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ISSUE 13  July 30, 1998

 

EVALUATE WHEAT MIDGE LARVAL INFESTATIONS

    Midge larvae are present in wheat heads, now. It is time to evaluate midge levels in those heads. An infested wheat head does not change its appearance so it is necessary remove the glume and expose the kernel or rub drier heads. We would suggest that random samples of wheat heads (approx. 20 heads per site, 4 to 5 locations) for this type of evaluation. This needs to be determined before midge drop from the heads after rainfall or heavy dews. So do not delay too long to look for the larvae.

    If you evaluate a field for potential yield reduction due to this insect, you want to establish the percent infestation level for the field, not just selecting some of the worst infested wheat heads.

    Research has shown that the percentage yield loss is closely related to the percentage of kernel infested with wheat midge. Entomologists estimate that an average of 13 larvae per head will reduce wheat yields to a point where control becomes profitable. For example, an infestation level of 2% reflects a larval population of 1-2 larvae per head (or 0.033 larva per kernel). The relationship between yield loss and the number of midge larvae and infestation level is summarized in the table below.

Larvae per kernel

% infested kernels

Estimated % Yield Loss

1

30-38%

40%

2

58-60%

65%

3

78%

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4

90-96%

79%

Janet J. Knodel
Area Extension Specialist Crop Protection

 

CORN BORER UPDATE

    Moth captures throughout the southeast quarter of the state have declined. The peak of moth activity occurred during the first half of July. With the decline in moth numbers, egg laying activity has also declined.

    At this time, if field scouting indicates larval populations are below threshold levels, treatment would not be recommended.

    If there are treatable corn borer larvae still present on plants, and the field is very close to threshold levels, treatment should be considered. All fields will continue to attract corn borer moths during the next flight period. The egg laying from these August moths will be spread out over the month. Treatments targeting these August larvae are difficult because only about 25% of this group ever seems to be controllable at any one time. Therefore, treating a field that is near threshold at this time, will minimize the cumulative infestation that could occur by September.

 

GRASSHOPPERS ON THE MOVE

    With small grain harvest underway, grasshopper movement from these maturing fields is occurring. In general, adult grasshoppers at numbers of 8 or more per square yard are capable of causing enough feeding injury to impact yield of field crops. When monitoring movement into fields, it is important to assess feeding injury before making a treatment decision. Field crops often require defoliation levels of 20% or more before effects on yield are realized. Direct feeding on pods in bean or ears in corn crops should also be considered.

    Treatment guidelines for grasshoppers on corn are not well established. Information on beans, specifically soybeans is a little better. Many of these infestations will be the heaviest on the field margins. Treating these areas may lessen the total numbers of grasshoppers successfully entering a field.

    Soybeans are most sensitive to defoliation during pod development (Growth stages R4 to R6). During this time, plants can tolerate up to 20% defoliation. Of greater concern, would be direct feeding damage to pods and seeds. Grasshoppers are able to chew directly through the pod walls and damage seed directly. If more than 5 to 10% of the pods are injured by grasshoppers, an insecticide application may be warranted.

 

WHEAT STEM SAWFLY AND HARVEST DECISIONS IN NORTH DAKOTA

    There has not been a great deal of focus on wheat stem sawfly over the past three seasons. When it has been a problem, it has been in the drier areas of western North Dakota. In fields with a history of problems, it is time to assess whether sawfly is present in fields and at what levels. By inspecting fields now, you may be able to plan cultural practices designed to limit losses and impact overwintering survival. Waiting till harvest or fall will limit your options, so don't delay.

    The larva is 1/2 to 3/4 inch long, dull white in color, has a distinct brown head and assumes an S shape when removed from the stem. As the stem dies, the larva move downward within the stem. When the larva reaches a point about one inch above the soil line, it cuts a V-shaped notch around the inside of the stem. When wheat lodges due to sawfly, the stem breaks at the point where the notch was cut.

    Surveying for sawfly infested stems: In areas where strip cropping is done, at least 2 samples should be taken per field or wheat strip. One sample is taken near the long margin of the field within the first few drill rows. The other should be near the center of the strip. At each site, examine 50 consecutive wheat stems of a drill row. Infested stems may still have a reddish brown spot below the second or third node. In some cases the stem will be discolored between the nodes and not form a distinct spot. When present, these spots are very reliable in identifying infested stems.

    Infested fields are placed into one of four stem infestation ratings: LIGHT, 0 to 5 percent; MODERATE, 6 to 24 percent; HEAVY, 25 to 39 percent; and, SEVERE, 40 to 100 percent. If your ratings are moderate or higher, you should be considering swathing instead of straight cutting to minimize lodged stems.

   Harvesting options: Growers have two options available to them to try and limit potential lodging losses. Swath infested fields prior to the occurrence of lodging or Straight-cut the most heavily infested fields first before significant lodging has a chance to occur. The weight of the head and the wind are what cause the stem to break at the notch cut by the larva.

 

PREPARATION OF GRAIN BINS

    It's that time of year for cleaning and preparing bins to receive this year's crop. This is an important program for preventing insect infestations.

1. Clean outside around the bin, beneath perforated floors and inside ducts in addition to cleaning the bin. Sweep or vacuum grain dust and old grain from floors, walls and ceilings where hiding places exist for stored grain insects. If you can tell what has previously been in the bin, it is not clean. In bins where the perforated floor cannot be easily removed, chloropicrin grain fumigant can be applied to control insects in the sub_floor area. Chloropicrin is a highly toxic chemical and as such, all label instructions and safety measures must be carefully adhered to. Debris and grain spills outside the bin also encourage rodents and insects which can then move in through openings.

2. When possible, avoid filling bins with new grain where some old grain is already present. This creates an ideal situation for insects in the bran bug group (sawtoothed grain beetles, flour beetles and the like).

3. Roof leaks commonly lead to columns of spoiled grain. Check for these leaks by looking for light coming into the bin. Moisture coming into the bin through the seal between the bin and concrete will cause spoilage around the perimeter of the bin at the base. Check the seal since sealants do deteriorate. Water will run away from the seal at the base of the bin wall if the concrete is sloped away from the bin. Also check the seals around the doors and hatches.

4. After cleaning and repairing, use a residual bin spray to treat the insect surfaces of the bins at least two weeks prior to filling. Recommended bin sprays are methoxychlor 2 lb EC, Reldan 4 lb EC or Tempo, applied according to label directions.

5. If grain is to be held in storage for a year or more, it should be treated with a grain protectant such as malathion or Reldan. Be aware, however, that malathion will not control Indian meal moths, which commonly infest stored commodities in the state, and neither Reldan nor malathion will control lesser grain borers.

Note: The American Malting Barley Association has a policy that does not allow for the use of any residual insecticide on malting barley. This would include the products malathion, and Reldan. These insecticides are allowable for use as residual bin sprays in bins that will be filled with malting barley. According to AMBA policy, the only chemical allowable for use on malting barley is phosphine (aluminum phosphide) fumigant to control an existing infestation.

It is very important to note that in situations where grain drying is necessary, an insecticide protectant should be applied after the grain has gone through the drier. Commercial grain driers generate enough heat to rapidly degrade insecticides applied to grain prior to the drying process.

6. In the fall, aerate to cool the crop and create a better storage environment in any bin larger than 2,000 to 3,000 bushel capacity. Determine if the aeration system in your bins will provide at least 1/10 cubic foot per minute (cfm) airflow for each bushel of crop being stored. Check the condition of the entire aeration system. You'll also need at least one square foot of opening in the bin for each 1000 cfm of airflow to allow the air to enter or exit from the bin.

Phillip Glogoza
Extension Entomologist


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