Sugarbeet root maggot, Tetanops myopaeformis
Adults of sugarbeet root maggot are 6.5 mm (0.25 inches) long with shiny, black bodies that do not have any obvious hairs, stripes, or bristles. The leading edge of each wing contains a smokey brown patch located about 1/3 the distance from where the wing originates on the thorax. The eggs are white, slightly curved, and about 1 mm (1/20 inch) long. Larvae are legless, white maggots that do not have distinct heads. Pupae are tan and smaller than mature larvae.
• Sugarbeet root maggot is a significant pest of sugar beets in Minnesota, North Dakota, Idaho, Nebraska, Montana, Wyoming, and Colorado. Other commercially grown plants susceptible to attack include spinach, table beets, and Swiss chard.
• Seedlings are the most susceptible to feeding of sugarbeet root maggots, which can sever the taproot and kill the plant.
• There are many cultural control recommendations for managing sugarbeet root maggot. For example, planting early will ensure plants have developed past the vulnerable seedling stage when maggot activity increases.
Adults of sugarbeet root maggot. Sugarbeet root maggot is a significant pest of sugar beets in the United States. Larvae are the damaging life stage and they feed on roots beneath the soil surface. Seedlings are especially susceptible to infestations because feeding of larvae can sever the taproot and kill the plant. In addition to sugar beets, this pest can feed on spinach, table beets, and Swiss chard. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Sugarbeet root maggot. Image credit: Ken Gray, Insect Image Collection. Oregon State University
Pupae of sugarbeet root maggot. Image credit: Ken Gray, Insect Image Collection. Oregon State University
Sugar beet root with feeding injury caused by sugarbeet root maggot. Note the dark lesions. Image credit: Ken Gray, Insect Image Collection. Oregon State University
Life history and habits
Eggs are laid in groups of up to 40 near sugar beets in soil, and each female can lay up to 120 eggs. Egg survival is highly dependent on soil conditions, with hot and dry soils promoting lower egg hatch and survival. After emerging from the egg, larvae develop through three larval stages while feeding on the roots of sugar beet. Mature larvae overwinter 12 inches or deeper in soil. When temperatures rise the following spring, larvae migrate upwards and begin pupating a few inches from the soil surface. Peak adult activity usually occurs in late May or early June. There is typically one generation each year in states like Idaho.
Maggots have rasping mouthparts and feed by scraping the root surface. Seedlings are particularly susceptible to feeding injury when the taproot is small and more easily severed. Yield loss is expected to be greatest in late planted fields where sugar beets are the most likely to be in the seedling stage during peak fly densities.
Feeding injury in older plants appears as black lesions that range in size from a pinhead to a dime. The entire root surface can be covered with lesions when infestations are severe, and the taproot will fork and branch when maggots feed on the root tip. Plants with injured roots wilt, especially between irrigations, and are more susceptible to root-rot pathogens. Fields with a history of root rot are especially vulnerable to minor infestations of sugarbeet root maggots.
Fly abundance can be monitored with yellow sticky traps. Over time, insects will cover the sticky surface and prevent additional captures, therefore traps should be checked a minimum of two times per week. After counting flies of sugarbeet root maggot, discard the old trap and replace it with a new one.
These traps should be deployed in spring along the fencerow or ditch bank along the sugar beet field, but not within the field. Traps should be spaced at intervals of several hundred yards and positioned for easy checking. Remove any weeds that might interfere with the trap. It is recommended that growers monitor and check each individual field since fly activity can differ between adjacent fields.
Recordkeeping is necessary to inform decisions related to management. Fly captures should be recorded for all traps and used to calculate a trap average. A daily average can be obtained by dividing the trap average by the number of days since traps were last checked. Control decisions should be made when captures in the daily average column have peaked. Compare the running tally, calculated by adding total trap averages, from the day of peak with thresholds in table 1 of the Integrated Pest Management Guide for Sugarbeet Root Maggot. Insecticides should be applied if the running tally exceeds the threshold. Chemical control is generally warranted if captures average 40-45 flies per trap from the start of the season.
There are several natural enemies of sugarbeet root maggot including ground beetles, predatory flies, birds, and some species of naturally occurring entomopathogenic fungi such as Metarhizium anisopliae. Aside from conserving natural enemies, there are no commercially available biological controls for management of sugarbeet root maggot.
Proper irrigation can decrease the severity of feeding injury on plants since frequent irrigation causes larvae to move closer to the surface where plants are less susceptible. Sugar beets planted in fine, sandy soils are more susceptible to feeding injury.
Crop rotation is recommended, but alone is ineffective since flies can migrate to and from neighboring fields. Planting early will allow sugar beets to grow beyond the seedling stage. After harvest, tilling deep into the soil will expose overwintering larvae to natural enemies and freezing temperatures, but may compromise soil health. The addition of cover crops prior to beet planting can help reduce feeding injuries caused by sugarbeet root maggot.
Chemical management of sugarbeet root maggot is challenging and requires several considerations, such as timing, weather, and product formulation. Both granular and liquid insecticides are available for managing sugarbeet root maggot and should be applied immediately after fly numbers exceed economic thresholds. For more detailed information on chemical control of sugarbeet root maggot, consult the North Dakota State University webpage.
Bechinski, E., R. Stoltz, and J. Gallian. (n.d.). Integrated Pest Management Guide for Sugarbeet Root Maggot. University of Idaho: Cooperative Extension. Available https://agresearch.montana.edu/wtarc/producerinfo/entomology-insect-ecology/SugarbeetRootMaggot/IdahoFactSheet.pdf
McDonald, S., L. Hofsteen, and L. Downey. 2003. Crop Profile for Sugar Beets in Colorado. Available https://ipmdata.ipmcenters.org/documents/cropprofiles/COsugarbeets.pdf
North Dakota State University. 2022. Monitoring and Controlling Sugarbeet Root Maggot. North Dakota State University. Available https://www.ndsu.edu/agriculture/ag-hub/ag-topics/crop-production/crops/sugarbeets/monitoring-and-controlling-sugarbeet-root-maggot
Oregon State University. (n.d.). Sugar beet-Sugar beet root maggot. Oregon State University: Extension. Available https://pnwhandbooks.org/insect/agronomic/sugar-beet/sugar-beet-sugar-beet-root-maggot
Stefan Jaronski. 2022. Sugar Beet Root Maggot Research. USDA. Available https://www.ars.usda.gov/plains-area/sidney-mt/northern-plains-agricultural-research-laboratory/pest-management-research/pmru-docs/sb-root-maggot/#:~:text=Sugarbeet%20root%20maggot%20(Tetanops%20myopaeformis,of%20a%20total%20of%20approx.