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ISSUE 11   July 13, 2006

ERRATUM TO INSECT SECTION LAST WEEK

Lorsban 4E was inadvertently left off of the list of organophosphate insecticides for control of soybean aphids in soybeans.

 

BERTHA ARMYWORM TRAP UPDATE

Accumulated trap catches for bertha armyworms continue to be below 500 indicating low risk of larval infestation to canola crop. Accumulated trap numbers can be viewed on ND map at:

http://www.ag.ndsu.nodak.edu/aginfo/ndipm/06IPMSur/HTML/CanolaTrapIPMsurvey.htm

 

DO I WAIT TO APPLY AN INSECTICIDE UNTIL APHID COUNTS ARE ABOVE THE 250 APHIDS THRESHOLD WITH THE DRY CONDITIONS?

The economic threshold represents the number of aphids per plant that will negatively impact yield and soybean quality. This research was conducted on healthy "unstressed" plants during the susceptible growth stages, late vegetative to R4 (full pod). Drought is another stress factor on top of the "aphid" stress. This is a difficult question and there is NO research data on aphid thresholds in drought conditions. One needs to weigh several factors to make a decision:

  • Number of aphids and length of time aphids have been on plants during susceptible growth stages. Currently, a population of 4,000 aphid days (Aphid day = one aphid on a plant for one day) results in a yield loss of 4% during the R1 (beginning of bloom) to R4 (full pod) growth stages.
  • Length of time remaining in susceptible soybean growth stages.
  • High temperature above 90 F will slow and eventually stop reproduction of soybean aphids (do not give birth to live young) and increase mortality rates (shorter life span of only 11 days compared to a typical 30-39 day life span).
  • How much will the drought conditions reduce the yield potential of soybeans? Will you see a net return if you do spray with an insecticide early (before 250 aphids per plant threshold)?
  • If you apply an insecticide early, you may need to apply a second application later in the season as aphid re-infest fields.
  • It is suggested to leave an unsprayed check strip for comparing against sprayed areas to determine the performance of the insecticide and the value of the treatment. At the end of the season, please send your 2006 field observations or any comments on economic threshold and insecticide spraying to:

    Email: Janet.Knodel@ndsu.edu

     

    BANDED SUNFLOWER MOTH EMERGING

    The banded sunflower moth, Cochylis hospes Walsingham, is a small ( inch long) tan-colored moth with a wingspan of about inch. The forewings are straw colored with a somewhat triangular dark brown band crossing the medial portion (see photo). The hind wing is light gray brown and has no distinctive markings. Within a week after emergence, the moths begin to lay eggs on the bracts of the sunflower heads. Females preferentially deposit more eggs on mid-size buds (R3) than on smaller or larger buds. Very few eggs are laid on plants at pollen shed (R5) and later. The majority of eggs are deposited on the outer whorl of bracts but a few eggs are placed on the inner bracts and the underside of the sunflower head. Eggs will be present through early August, hatching occurs five to eight days after the eggs are deposited. Larvae (see photo) feeding on florets may prevent pollination and reduce the total number of seeds produced by the sunflower head. Once the larvae reach the third instar they feed on seeds, usually consuming the entire kernel of the seeds fed on. Small areas of silken webbing on mature sunflower heads are indicators of banded sunflower moth larvae.

    Adult banded sunflower moth
    Adult banded sunflower moth

    Banded sunflower moth larvae
    Banded sunflower moth larvae

    Banded sunflower moth eggs
    Banded sunflower moth eggs

    Two procedures are available to determine when populations of banded sunflower moth are above treatment threshold.

    A new Egg Sampling Procedure has been developed by Mundal and Brewer for determining economic injury levels based on the number of egg found in the field edge. Randomly select five buds at each sample site and sample two sites from each side of field. From each bud, count the number of eggs per bract from six bracts. Then, calculate an average egg count for the field using the "ED Calculator" from the following website. This determines the egg threshold and economic distance. Economic distance is the distance of infestation from the field edge.

    http://www.ndsu.nodak.edu/entomology/econ_calc.xls

    Adult Moth Sampling Procedure: Randomly sample 20 plants per 5 sites for a total of 100 plants during the late bud stage during late morning to early afternoon when moths are resting on leaves and easily disturbed. If treatment is warranted, it should be applied at the R5.1 sunflower plant growth stage. Treatment thresholds are based on treatment cost, market price, plant populations using the following formula:

    EIL

    = (

    (Treatment Cost ($) / Market Price)

    ) x 582.9) - 0.7

    (moths per 100 plants)

    Plant Population

    Using $8.00/acre for insecticide treatment cost, $0.09/lb for market price, and 20,000/acre for plant population, the threshold would be 2 moths per 100 plants. If adult moth population has exceed this level, one should consider the use of a chemical insecticide to prevent larval seed damage.

    A list of insecticide registered for sunflower insect control is available at:

    http://www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates/ICG_06/09c_Mustard_Sunflower06.pdf

     

    LEAFY SPURGE HAWKMOTH

    The leafy spurge hawkmoth, Hyles euphorbiae (Sphingidae) has been observed feeding on leafy spurge. It was first introduced into U.S. in 1966 in Montana for control of leafy spurge and has spread into ND and other states. This species seems to like areas of leafy spurge near trees. The moth is generally quite abundant in areas where it has been established for a number of years. The larvae changes colors as it matures and it is also believed to have two different color forms (see photos). The first instar (life stage) are a dark black or blackish green, and later have a longitudinal yellow and dark brown stripes that change to green with white spots by two weeks of age. The last instar is green, black and red with yellow spots. Coloration on larvae can also vary depending on temperatures, relative humidity and genetics.

    Hawkmoth larvae

    Hawkmoth larvae
    Hawkmoth larvae

    Adult hawkmoth
    Adult hawkmoth

     

    INSECT IDENTIFICATION QUIZ

    Question: What larvae is this insect from? It has been observed feeding on corn, soybean, and other garden / weed plants in North Dakota this year. It looks like a lady beetle larvae. Be sure to look closely!

    Tortoise beetle
    (courtesy G. Bradburg, Williston Research Ext. Center)

    Tortoise beetle
    (courtesy K. Brown, Divide County County Ext. Office)

    Answer: This is a larvae of a Tortoise beetle (Coleoptera: Chrysomelidae: Hispinae). It feeds on sunflower, corn, morning glory, bindweed, sweet potato, cabbage, strawberry, milkweed, and plantain. Although tortoise beetles are usually not a pest, both larvae and adults feed on leaves causing them to be riddled with holes. This type of injury is most damaging to seedlings. The larvae has an interesting habit of carrying its tail over the body and holding large masses of excrement. This is probably a defensive mechanism against predators and parasitiods.

    Janet Knodel
    Extension Entomologist
    janet.knodel@ndsu.edu


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