NDSU Crop and Pest Report

Entomology


ISSUE 14   August 5, 2004

SOYBEAN APHID SITUATION

Surveys for soybean aphid are finding a few fields with small colonies present, generally on a small percent of the plants sampled (20% or less). One exception in our IPM crop and pest survey was a field in southeast Grand Forks County where that field had greater than 80% of the plants with aphids. This field is in line with a series of fields sampled on both the North Dakota and Minnesota sides of the river that have had aphids detected. In addition, fields along the Red River in the southern end of the valley have had aphids detected on field margins, but at levels of less than 20% of the plants.

The next two weeks will be critical in determining whether soybean aphid will cause problems this season. Scout fields, focusing on the field margins to determine if aphids are present. Remember, the treatment threshold is 250 aphids per plant when present on 80% of the plants.

The recent rains in the region should have a suppressive effect on the aphid populations. Hard rains have been observed to physically wash aphids from plants. Also, fungal pathogens that infect the aphids and kill them become active under these wetter conditions. Predators are being found in the fields where the aphid colonies have been found. These predators will also help to suppress the small aphid populations that are present.

 

SOYBEAN APHID GROWTH ESTIMATOR: The SAGE Model

The University of Minnesota soybean aphid research group has made another valuable tool available that should aid in soybean aphid management. They have prepared a population aphid growth model that runs in an Excel spreadsheet. By inputting aphid density and temperature data, the model will provide a forecast for population growth. The model can be downloaded at:

http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sagemodel.htm

To use the SAGE model, enter the average aphid density (aphids/plant) and today's high and low temperature. Make sure to include the zeroes (no aphids on a plant) when you calculate the average. As you enter the forecasted high and low temperatures for the next 6 days, the model will predict an average rate of population build up (expressed as the time required for populations to double).

The model will also predict when the population is expected to reach a threshold of 250 aphids per plant. The model is likely to over-estimate aphid populations in the field. Use this model as a guide to determine when to resample a field (preferably at least 2-3 days before threshold is reached).

Your entries into SAGE can be saved using the "Save As..." function in Excel. To track populations more carefully, replace forecasted temperatures with observed temperatures as time progresses since you last sampled a field. If you need to move temperatures within the spreadsheet, use Copy and Paste. DO NOT use Cut and Paste; SAGE won't like it.

This information is provided by R.C. Venette, B.P. McCornack, and D.W. Ragsdale. 2004. Soybean aphid growth estimator. ver. 1.2, University of Minnesota, St. Paul.

 

SUNFLOWER INSECT UPDATE

Banded moth should be in fields, now. Red Seed Weevil are beginning to move to any flowering fields present in an area. The IPM crop survey of sunflower fields only had detections in the southwest quarter of the state. More reports of finding weevils are expected with this weeks survey. Last year’s National Sunflower Association regional survey detected the greatest levels of seed damage from weevil in the south central to western counties. Lower damage levels were found in other areas, but keep in mind damage was found. Scouting seed weevil in confection and oilseed fields is important this season.

Red Seed Weevil Management in Oilseeds

To decide whether to use an insecticide treatment to control red sunflower seed weevils, it is necessary to determine the economic threshold for your situation this year. The economic threshold (ET) for red sunflower seed weevil depends on the following variables:

  • the cost of insecticide treatment per acre
  • the market price of sunflower in $ per lb.
  • the plant population per acre
  • ET =                         Cost of Insecticide Treatment                                   
    Market Price x 21.5 ((0.000022 x Plant Pop/n) + 0.18)

    Red Seed Weevil Economic Thresholds (Weevils per head) should be approximately 7 weevils per head.

    Sunflower plant stage is used to time insecticide treatments. For oilseeds, the ideal plant stage to treat is when most plants in the field are at 40 percent pollen shed. However, we recommend that treatment be considered when more than half of the plants in the field are just beginning to show yellow ray petals to 30 percent of the heads shedding pollen and the rest of the plants in the field are still in the bud stage. The consideration of treatment at the early bloom stage should allow growers a sufficient cushion of time to have their fields treated. Growers must be aware, however, that if weevil populations are high and/or spraying is done too early, a reinfestation may occur and a second insecticide application may be necessary.

    Insecticides registered for seed weevil control include Asana, Baythroid, Lorsban, Methyl parathion, Scout X-TRA, and Warrior.

    Confection or Hulling Market Treatments

    The banded moth, seed weevil, and the Lygus bug have all impacted quality of these sunflowers the past five seasons. It is recommended at this time, that sunflowers grown for these markets be treated a minimum of two times, once at early flowering and again 5 to 7 days later. With this type of program, a window of protection should be provided to minimize impact from all three of these seed damaging insect pests.

    Growers should be planning now for treatment schedules. When flowers begin blooming across the region, competition for access to aerial applicators increases.

    Reducing Insecticide Rates or Volumes ?

    Insecticide labels have recommended rates and volumes that users should follow. The rates are based on trials and have been set to provide consistent results over a range of environmental conditions. For many of our sunflower head feeding insects, the best field results have often been obtained with mid to upper rate ranges. In light of some of the large populations of seed feeding insects last year, it is prudent to use the most effective rates.

    Volume is also important. When reducing the total volume of spray per acre there is the risk of reducing the effective coverage of our treatment target. If the droplet coverage on the plant surface is reduced, there is less chance of contacting an insect during the application and with droplets leaving residual for insects to contact later. Reduced volume further risks evaporation of the spray under hot dry conditions, limiting the amount of product that ever reaches the target.

    Phillip Glogoza
    Extension Entomologist
    pglogoza@ndsuext.nodak.edu


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