Sugarbeet root maggot, Tetanops myopaeformis, has been an increasing production challenge for Red River Valley sugarbeet growers over the past few years. Areas seeing the highest population numbers, or hot spots, over that span, could see even higher numbers during the 2021 growing season.
There have been three years of consecutive sugarbeet root maggot population increases since 2017.
“The trend is very much upward,” said Mark Boetel, North Dakota State University Research and Extension Entomologist. “In 2020, the overall valley-wide population level was the second-highest seen in 14 years. This suggests that control efforts between 2017 and 2020 were unsuccessful in reducing overall population levels for many producers.”
Root injury ratings between 2017 and 2020 have increased by 128%.
Boetel said a factor contributing to the increasing population numbers could be the weather.
“Some data suggests that if there is heavy soil saturation during the transition period between the pupa stage to adults, that can push population numbers down,” said Boetel. “We haven’t had the long-standing, severe wet springs in this area for quite some time.”
With the severely dry conditions seen thus far in 2021, population suppression due to soil saturation will again be at a minimum.
Areas at highest risk of damaging SBRM infestations include rural Auburn, Bathgate, Buxton, Cavalier, Crystal, Drayton, Glasston, Hamilton, Hoople, Reynolds, St. Thomas and Thompson in North Dakota and Argyle, Crookston, Donaldson, East Grand Forks and Warren in Minnesota.
Moderate risk is expected in areas bordering high-risk zones.
Proximity to previous-year beet fields where populations were high and/or control was unsatisfactory can increase risk, Boetel noted. Areas where high fly activity occurred in 2020 should be monitored closely in 2021. Growers in high-risk areas should use an aggressive form of at-plant insecticide treatment (granular insecticide) and expect the need for a post-emergence rescue insecticide. Those in moderate-risk areas using insecticidal seed treatments for at-plant protection should monitor fly activity levels closely in their area and be ready to apply additive protection if justified. Boetel also suggested to pay close attention to fly activity levels in late May through June to decide if post-emergence treatment is needed.
The insect overwinters in the previous year's beet fields and, once they emerge from those fields when their wings are hardened enough, they will move into a current year sugarbeet field. The peak fly or peak activity is a term used for the highest activity point seen in the current year beet fields.
Over the past 15 years, the average peak fly activity date is June 13, but over the last three years consecutive, peak fly dates have occurred in the first week of June.
“That is something of concern. What I’m wondering if this is the new normal or just an aberration,” said Boetel.
When moving to the current year’s beet field, the flies will lay eggs. About six days later, those eggs will hatch. The larvae will then progress through its stages of development. When they get to the third instar, they are ready for overwintering and burrow down to about 12-14 inches below the soil surface. From there, the process is ready to cycle around again once spring comes around.