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Sugarbeet Root Maggot Update: Hot Weather Will Accelerate Fly Emergence (06/03/21)


Despite relatively low sugarbeet root maggot (SBRM) fly activity at this time, the anticipated streak of hot weather is likely to produce major surges in fly emergence and subsequent movement into beet fields in the next couple of days. Until about one week ago, weather patterns had SBRM development following a very normal pattern, however, the processes of pupation and adult emergence are very flexible and are easily accelerated under warm conditions such as those anticipated in both short- and long-term forecasts for the Red River Valley (RRV). It will be important to closely monitor SBRM fly activity over the next couple of weeks to determine where the most concerning hotspots begin emerging. Fly counts for all monitoring sites can be viewed at:

An adult sugarbeet root maggot (fly) on a sugarbeet seedling. The adult sugarbeet root maggot has characteristic smoky-brown patches on its wing margins.

As shown in Table 1, SBRM degree-day (DD) accumulations in much of the RRV vary slightly among latitudes, with the highest accumulations occurring in southern parts of the growing area. However, the expected hot weather is likely to have somewhat of an equalizing impact on DD accumulations, which is illustrated by the similar forecasts for “Maximum Likelihood Peak Fly Date” in the table.

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 May 31) High Fly Activity Period Maximum Likelihood Peak Fly Date*
Fargo, ND 525 June 4-6 (+80 F, dry, and low winds) June 5
Ada, MN 486 June 5-7 (+80 F, dry, and low winds) June 6
Grand Forks, ND 466 June 6-8 (+80 F, dry, and low winds) June 7
St. Thomas, ND 447 June 6-9 (+80 F, dry, and low winds) June 7

* 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.

Map: Accumulated Sugarbeet Root Maggot DD (F) (2021-05-31)

Peak fly activity typically occurs when an area reaches about 650 DD. However, flight activity is significantly impacted by weather. Warm weather (around 80°F) and calm to low-wind (i.e., less than 12 mph) conditions are most conducive to fly activity, whereas, flies tend to remain relatively inactive in cool, rainy, or windy conditions. Since the likelihood of rainfall events and associated winds is relatively low in the short-range forecast, a quick rise and a single, large peak in fly activity is expected for most areas where SBRM infestations are likely to occur.

Control. Growers in areas at high risk for damaging SBRM infestations should plan on applying a postemergence insecticide, especially if a moderately performing insecticide (i.e., insecticidal seed treatment, at-plant liquid insecticide, or a low to moderate rate of a granular insecticide) was used at planting. Although rare this year, any fields in which heavy rainfalls (> 3 inches) occurred after at-plant insecticides were applied also may need additional postemergence protection, regardless of the insecticide used at planting time.

The entire RRV growing area has reached the 440 DD mark that is recommended for beginning to apply postemergence granules (Figure 2). Although granular products can provide excellent postemergence SBRM control, they are less effective under dry soil conditions such as those commonly found in much of the growing area. Producers planning on using a granular product for root maggot control this year should do so immediately; however, if rain is expected within a few days, it will be advantageous to apply the material shortly (i.e., a day or less) before the anticipated rain. Effectiveness of granular applications can also be improved by lightly incorporating the product into the soil during the application.

Postemergence liquid insecticides work well under either moist or dry soil conditions. Applications should be targeted at 2 to 5 days before peak fly, but should provide some control if applied within a few days after peak as well. Sprayed fields should be monitored for potential resurgences of flies, and may require retreatment if subsequent infestations reach or exceed 0.5 flies per plant. For more guidance on postemergence control strategies, consult the “Insect Control” section of the 2021 Sugarbeet Production Guide or the “Sugarbeet Insects” section of the 2021 Field Crop Insect Management Guide

Mark Boetel
Research & Extension Entomologist