Final Regular Update for 2006

This is the last of the regular Manitoba Insect and Disease Updates for 2006. Should there be the need for additional updates over the fall and winter (due to additional pest problems, problems in stored grain, new product registrations, etc.) these will be provided on an as needed basis.

Soybeans

Spider mites: High populations of spider mites continue to be found in some soybean fields in the Red River valley. However, in most of these fields the soybeans are advanced beyond the point where leaf damage due to spider mites can have significant economic consequences.

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.

All of the soybean fields we looked at last week were at least in 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.

Our conclusions were that the soybean plants had advanced to the stage where applying insecticide to control high mite population would not likely result in a significant increase in yield and would be uneconomical at this point in the season.

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.

Two-spotted spider mites on soybean leaf

Grasshopper Survey Update

So far data from 125 sites has been reported for the grasshopper survey for 2006. Numbers have been low so far from the data that has come in; 6 sites have had counts in the moderate risk category (8-12 / m²), and the rest of the sites have all been in the light or very light categories. There have been no sites with numbers in the severe or very severe categories yet.

A reminder for those collecting data for the survey that estimates of grasshopper numbers should be completed in August. Even for sites where numbers are known to be low, low numbers are still good data as it means the area will be represented on the forecast map that comes out and provides farmers and agronomists with some estimate of grasshopper numbers in the region.

The protocol and data sheet for the grasshopper survey are found at: http://www.gov.mb.ca/agriculture/crops/insects/fad95s00.html

General Disease Update

I hesitate to even say it (for fear that things could still change) but this has been a banner year for a LACK of major disease concerns in field crops. This all began mid-season when things turned generally drier and since then rainfall has been very localized. We have not been reporting on disease issues for several weeks because there have NOT been significant findings from the field. One minor concern is that anthracnose symptoms have been documented on Envoy beans, a variety that has, for a long time, had apparently durable resistance to the disease. We have yet to see symptoms in Envoy that end up on seed.

Fusarium head blight in cereals has been conspicuous by its absence. Reports from the field and discussion with Canadian Grain Commission officials suggest that levels in all cereals harvested to date have been very low. Interestingly, it is the first time in 5 years that levels in barley have been so low that there has actually been some malt selection from barley grown in Manitoba. That’s good news! The lack of infection is primarily due to a lack of the conditions that are conducive to infection – the coincidence of heat and high humidity at flowering. We cannot say that it’s a thing of the past. However the positive benefit from fungicide spraying is more likely attributable to the control of leaf rust then Fusarium head blight.

Insect Identification Quiz

Question: While looking along the roadside vegetation at the edge of the fields, you notice a lot of these grasshoppers with long antennae. There are quite a few; should you be concerned?

Answer: These are called long-horned grasshoppers or more commonly katydids. You may even notice several species of long-horned grasshoppers in a grassy ditch. They are not crop pests, but are often abundant in the grassy areas outside of some fields. The males of grasshoppers belonging to this family are noted songsters, and you will hear the drawn out bzzzz of many of them while walking along a field edge this time of year (although many mistakenly think the sound is crickets, which make more of a chirping sound).

Our grasshopper species that are potential pests all belong to a family of grasshoppers known as short-horned grasshoppers (Acrididae). As the names would suggest, the long-horned grasshoppers (non-pest ones) have antennae that are almost as long as the body, and sometimes longer, whereas the short-horned grasshoppers (which include our potential pest species) have much shorter antennae, usually less than half the length of the body. If you can see the type of grasshoppers that are jumping away when you are doing grasshopper survey, make sure you are not counting the katydids, since these are not potential pests.

While using a sweep net to collect and look at the grasshopper species present in the ditches last week, in many samples there were more katydids than pest species of grasshoppers, although there were some exceptions. This ratio will vary greatly from site to site and year to year.