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Sugarbeet Root Maggot Fly Emergence is off to a Strong Start: Again! (06/06/19)

Despite the slow start to spring this year, warm afternoon temperatures that occurred during the past couple of weeks have resulted in early emergence of exceptionally high sugarbeet root maggot (SBRM) fly activity at a few locations in the central and northern Red River Valley (RRV).

Despite the slow start to spring this year, warm afternoon temperatures that occurred during the past couple of weeks have resulted in early emergence of exceptionally high sugarbeet root maggot (SBRM) fly activity at a few locations in the central and northern Red River Valley (RRV). Although unusual, this is reminiscent of the 2018 growing season when major fly outbreaks began in late May and continued at relatively high levels for nearly three weeks. 

While walking in a field in southern Pembina County earlier this week, I observed a lot of flies on the ground and in flight, but very few were visiting sugarbeet seedlings. Also, most (over 90%) of the flies I observed were males. This is not unusual, as males typically emerge earlier than females. With that in mind, I urge growers and pest management advisors to avoid panicking and treating fields too early. The NDSU root maggot model suggests that peak fly activity (the point at which the largest numbers of both male and female flies are present) is still at least 8 to 12 days away. I expect this year to be a bad one in relation to root maggot outbreaks.

Expected high- and moderate risk areas for this year.  Areas at highest risk of damaging SBRM infestations this year include rural Auburn, Bathgate, Bowesmont, Cavalier, Drayton, Grand Forks, Reynolds, St. Thomas, and Thompson, ND, as well as Argyle, Crookston, East Grand Forks, Euclid, and Fisher, MN. Additionally, moderate risk of economic loss is expected in areas bordering all high-risk zones, as well as near Buxton, Cashel, Crystal, and Grafton, ND, and Ada, Eldred, and Fisher, MN.

Current fly counts.  NDSU research and Extension personnel are monitoring SBRM fly activity at 37 sites throughout the RRV.  American Crystal Sugar Company personnel are monitoring an additional 72 fields, which brings the grand total for the fly monitoring network to 109 fields. Fly counts (totals from 2 stakes per field) are posted online for each location on a same-day basis every Monday, Wednesday, and Friday. Current hotspots include the following: Auburn, Cavalier, Crystal, Grand Forks, Merrifield, and St. Thomas in North Dakota, and Argyle, East Grand Forks, and Stephen in Minnesota.

Long-term peak fly forecast.  Peak fly activity typically coincides with the first rain-free, warm (about 80°F), low-wind (< 10 mph) day at 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 after this weekend. That will likely contribute to later-than-average emergence and activity peaks.

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 between June 13 and 16 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 between June 15 and 18, and peaks in the northern Valley, including Grand Forks, Crookston, St. Thomas, and Cavalier) is likely to occur between June 16 and 19. However, in light of this seemingly unprecedented timing and intensity of fly activity at this point in the growing season, it is impossible to know how long flies will persist. Therefore, growers and crop advisors should carefully monitor this situation for the next few weeks. 

Concerns.  Late-planted fields are common throughout the 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 peak (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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