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ISSUE 9   July 5, 2007

PEST ALERT: WHEAT MIDGE IN NORTHERN TIER OF NORTH DAKOTA

Field scouting has found that economic populations of wheat midge are present in wheat fields in susceptible crop stages - heading through mid-flowering. Early planted fields that are past mid-flowering were not at threshold levels. So, continue to scout for adult midge to avoid unnecessary losses, especially with the current high price of wheat. The weather conditions have been unusually favorable for wheat midge: moist soils, high humidity and warm night temperatures. Monitor for adult midge during late evenings (after 9:00 PM) with warm night temperatures >60 F and light winds <6 mph. Economic thresholds are:

  • Hard Red Spring Wheat = one wheat midge per 4-5 heads
  • Durum = one wheat midge per 7-8 heads.
  • The degree day model for wheat midge (see map) indicates that the northern tier is close to 50% emergence completed for the female wheat midge, while the southern tier is at the end of wheat midge emergence. The estimated high-risk planting dates for economic infestations of wheat midge include: May 7 to May 17 for the northern tier; May 3 to May 13 for the central tier; and May 1 to May 10 for the southern tier. These dates are only estimated and will vary with your local environmental conditions. For example, favorable weather conditions for wheat midge can extend the period of egg laying and longevity of adult midges.

    wheat midge map

     

    BERTHA ARMYWORM EMERGENCE UNDERWAY IN CANOLA

    Bertha armyworms are beginning to emerge and adult moths are being captured in pheromone traps (see photo) in the north central and the northeastern North Dakota. So far, accumulated trap catches have been low, <200 moths per trap per site. These traps can be used to forecast the risk of Bertha armyworm outbreaks in a general area. High trap catches over a long trap period generally indicate the level of larval populations to follow. Result of the trapping network are available on the NDSU IPM website.

    pheromone trap
    Pheromone trap

    The adult moth is about 1˝ inches and mainly gray-black with a silvery/whitish kidney-shaped spot and silvery/whitish fringe on each forewing (see photo).

    Bertha armyworm adult
    Adult Bertha armyworm moth

    Moths overwinter in North Dakota and emergence begins in late June and continues through early August. These night fliers are particularly attracted to blooming canola fields for their nectar and egg laying sites. Eggs are laid on the lower side of leaves in clusters of 50-500 eggs in a typical honeycomb pattern, and hatch in about one week. The emerging larvae (1/10th of a inch) are usually green in color. Mature larvae are about 1˝ inch long and vary in color from green, brown to velvety black (see photo).

    Bertha armyworm larva
    Bertha armyworm larva

    Larvae often hide underneath leaf litter and clumps of soil during the day, which makes them difficult to see. As the canola plant drops its leaves, the mature larvae (>˝ inch) begin to feed directly on the pods which causes economically important yield losses and premature shattering. More on scouting and thresholds for Bertha armyworm in upcoming issues of Crop & Pest Report.

     

    MORE POTATO LEAFHOPPERS IN DRY BEANS, SOYBEANS, AND POTATOES

    High populations of potato leafhopper have reported in dry beans, soybeans, and potatoes in eastern North Dakota with some fields being treated with insecticide. Scout fields and use these economic thresholds to help make spray decisions:

    Dry bean = 1 leafhopper per trifoliate leaf;
    Alfalfa = 1-2 leafhopper per sweep when alfalfa is 8-14 inches high;
    Soybeans = 5 leafhoppers per plant in vegetative stage and 9 leafhoppers per plant in early bloom stages
    Potatoes = 10-20 adults per 20 sweeps or 1 nymph per 10 leaves

     

    COLORADO POTATO BEETLE IN POTATO

    Colorado potato beetle is the most common and destructive leaf feeding pest of potato. Both adults and larvae feed on foliage. In our potato insecticide trial in Glyndon, egg hatch is over 50% completed and foliage defoliation is at 10%. In the untreated plots, defoliation will probably be over 30% by next week! The adult is 3/8 inch long, with oval body and a yellow-brown color with 5 black stripes on each wing cover (see photo). The larvae are 1/8 to 3/8 inch long, brick red to light orange in color (see photo). Eggs are laid on the underside of leaves in clusters of 10 to 30 and are orange colored when ready to hatch. In North Dakota, overwintered beetles emerge late May through June. The first-generation larvae are present in the fields from June through July. From these larvae, the second generation of adult beetles usually begins to emerge in fields during July, feeding and laying eggs.


    Colorado potato beetle adult

    Colorado potato beetle larvae

    One of the greatest concerns with management programs for beetles is resistance to insecticides. The best way to manage the development of resistance in an insect population is the reduced use of compounds, limiting the selection of surviving (resistant) individuals. In North Dakota, resistance to the pyrethroid insecticides has been documented and the use of these compounds should be limited to one application per season. If control failures occur following the application of any product, switching to a different class of insecticides is recommended.

    Threshold: The current recommendation is that spraying be initiated at first egg hatch. Best results have been achieved by flagging the first egg masses that can be located, monitoring these daily, and spraying at 15 to 30% hatch. If the insecticide used is effective but not persistent, a second application should be made 5 to 10 days later. With this approach, the first-generation beetle larvae should be controlled with one or two applications.

     

    BANDED SUNFLOWER MOTH EMERGENCE UNDERWAY IN SUNFLOWER

    Banded sunflower moths (BSM) have been captured in pheromone traps located in Fargo, Prosper, and Mapleton (Cass County). This is an early emergence for BSM. In the northern tier, BSM should begin to emerge from the soil about mid-July. Peak adult activity normally occurs about the last week of July or the first week of August. Adult moths fly from last years field to the current year’s field. At this time, moths congregate around field margins. The moths move to fields during the bud stage, with a preference for the mid-bud stage for egg laying. Eggs are laid on the back of the bud and the outside of the bracts. This year the early planting sunflower fields will be more attractive for egg laying than later planted fields. Larvae hatching from the eggs move to the face of the flower and begin feeding on bracts and florets. More on egg scouting in upcoming issues of Crop & Pest Report.

     

    DIAMONDBACK MOTH DAMAGE IN FLOWERING CANOLA

    There’s more reports of diamondback moth larvae causing aborted flowers in canola field located in the north central and the northeastern regions of North Dakota. Some areas that have been reporting high numbers include Mohall in Renville County, Bottineau County, Kenmare in Ward County, and Cando in Towner County. High densities of larvae are being observed in late planted fields and indicates that the late planted canola fields are at a higher risk than earlier planted canola fields. Injury often looks like heat blasting, which is actually the feeding from the larvae of diamondback moth causing aborted flowers (see Bertha armyworm larva photo, aborted flower above larva). Be vigilant with your scouting until end of podding for these tiny green worms.

    Janet Knodel
    Extension Entomologist
    Janet.Knodel@ndsu.edu


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