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Red Seed Weevil Warrants Timely Monitoring in '98

Sunday, March 1, 1998
filed under: Insects

An old enemy showed up in some Upper Midwest sunflower fields last year, and there’s a good chance growers will see its offspring in 1998.

The red seed weevil, which caused severe damage to numerous fields during the late 1980s, was seen increasingly in sunflower fields in 1997. In some cases, the insect was there but not at a level that required spraying. In other cases, growers had to treat.

“I did treat some confectioneries last year . . . more than usual,” notes Cameron Wischer, owner and pilot for Wischer Aviation, Inc., of Grandin, N.D. “I had a three-year stretch just a few years ago where I didn’t make a single application [for seed weevil].”

“We’ve gone from not seeing any seed weevil to seeing some, but not at threshold levels,” reports Carter Medalen, agronomist with Cenex Land O’Lakes in Valley City, N.D. “We follow the data from North Dakota State University (NDSU), and I know they’re seeing a trend for increased seed weevil.”

That’s true, says Phil Glogoza, extension entomologist at NDSU. “It still wasn’t like what we saw in the 1980s, but there was increased spraying for red seed weevil in 1997,” he reports.

Glogoza says the pest has been more of a problem in South Dakota and Minnesota the past few years because of drier weather patterns in those states’ sunflower districts.

What it all means for 1998 is yet uncertain. As with most insects, higher seed weevil populations one year make it likely that growers will see more of their offspring the next year. And the red seed weevil, say entomologists, is an insect that tends to run in cycles even more than most. But each year’s weather still plays an important role in the outcome.

“We believe cool, wet, late springs generally are not favorable for seed weevil. This insect tends to prefer drier conditions,” according to Glogoza.

In a puzzling turn, though, in 1997 the Dakotas had one of the colder, wetter springs in quite some time — yet seed weevil populations trended higher.

“You can’t just watch the weather,” says Medalen. “I would urge all growers to scout their fields in 1998.”

Red seed weevil historically has been the number-one economic pest in sunflower in the Upper Midwest. Sun-flower beetles generally are more numerous, but they don’t inflict the damage that weevils do. Weevil larvae feed in a flower’s seeds, where they hatch. The seeds are tender at that stage, and the larvae feed on the seed for about one month. By harvest, there’s a seriously damaged seed.

The problem is especially touchy for confection sunflower growers, who start getting docked at anything greater than one percent bug damage. But oil-type growers also have reason to be concerned. “For ‘oilers,’ the problem is that [when] they lose those seeds, they lose poundage,” Wischer points out.

Scouting for seed weevils should begin at early bloom — typically late July or early August in Dakota. A minimum of 50 plants should be scouted per field, preferably in four separate areas of the field, Glogoza recommends. Closely inspect the face of the plant head and count the number of seed weevils present.

NDSU’s economic threshold for confection sunflower is one to two weevils per plant. “Seed weevils are difficult to find in the field. For this reason, most growers are careful to scout thoroughly and to treat as soon as they reach economic thresholds,” says Tom Halvorsen, product service lead for Zeneca Ag Products.

For oil-type sunflower, the economic threshold developed by NDSU is between seven to 11 weevils per plant, depending upon the cost of treatment and sunflower prices at the time, according to Glogoza. (Additional information on NDSU’s recom-mended threshold levels and how to calculate those levels is available in the publication mentioned on page 16.)

Treatment is straightforward. A single application of methyl parathion or a registered pyrethroid applied by air is usually sufficient. The NDSU 1998 Crop Production Guide lists Warrior, Asana, Scout Extra and Baythroid as possible pyrethroids. “Pyrethroids are developed from pyrethrin, a natural insecticide compound found in chrysanthemum flowers,” reports Dale Chaney, technical business lead for Zeneca. “With Warrior, we have increased the efficacy over first-generation pyrethroids 10-fold.”

A full-label rate (2.56 ounces per acre) of Warrior, recommended by Zeneca for seed weevil control, “works out to 50 acres to the gallon,” Halvorsen explains. “Coverage is important, so we recommend a rate of five gallons of water per acre.”
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