Sugarbeets Under Pressure from Lygus Bug

Entomologist urges close monitoring in August.

Lygus bugs have been found in many Red River Valley sugar beet fields.

"Thus far, most infestations are relatively low, but our hot and dry weather will hasten the drying down of other area crops and small-seeded broadleaf weeds that often harbor lygus bugs," says Mark Boetel, North Dakota State University entomologist. "Lygus bugs have caused significant injury to sugar beets in late July through mid-August in past years. Fields should be monitored during the next few weeks to determine if an insecticide application will be needed."

Lygus bug adults are about 1/4 inch long and 1/8 inch wide. Their color can range from dark greenish yellow to a dingy, mottled brown. Most have a pale, yellow, V-shaped mark near the middle of their back and two faint, light yellow patches near their hind end.

Immature lygus bugs are called nymphs. They pass through five nymphal molts before reaching adulthood. First-instar nymphs are very small (1/25-inch long) and wingless, and look like a large, bright green aphid. Also, they have a faint black spot (actually a scent gland) on the center of their back. Older nymphs have four additional spots.

Lygus adults and nymphs use piercing and sucking mouthparts to feed on the plant. They pierce the plant and inject a salivary toxin that liquefies and kills plant tissue. The bugs then suck up the resulting liquid. Feeding injury in sugar beets usually is concentrated in new leaves and petioles.

Symptoms include leaf curling and wilting; seepage of a black, oillike exudate; swollen feeding scars on petioles; and often a gray to black sooty appearance on new growth near the plant crown. Leaf tip burn also can be a symptom of lygus feeding injury.

Lygus injury can cause plants to use carbohydrate reserves to produce new leaves and stems. This can lead to major sugar yield losses if it occurs later in the season, when the reserves should be building up in the root.

Sampling should be done with care because adults can fly away and nymphs usually hide or drop from the plant when the canopy is disturbed. Young nymphs blend in well with the beet canopy because of their green color, so they can be difficult to detect.

"An insecticide application is probably needed if the harvest is at least three weeks away and if the infestation exceeds one lygus bug per plant (adults and nymphs combined)," Boetel says. "An insecticide is not likely to be economically justified if harvest will occur within three weeks after the application."

For more information on lygus bug control, see www.sbreb.org/Production/production.htm.  

NDSU Extension Communications

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