insects in stored canola will not feed on sound, healthy seed. Moisture-loving fungus feeders such as foreign grain beetles, psocids and mites may be found in canola if it is in poor condition. Primary stored product insects can be found if there is cereal grain mixed in with stored canola.

Alternatives to insecticide - Growers should make sure the bin is free of chaff, seeds, and foreign material. Combine settings should be set to harvest sound seed. Bins should be well sealed to prevent moisture infiltration. It is important to keep canola cool and dry in the bin (for long term storage, temperatures should be kept at less than 15 °C and grain moisture levels below 8 %.)

Cereal Crops

Disease vector considerations when planting winter wheat: While there are many factors to consider when trying to determine the best time to seed winter cereals, another thing to add to the list of considerations is the risks of wheat streak mosaic virus, which is spread by mites, and barley yellow dwarf, which is spread by aphids. Both can be transmitted to winter cereals in the late-summer and overwinter in the crop. Delaying seeding to later in the range of recommended seeding dates (which is August 20th to September 20th if the crop is to be insured) can reduce the risk. To manage wheat streak mosaic virus it is important that a field be free of all volunteer wheat plants and grassy weed hosts for at least 2 weeks before planting winter wheat. This deprives the mites of food, ensuring that the field is starting free of the mites that can transmit the virus.

General Crop Scouting

Spider mites: Being a drier year, we are noticing some feeding by spider mites, particularly on soybeans and in some corn fields. In many instances there are edge effects. In addition to the level of feeding on the leaves, distribution in a field and staging of the crops are a couple of other things to consider.

In soybeans, feeding by spider mites reduces the photosynthetic area of the leaves. A significant amount of this type of damage during the full pod (R4) and beginning seed (R5) stages can result in a reduced number of seeds and smaller seeds. Some soybean fields have already reached the full seed (R6) stage, where the seeds in the upper pods fill the pod cavity. So seed size and number has already been determined in these fields. To complicate the field scouting, leaf yellowing naturally begins during the R6 stage, so distinguishing loss of photosynthetic area due to spider mites from natural leaf color change becomes difficult. In most instances the feeding by the mites will on average not be severe enough or the crop will be advances enough that control would not be economical.

Regarding what to expect for next year with spider mites; it is hard to predict. Spider mite populations are influenced greatly by the weather. Hot, dry conditions generally favour higher mite populations. In most years spider mite populations are controlled by natural factors to the point where they are barely noticeable.

Assessing mite populations can be very frustrating because the mites are very small (< .5 mm), and hard to see on the plants. Also note from the picture below that there can be some variation in colour. It is easier to assess the feeding damage than to estimate mite populations, but in the later plant stages natural colour change and plant pathogens can complicate estimating mite damage.

Figure 2 Two-spotted spider mites on soybean leaf