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Scientists Collar Mystery Sunflower Pest

Agricultural Research Service scientists in Fargo, N.D., may have found the mystery culprit behind unsightly blemishes that began turning up on confection sunflower seeds in 1998.

At first, the prime suspect was a species of disease-causing Alternaria fungus, a common pathogen of sunflower crops. But studies by ARS scientists Thomas Gulya and Laurence Charlet with the agency’s Northern Crop Science Laboratory in Fargo exonerated the fungus. When they sprayed sunflower plants with fungicide and then examined the seeds later in the season, many still harbored spots, otherwise known as kernel brown spot. When the plants were sprayed with insecticide, however, the incidence of blemishes dropped sharply.

Acting on a tip from commercial growers who reported an insect damaging nearby canola crops, Gulya and Charlet focused their attention on the Lygus bug. Its main claim to fame is pestering cotton, even though dozens of crops can serve as hosts. In the Northern Great Plains, where sunflowers are king, the bug’s mischief has primarily been confined to alfalfa and canola crops--or so it was thought.

To find out, Gulya and Charlet conducted greenhouse experiments in which Lygus bugs were placed on sunflower seedheads. They then sealed cloth bags around the seedheads to confine the insects. Checks of the seeds at maturity revealed tell tale brown spots identical to those seen in field-grown seeds. The spots caused by Lygus bug feeding are most likely the result of digestive enzymes the pest injects to extract nutrients.

For growers who produce sunflowers for the confectionary snack market, such damage can doom seed harvests to a less profitable outlet: birdseed. A longer story about the scientists’ detective work appears in the February issue of Agricultural Research magazine on the World Wide Web.

ARS is the U.S. Department of Agriculture’s chief scientific research agency.