Mark Boetel, research and extension entomologist at NDSU, reports that significant infestations have been detected in the Becker County, Minn., area. Small numbers also have been observed as far north as St. Thomas, N.D., but economically significant infestations appear to be most prevalent in the southern portion of the valley.

About the pests

He says two species, the beef leafminer and the spinach leafminer, are capable of causing economic losses in sugar beets. The two are similar in appearance, life history and in the damage they cause to sugar beet plants.

Adults are small clear-winged flies with brown or grayish brown to green bodies that look like small house flies. They also have numerous hairy spines on their backs. The adults observed in Becker County were dark brown in color.

Eggs hatch within three to 10 days, depending on temperatures. Newly hatched larvae (pale-green to whitish worms tapered from front to back) tunnel into and feed between the upper and lower surface of the leaf, creating characteristic serpentine paths or "mines." As feeding progresses, the mines expand and run together. Eventually, the damage results in necrotic leaf blotches. Although the blotches are most common near the leaf tips, they can appear at any point throughout the leaf.

Boetel says the larvae easily can be detected by holding an infested leaf up toward a source of light, such as the sun.

Scouting fields

These leafminer species are capable of producing up to three generations per year, but it usually is the first generation that causes the most significant problems. Boetel says we are likely at or near the start of the second generation.

"Although major problems are not expected to develop much later into the season, fields should be monitored immediately to determine if treatment is necessary, and for the next few weeks for further flare-ups, especially fields that had borderline infestations but were not treated," he says.

Scouting is important because leafminers only may be a problem in isolated fields. Early detection is crucial because insecticides are most effective when applied at or just before egg hatch. However, Boetel says, very good control has been achieved after larvae already have tunneled into leaves.

To scout a field, he says, sample several sets of 10 random plants in several representative areas within a field. The more samples, the more reliable the population estimate will be.

"The current recommendation is for fields to be treated if 50 percent or more of the plants have egg masses and small mines are visible," Boetel says.

Insecticide options include diazinon, Lannate and Lorsban. Boetel notes that products that do not specifically list leafminers on the label can be used as long as they are registered for use on beets.

If planning to tank mix when spraying for leafminer, liquid formulation of insecticides containing oil may increase the chance of injury from the herbicides Betamix and Progress, so the amount of crop oil used should be reduced by 50 percent to minimize the chance of crop injury.