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Sugarbeet Root Maggot: The Calm before a Pending Storm (06/04/15)

Although sugarbeet root maggot (SBRM) fly activity is currently at low levels throughout most of the Red River Valley (RRV), all indicators thus far suggest that some sites within the Valley will experience very high populations this year.

Sugarbeet Root Maggot:  The Calm before a Pending Storm

Although sugarbeet root maggot (SBRM) fly activity is currently at low levels throughout most of the Red River Valley (RRV), all indicators thus far suggest that some sites within the Valley will experience very high populations this year.  Particularly concerning this year is that SBRM populations have steadily increased during each of the past four growing seasons.  In 2011, our fly activity monitoring program captured an average of 42 flies per trap.  In 2014, the capture rate was 111 flies per trap, which is a 164% increase over that observed in 2011.  The number of fields surveyed and their locations sometimes change significantly among years of this program.  As such, these surveys are more informational than they are scientific.  However, this trend at least suggests that some fields could play host to high or even severe SBRM infestations this year.

Peak fly forecast.  According to the NDSU root maggot developmental model, peak SBRM fly activity typically coincides with the first rain-free, warm (about 80°F), low-wind (< 10 mph) day at the accumulation of 650 degree-day (DD) units.  To monitor SBRM DD accumulations for your area, consult the Root Maggot model application on the North Dakota Agricultural Weather Network (NDAWN). The site also includes a “help sheet” with information on how to use the model, recommendations for when insecticide applications are justified, and tips on when to apply them.  American Crystal Sugar Company, in cooperation with NDSU, has developed a mobile device app based on our SBRM developmental model.  The app is free, and available for both Android- and iOS-based mobile devices. 

This year’s DD accumulations suggest that fly activity will intensify very soon, and that notable activity increases are expected over the weekend of June 7-8.  Latitude typically impacts the timing of peak fly activity, with infestations in southern latitudes peaking two to five days before those in northern areas.  The current forecast for four representative RRV locations is presented in Table 1.

 Table 1. Degree-day (DD) based predictions for timing of high SBRM fly activity periods and peak fly activity in the Red River Valley

Location

Total DD
(as of June 1)

High Fly Activity Period

Maximum Likelihood Peak Fly Date*

Sabin, MN

544

June 7-9 (+80°F and low winds)

June 8

Ada, MN

514

June 9-11 (+80°F and low winds)

June 9

Grand Forks, ND

535

June 8-10 (+80°F and low winds)

June 9

St. Thomas, ND

489

June 10-12 (+80°F and low winds)

June 11

*Maximum likelihood for peak fly activity is based on extended weather forecasts for wind speed, air temperature, and precipitation. 
                Peak fly in current-year beets usually coincides with the first rain-free, calm/low-wind day to reach 80°F after 650 DD are accumulated.

IMPORTANT:  a new feature of the SBRM forecast this year is the “Maximum Likelihood Peak Fly Date”.  It is important to realize the following regarding this estimate: 1) it is based on the 10-day extended weather forecast; 2) it includes considerations for impacts of wind speed, air temperature, and precipitation events on SBRM flight behavior; and 3) it has the same intrinsic limitations that any 10-day weather forecast can have.

Root maggot fly counts.  NDSU, in cooperation with the American Crystal and MinnDak sugar cooperatives, is monitoring SBRM activity at 45 RRV sites in both ND and MN this year.  The traps are checked three days per week (MWF), and the counts are posted online late-afternoon each count day.  To view counts in your area, visit: http://www.ndsu.edu/entomology/people/faculty/boetel/flycounts/.

Postemergence SBRM Control.  Growers in areas at high risk of having damaging SBRM infestations should plan on applying a postemergence insecticide for additive protection, especially if an insecticidal seed treatment or a low to moderate rate of a granular soil-applied insecticide was used at planting.  Fields in which heavy rainfalls (> 3 inches) occurred within two to three days after at-plant insecticides were applied also may need additional postemergence protection.

Growers choosing to use a granular insecticide for postemergence SBRM control should apply it as soon as practical, because granules are most effective if applied between 4 and 14 days before peak fly activity.  Postemergence liquid insecticides should be applied as close to peak fly activity as possible. However, applications made within two to three days before or after the peak should provide good results as well. Treated fields should be monitored for potential fly resurgences, and may require retreatment if subsequent infestations reach or exceed 0.5 flies per plant

IMPORTANT:  If a chlorpyrifos-containing liquid spray (e.g., Lorsban 4E, Lorsban Advanced, or any generic chlorpyrifos product) is applied, 10 days must pass before another chlorpyrifos-containing liquid can be made to the same field.  If retreatment is deemed necessary within 10 days of the initial chlorpyrifos application, an insecticide product containing a different active ingredient must be used.  For more guidance on postemergence control strategies, consult the “Insect Control” section of this year’s Sugarbeet Production Guide.  Always remember to READ, UNDERSTAND, and FOLLOW the label of your insecticide product – it’s the law.

Mark Boetel

Research & Extension Entomologist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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