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Sugarbeet Root Maggot Update: Peak Fly Expected in Two Weeks (06/01/17)

Most overwintered sugarbeet root maggots (SBRM) are in either the larval or pupal stage in last-year’s sugarbeet fields, and nearly all are within the upper two inches from the soil surface. However, it will not be surprising to see an occasional fly in the next couple of days, with activity picking up significantly by the end of next week.

Sugarbeet Root Maggot Update:  Peak Fly Expected in Two Weeks

Most overwintered sugarbeet root maggots (SBRM) are in either the larval or pupal stage in last-year’s sugarbeet fields, and nearly all are within the upper two inches from the soil surface.  However, it will not be surprising to see an occasional fly in the next couple of days, with activity picking up significantly by the end of next week.

NDSU is monitoring root maggot fly activity this year at 36 Red River Valley (RRV) sites in cooperation with American Crystal Sugar Company and the MinnDak Farmers Cooperative. Based on spring weather patterns, it appears that fly emergence and subsequent movement into sugarbeet fields will occur at dates that will closely mirror 15-year historical averages. To monitor what is happening with respect to fly activity in a particular area, visit the NDSU Entomology Root Maggot Fly Count web page for daily and cumulative fly counts for all monitoring sites

NOTE: this monitoring project is aimed at providing a general idea of where potential root maggot fly hot spots are developing throughout the growing season; however, they are not a substitute for monitoring specific activity in individual fields.

Fly emergence and activity are highly dependent on weather patterns, and the key determining factor for emergence is prevailing temperatures.  As such, fly emergence and subsequent activity typically occur slightly earlier in southern portions of the growing area than in northern locations.  Peak fly activity in beet fields occurs, on average, at about 650 Degree-Day (DD) units.  As suggested above, DD accumulations throughout the RRV continue to be at about average for this time of year (see Fig. 1 for accumulations as of 5-30-17). 

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An application based on our model for monitoring DD accumulations and forecasting peak fly is located on the NDAWN site under Sugarbeet Root Maggot. The site also includes a “help sheet” with information on using the model and how to optimize root maggot control.  It is important to note that warm weather (around 80°F), and calm to low-wind conditions are most conducive to fly activity.  Flies will be relatively inactive in cool, rainy, or windy conditions.  Courtesy of American Crystal Sugar Company, the NDSU root maggot model is also available as a free app for iPhone and Android mobile-enabled devices. 

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CONTROL:  Growers in hotspots or high-risk areas for SBRM infestation should plan on applying a postemergence insecticide, especially if an insecticidal seed treatment or a low to moderate rate of an at-plant soil insecticide was applied.  This is especially the case this year, because many fields in root maggot high-risk areas were planted late and plants in those fields are atypically small and vulnerable to attack by root maggot larvae.

Postemergence granules are usually effective if applied between 1 and 2 weeks before peak fly, but will also likely be beneficial if applied as close as 3 to 5 days before peak.  Granules should be incorporated into soil, and a postapplication rainfall will usually enhance control.  If postemergence granules will be used, they should be applied as soon as possible.  Granular applications at this time are more practical in central and northern parts of the RRV (Grand Forks, Walsh, and Pembina Counties in ND, as well as and Polk County, MN), whereas there is probably not sufficient time to effectively apply granules in the southern RRV. 

Postemergence liquid insecticides work best if applied close to (i.e., 2-3 days before, during, or within 2 days after) peak fly activity.  As such, growers expecting to use a liquid spray should monitor fields closely during the next two weeks for fly infestation buildups.  It is also advisable to monitor local agricultural media sources (radio, Crop & Pest report, and the Fly Counts web page) for updates on root maggot control and other insect pest management topics.

Remember to always READ, UNDERSTAND, and FOLLOW all label directions and precautions.  It is illegal to use a pesticide in a manner inconsistent with its label.  For more guidance on postemergence control strategies, consult the “Insect Control” section of this year’s Sugarbeet Production Guide

Mark Boetel

Research & Extension Entomologist

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