Predicting Stem Weevil Emergence Via GDD
Will High Plains sunflower producers soon be able to utilize an accumulated growing degree day (GDD) model formula to reliably predict emergence patterns of the sunflower stem weevil? Scott Armstrong believes it’s a very real possibility.
Armstrong, a Colorado State Univer-sity research entomologist, is heading up the second year of a planned three-year project to determine the correlation between GDD and stem weevil emergence patterns in the region. The project was initiated in response to significantly increased damage from the stem weevil — whose scientific title is Cylindrocopturus adspersus (LeConte) — to sunflower in northeastern Colorado and adjacent High Plains areas in recent years. According to Armstrong, the sunflower stem weevil problem reached “epidemic” proportions in some parts of the region during the 1993 and 1994 seasons.
Because most High Plains growers had not previously experienced economic loss from stem weevil, however, Armstrong says there’s limited regional experience in scouting for or treating this insect. Also, the stem weevil’s small size, coupled with its habit of dropping to the ground when the host plant is disturbed, can make scouting quite difficult.
“Until improvements are made in the detection and scouting for the sunflower stem weevil, the most reliable method of predicting adult emergence in the spring would be with a heat unit-based degree-day model,” Armstrong believes. He notes that a base temperature of just over 40 degrees F. is needed for growth and development of the stem weevil. By adding in the daily maximum and minimum temperatures for a given area, along with numbers of emerging stem weevil adults, one can correlate growing degree days with weevil emergence, Armstrong says.
This is not the first time entomologists have investigated the correlation between GDD and stem weevil emergence. North Dakota-based USDA entomologist Larry Charlet researched the subject in the 1980s, but found that a GDD-based emergence prediction model was no more or less accurate than a calendar-based prediction. “There was no significant difference between [calendar date and degree day] for any of the emergence periods,” Charlet reported. “[In] fact, both methods for 50% and 90% emergence provided almost the same results.” Based on his research over a three-year period, Charlet concluded there was no particular advantage to using accumulated GDD as the basis for predicting when stem weevils would emerge in a grower’s field during late spring/early summer.
Armstrong is aware of the North Dakota research, but he believes the different climactic conditions found in the High Plains sunflower region will result in a GDD-based model being more useful in predicting weevil emergence than it would be in the Northern Plains.
“We accumulate heat units a little faster here,” Armstrong says, noting that there’s also significant variation in temperature from locale to locale within the High Plains. For example, the average number of annual growing degree days for Akron, Colo., during the five-year period of 1990-94 was 2,530; for Burlington (about 80 miles to the southeast), it was 2,703; for Sterling (just 35 miles north of Akron), the average GDD was 2,797; while for Fort Collins (110 miles to the west), annual GDD was 2,445 when averaged across those five years.
“Because of this variation, we are going to pick up some differences (in weevil emergence),” Armstrong contends. “The stem weevil is going to emerge down at Burlington before it emerges at Akron simply because there is a difference in degree-day accumulation. It’s warmer down there.”
Armstrong, who is based at the USDA-ARS Central Great Plains Research Station near Akron, began his GDD/ emergence study in April of 1995 by placing six sets of 10 weevil-infested sunflower stalks (from ’94 crop residue) beneath six screen cages. The cages were monitored every two to three days from mid-May through July for adult sunflower stem weevils. All detected adults were removed from each cage on each sample date so that none would be counted more than once. Meanwhile, maximum and minimum daily temperatures for the period of January 1 through July 31 were taken from a weather station located slightly more than 100 yards from the cages.
Armstrong found that the first stem weevils emerged on June 6, when accumulated GDD registered 379. The largest number of weevils emerged between June 25-29. As of the 25th, emergence was 50 percent complete; by the 29th, 90 percent of the weevils inside the cages had emerged from the sunflower stalks. All had emerged by July 11, by which time accumulated GDD had reached 744. Precipitation was above normal during the 1995 emergence period, while temperatures were below normal, resulting in what Armstrong says was a later-than-normal emergence pattern.
Still, the 1995 Akron data on stem weevil correlation with GDD were “good enough that we felt we could spread out these cages,” the CSU entomologist explains. So in 1996 the project is being expanded to include 11 cages and several sites: Akron and Burlington, Colo.; Sidney, Neb.; and St. Francis, Goodland and Hays, Kan. Cooperators at the other sites will be taking emergence counts and also sending temperature data to Armstrong. If the information arrives on a timely basis, he hopes to make it available to growers and others on DTN.
Armstrong adds that there is a difference between the temperature inside the screen cages and that outside the cages. So he has a correction factor to ensure that the readings accurately reflect each locale’s ambient temperature.
The project is expected to continue through 1997, thereby providing three years of data. At that point, Armstrong is hopeful he’ll have a reliable model to help High Plains sunflower producers predict the timing of stem weevil emergence. Coupling that with more education and experience in scouting for this hide-and-seek insect should improve producers’ ability to manage it, he believes.
“As we collect more data, I hope farmers will be in tune with what we’re doing and then use this information to their benefit,” concludes the Colorado entomologist. “I think they can.”
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