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Sugarbeet Root Maggot Flies Emerging, Early Hotspots Developing (6/04/20)

Warm weather during the past couple of weeks has accelerated sugarbeet root maggot (SBRM) development, and some hotspots of early fly activity have been detected in the collaborative surveys being conducted by NDSU and American Crystal field personnel.

Warm weather during the past couple of weeks has accelerated sugarbeet root maggot (SBRM) development, and some hotspots of early fly activity have been detected in the collaborative surveys being conducted by NDSU and American Crystal field personnel. A total of 136 fields throughout the Red River Valley production area are being monitored this year. Currently, the highest counts, ranging from 25 to 270 cumulative flies per field (2-stake per field total) have been observed in fields near Grafton, Warren, Thompson, Buxton, Reynolds, and St. Thomas (listed in ascending order of total flies captured). Despite these high counts, many more flies are likely to appear during the next few weeks.  Fly counts (totals from 2 stakes per field) are posted online here for each location on a same-day basis every Monday, Wednesday, and Friday.

Expected high- and moderate-risk areas for 2020.  Areas at highest risk of damaging SBRM infestations this year include rural Auburn, Bathgate, Cavalier, Crystal, Glasston, Grand Forks, Merrifield, St. Thomas, Thompson, and Walhalla, ND, as well as Argyle, Crookston, Donaldson, East Grand Forks, Eldred, and Stephen, MN. Moderate risk of economic loss is expected in areas bordering all high-risk zones, including Buxton, Drayton, Forest River, Hamilton, Nash, Oakwood, and Reynolds, ND, as well as Ada, and Fisher, MN.

Long-term Peak Fly forecast.  Degree-day accumulations are tracking at a fairly average pace this year and, as shown below, the southern RRV is slightly ahead of most of the northern Red River Valley (RRV). Peak fly activity typically occurs on the first rain-free, warm (about 80°F), low-wind (< 10 mph) day around the accumulation of about 650 degree-day (DD) units. Extremely high activity is expected in the high-risk areas listed above, and it will continue to rise during the next two weeks. The extended weather forecast suggests more moderate, to even unseasonably mild, temperatures during the middle/later part of next week. That, combined with the possibility for scattered showers could spread fly activity out over several days.

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The current forecast for peak fly activity dates at four representative RRV locations appears in Table 1. NOTE:  this is an extended forecast, and it is based on the 14-day extended weather forecast.  Please stay tuned for updates in the Crop & Pest Report, as well as other agricultural media outlets (e.g., radio) for ongoing adjustments as we zero in on peak fly. Peak fly activity in current-year beet fields is expected to occur on or within 2 days of June 9 in the southern Valley, although low infestation levels are expected for this part of the growing area. Activity in the central RRV (i.e., Hillsboro, Ada, etc.) is expected to peak on or within 2 days of June 13, and peaks in the northern Valley, including Grand Forks, Crookston, St. Thomas, and Cavalier) is likely to occur between June 13 and 18.

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Concerns.  There are several late-planted fields in the central and northern RRV. Those fields will be especially vulnerable to SBRM attack, because most sugarbeet plants within the insect’s range will be atypically small when SBRM larvae begin feeding. Fields that were treated with a planting-time granular insecticide will likely have a good base of protection because most of the active ingredient from those applications should be present when larval feeding injury begins. Conversely, these early infestations could be especially damaging to fields that were not treated with a planting-time insecticide. This underscores the need to be vigilant about monitoring fly activity within individual fields. It also suggests that growers in affected areas should be prepared to make postemergence rescue insecticide applications, especially if fields were planted under any of the following SBRM management approaches: 1) no at-plant insecticide; 2) a granular insecticide at a low to moderate rate; 3) a planting-time liquid insecticide; or 4) an insecticidal seed treatment.

Postemergence SBRM Control.  Growers in high-risk areas for damaging SBRM infestations, or especially those that observe high fly activity in their fields, should plan on applying a postemergence insecticide for additive protection, especially if their insect management program involved any of the above-mentioned scenarios. Fields in which heavy rainfalls (> 3 inches) occurred within two to three days after at-plant or postemergence insecticides were applied also may need additional postemergence protection.

Postemergence insecticide options for root maggot control include both granular and sprayable liquid formulations. Postemergence granular insecticides perform best if applied at least five days ahead of anticipated peak fly activity, but work just as well if applied over two weeks ahead of peak fly. Sprayable liquid insecticide applications, which can either be applied by ground-based equipment or aircraft, perform best if applied close to (within 2-3 days of peak fly; either on, before or after peak).  Treated fields should be monitored closely after a postemergence application to determine if fly activity resurges. Some fields could 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 liquid can be made to the same field.  If retreatment is necessary within 10 days of the initial chlorpyrifos application, an insecticide containing a different active ingredient must be used.  For more information, 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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