NDSU Crop and Pest Report

Entomology


ISSUE 5  May 29, 2003

 

SUGARBEET ROOT MAGGOT: PEAK ACTIVITY WITHIN 14 DAYS

Although our 2003 monitoring sites have been slightly ahead of schedule in degree-day (DD) accumulation for much of this spring, recent rainfall events have cooled soils off and have likely slowed the development of overwintered sugarbeet root maggot populations. Based on current accumulations and the extended weather forecast, peak fly activity is projected to occur between June 2 and 12. However, it is important to note that a warm (80 degrees Fahrenheit or above) day is usually needed after the necessary DD accumulation is reached for the actual peak to occur – Peaks are expected to occur slightly earlier in the southern 1/3 of the valley due to moderately higher recorded DD accumulations; however, root maggot populations are not expected to be at threatening levels in that area. Watch for an updated forecast in next week’s issue of Crop & Pest Report.

 

CONSIDERATIONS FOR POSTEMERGENCE CONTROL OF SUGARBEET ROOT MAGGOT

Most Red River Valley sugarbeet producers got off to a good start this spring and were able to plant their beets somewhat early for the 2003 growing season. This will make plants more able to withstand light levels of sugarbeet root maggot feeding injury. Fields in low-level risk areas (central and southern areas of the Valley) should be adequately protected with a planting-time soil insecticide. However, growers in areas of anticipated high maggot populations are advised to consider applying a postemergence liquid or granular insecticide, especially if seedling development is significantly behind normal. Granular treatments usually perform better if applied slightly before anticipated peak fly activity and liquid products provide the best activity if applied within three days (either before or after) peak. Soil moisture and the severity of fly population levels should be considered in choosing whether to use a liquid or a granular formulation.

Postemergence granular insecticides are most effective under moist soil conditions or if applied within a few days prior to a rainy period. Granules are also advised in fields that have been re-seeded without a second planting-time insecticide.

Liquid insecticides will perform better than granules if soil conditions are dry. In addition to fly control, a liquid formulation of an organophosphate such as Lorsban 4E may provide fly control as well as residual larval control, especially if rainfall is received within 1 or 2 days of application to adequately incorporate it into the soil. Liquid insecticides also may perform better if an unusually high flare-up of fly activity occurs. Research suggests that adult fly control can also be achieved with Asana; however, it and other pyrethroid products are not particularly active against larvae when applied postemergence. Refer to the "Insect Control" section of the 2003 Sugarbeet Production Guide or the "Sugarbeet Insects" section of the 2003 Field Crop Insect Management Recommendations for more detail and specific product recommendations. The respective WWW locations for online versions of these publications are:

http://www.sbreb.org/Production/production.htm

and

http://www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm

Mark Boetel
Research & Extension Entomologist
mboetel@ndsuext.nodak.edu

 

WHEAT MIDGE DEGREE DAY ACCUMULATION

For most of the month of May we have been in the 200 to 600 degree day range, the range that when wheat is planted it is likely to be heading when wheat midge are emerging. Fortunately, most of the state is witnessing low populations of midge at this time.

It is interesting to note that the "risk window" is very similar for the entire state. We reached 200 DD throughout the state during the last week of April. We will reach the 600 DD mark everywhere in the state by the end of May. In the past, we would see as much as a two weeks difference between start and finish of this event, often reaching 600 DD in early June for the northern counties.

Degree Days as a tool for Wheat Midge Management

Based on data from Canada, the threshold temperature for wheat midge development is 40E F. Observations indicate the following DD accumulations for events in the midge population.

DD Biological Event

450

the midge breaks the larval cocoon and moves close to soil surface to form the pupal cocoon

1300

10% of the females will have emerged

1475

about 50% of the females will have emerged

1600

about 90% of the females will have emerged

Identifying at Risk Wheat

Based on North Dakota field observations, midge larval infestations were the greatest when heading occurred during peak female emergence (1475 DD). When using 40E degrees as a threshold for wheat development (normally wheat development is monitored with 32 degrees), heading occurs around 1000 _ 1100 DD. Using this information, the following midge activity is expected based on degree day accumulations at time of wheat planting. (Note: remember that the North Dakota Ag Weather Network now can run the wheat growth and midge models separately, with specific planting dates, to identify which fields are at risk to midge. See Crop and Pest Report, Issue 1)

Wheat Midge Degree Days Used as a Guideline for HRSW Risk Assessment

HRSW planted PRIOR to accumulating 200 DD will head before wheat midge emerge.

HRSW planted FROM 200 to 600 DD will be heading at the time wheat midge are emerging.

HRSW planted AFTER 600 DD will head after peak emergence and should be at low risk to midge infestation (higher risk of frost, however).

Limitations of Degree Day Models

Although DD are useful in predicting development of many insect species, these predictions are only estimates. The accuracy of a DD estimate is dependent on the temperatures used in calculating degree days. DD should be calculated with temperatures that represent the environment where insects are developing. Temperatures at one site give only a rough estimate of insect development at another site miles away.

Using Degree Day Models 

The primary use of DD in IPM is to time scouting for pest species. DD can also be used to predict when an event will occur. By knowing the number of DD accumulated to date, we can estimate future DD by using average maximum and minimum temperatures, such as 5 or 30 year averages. These averages give only an approximation of when an event occurs, but they can be useful in planning our sampling and control activities. By using DD we can eliminate unnecessary scouting, we can avoid missing injurious pest populations, and we can make better management decisions. Thus these techniques help prevent economic losses and excess use of pesticides.

 

GRASSHOPPER HATCH UNDERWAY

With the arrival of some warm temperatures over memorial Day weekend, grasshopper hatch has gotten underway . . . again. There were reports of grasshoppers hatching in early May. However, the cold wet weather that lingered much of the month had a negative impact on those early risers. Now it is time to watch this next batch and determine how successful they are at surviving.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu

 


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