NDSU Crop and Pest Report

Entomology


ISSUE 1  May 1, 2003

 

NORTH DAKOTA GRASSHOPPER OUTLOOK FOR 2003

Grasshoppers have the potential to be the dominating insect pest problem across the region this season. The southeastern and central counties experienced outbreak situations last season. Rangeland in the south central and southwest had large populations of adults, as well. If weather favors grasshopper survival and development this season, additional field crop and range areas could have troubles. See the 2002 survey map for details.

Grasshopper treatment guidelines are:

 

Nymphs (Young hoppers)
per square yard
Adults
per square yard

Rating

Margin Field Margin Field

Light

25 - 35

15-23

10 - 20

3 - 7

Threatening

50 - 75

30-45

21 - 40

8 - 14

Severe

100 - 150

60 - 90

41 - 80

15 - 28

Very Severe

200+

120

80+

28+

Whenever grasshopper populations reach the threatening level, feeding damage to crops should be anticipated. Directing control efforts at nymphs in hatching sites is recommended to minimize the total area requiring insecticide treatment, permits lower insecticide rates for effective control of small nymphs, and minimizes the potential for future crop damage.

If you are further interested in the rangeland situation, the USDA-APHIS-PPQ rangeland grasshopper survey map can be viewed in color at:

http://www.sidney.ars.usda.gov/grasshopper/Extras/map03.htm

 

ALFALFA WEEVIL MANAGEMENT: BIG CONCERN IN SOUTHWEST ND

Last season, alfalfa growers in the southwestern part of the state reported significant infestations of alfalfa weevil. One of the key points that needs to be emphasized this year is early scouting . . . well before leaf damage becomes apparent.

There are temperature, or degree day, models that have been developed for alfalfa weevil predictions, though they have not been evaluated in our state. The alfalfa degree day guidelines used by Iowa State University are based on a 48EF base temperature. Remember, degree days are used to predict when important biological events are likely to occur. With these events predicted, we should be able to better time field scouting to determine insect populations and have confidence in knowing where we are in the development of the population.

LIFE STAGE ACTIVITY DEGREE DAYS
Egg hatch   300
1st - 2nd instar

Light leaf feeding

301-438

3rd - 4th instaf

Major leaf feeding

439-595

pupa - adult

Feeding stops

596 - 810

Begin scouting for alfalfa weevil larvae when 250 DD have accumulated from January 1 (we use March 1 as a practical starting date for our insect models). Currently, we have only reached the low 100's in areas where weevils were a problem last year. Monitor tip injury to assess infestation levels and damage. This method is relatively simple and provides adequate estimates for the pre-harvest damage potential from alfalfa weevil when planning management decisions.

Select 50-100 alfalfa stems, (10 to 20 randomly selected stems from each of 5 locations) and examine for signs of feeding damage in the leafbuds and growing tip leaves. Divide the number of stems with recent tip injury by the total stems collected, convert to a percent, and compare with the threshold.

Many of the problems in most years come with weevils have come after the first cutting. It is important, when cutting alfalfa that has weevil larvae feeding, to assess the need for post-harvest weevil management. Monitor regrowth for potential stubble infestations, particularly beneath windrows. After the hay has been picked up, sample the stubble and early regrowth in 20 one square foot samples, 4 chosen randomly from 5 locations. When regrowth after harvest is sufficiently tall, go back to monitoring tip injury.

Insecticides labeled for alfalfa weevil include Baythroid, carbaryl (Sevin), Furadan, Imidan, Lorsban, malathion, methyl parathion (including Penncap M), Mustang, permethrin (Ambush and Pounce), and Warrior.

ALFALFA WEEVIL TRESHOLDS
Before 1st Cutting
35% (weak stand) plants with feeding damage
40% (vigorous stand) plants with feeding damage and/or 2 live larvae/stem
After 1st Cutting in stubble
8 or more larvae/ft2, (6/ft2 on sandy soil); or larvae are supporting regrowth

For a nice discussion providing an overview of different weevil management strategies that have been evaluated by South Dakota State University entomologists, you can access the document , Alfalfa Weevil: Evaluation of Control Practices in South Dakota, by Dr. Mike Catangui (2001), at:

http://plantsci.sdstate.edu/ent/entpubs/EEFS011.pdf

 

CORN ROOTWORM RESISTANT GM CORN APPROVED BY EPA

On February 25, 2002, the Environmental Protection Agency (EPA) announced that they had approved for commercial use transgenic corn that controls corn rootworm larvae. Registration was granted to Monsanto for YieldGard Rootworm corn. YieldGard Rootworm corn should not be confused with YieldGard Corn Borer corn or any other Bt corn hybrids that are resistant to European corn borer. The YieldGard Rootworm corn hybrids contain a beetle-specific Bt protein (Cry3Bb) that is toxic to corn rootworm larvae. This toxin has no effect on corn borers or other caterpillars.

As with past Bt corn hybrids, insect resistance management (IRM) will be required of farmers who grow YieldGard Rootworm corn. Although the IRM program is similar to those for earlier Bt corns that targeted European corn borer, there are some significant differences. The differences are needed since the biology of the corn rootworm is much different from that of the European corn borer.

Though we won’t see much use of the technology in North Dakota, yet, here are the resistance management requirements for using YieldGard Rootworm corn are:

  • Growers must plant a structured refuge of at least 20% non-YieldGard Rootworm corn that may be treated with insecticides as needed to control corn rootworm larvae. Growers will not be permitted to apply insecticides labeled for corn rootworm to the refuge for control of insect pests while adult corn rootworm are present unless the YieldGard Rootworm field is treated similarly. Refuge acres should be planted as blocks in or adjacent to YieldGard Rootworm cornfields or as in-field strips.
  • External refuges must be planted adjacent to YieldGard Rootworm fields.
  • When planting the refuge in strips across a field, refuges must be at least six rows wide, preferably 12 consecutive rows wide.
  • In addition, the refuge must be planted in similar ground as the YieldGard Rootworm corn. If the YieldGard is planted in ground that was in corn the previous year, the refuge must be planted in ground previously planted to corn. General management of the YieldGard Rootworm corn and the refuge should be similar.
  • Phillip Glogoza, Extension Entomologist
    pglogoza@ndsuext.nodak.edu

     


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