May 23, 2002

Cool Weather Slows Sugarbeet Root Maggot Development

The cool temperatures that have predominated in most of the Red River Valley this spring have likely slowed development of the overwintered sugarbeet root maggot populations according to Mark Boetel, North Dakota State University Research and Extension entomologist.

"NDSU has, for several years, been collecting root maggot emergence and adult fly activity data and correlating it back to soil and air temperature accumulation to develop a predictive model," says Boetel. "Typically, peak fly activity occurs on the first warm (80 degrees Fahrenheit or above) day following the accumulation of 600 degree-day units."

As of May 15, degree-day accumulations in the valley ranged from 93.3 at Baker, Minn. to 102.1 and 140.7 at St. Thomas and Wahpeton, respectively. In more normal springs over the past few years, these sites have exceeded 200 DD and have approached 300 units by this time.

"With past data in mind we would expect peak emergence to be delayed," Boetel says. "Unfortunately, right now, we don’t know by how much. The answer will come as the weather runs its course over the next few weeks. Given the current DD levels and anticipated daily accumulation rates, peak activity is not likely to occur before mid-June regardless of location within the valley. Red River Valley soils can warm up quickly so maggot development could be accelerated if a warm trend were to occur within the coming weeks."

A few small pockets of early-planted Red River Valley sugarbeets were replanted due to losses from frost damage, wind-blown soil or rotting seed in cold, moist soils. Boetel notes that replanted beets will be smaller and more vulnerable to attack during the root maggot larval feeding period than older, more established plants.

"Growers that are replanting in areas where high maggot populations are anticipated should consider a couple of options," Boetel says. "Growers can apply another granular material at replanting or they should consider applying a postemergence treatment of either a liquid or granular insecticide. Postemergence granules perform better under moist soil conditions and should be applied slightly before peak fly activity occurs. Liquid products generally provide better results if dry conditions prevail and when applied within three days of peak fly activity."

Use of Counter 15G, Counter CR and Lorsban 15G is limited to one application per year. If one of these products was applied on a particular field at initial planting, another material must be used for the succeeding application, Boetel says

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Source: Mark Boetel, (701) 231-7901, mboetel@ndsuext.nodak.edu
Editor: Rich Mattern, (701) 231-6136, rmattern@ndsuext.nodak.edu