According to Mark Boetel, research and extension entomologist at North Dakota State University in Fargo, the tarnished plant bug often is referred to as the "Lygus bug," but Lygus actually refers to a complex of plant-feeding insect species in the Lygus genus.

Can cause production reduction

As tarnished plant bugs feed on plants, they inject a toxin with their saliva that liquefies and kills plant tissue, Boetel says. Feeding injury in sugar beets usually is restricted to new leaves and petioles. Symptoms include curling and wilting leaves, tumorlike feeding scars on petioles and blackening of new plant growth near the center of the crown. Injury often causes the plant to use carbohydrate reserves to produce new leaves and stems.

"Unfortunately, this occurs at a time of the season when these reserves should be building up and can result in a reduction in sugar production," Boetel says.

Both adult and immature stages of tarnished plant bugs are capable of injuring plants. Adults are about a quarter inch long and one-eighth inch wide. Color can range from dark greenish yellow to a dingy mottled brown. Older adults usually have a distinctive mottled coloration with lightened wing tips and a pale yellow V-shaped mark near the middle of the back.

The tarnished plant bug passes through five nymphal stages, each progressively larger, before becoming adults. First stage nymphs are very small, wingless and look like a bright green aphid. They also have a faint black spot in the center of the abdomen. Later-stage nymphs will have a total of five spots on their backs.

Both adults and nymphs are elusive. Adults often fly away and nymphs usually hide or drop off the plant when the beet canopy is disturbed. Scouting needs to be done carefully to obtain accurate population estimates.

Treatment

Boetel says there is no established economic threshold for tarnished plant bug control, but treatment may be justified if checking 30 to 50 plants in a field shows at least one-third of the plants with one or more adults or nymphs.

Preharvest interval may be a critical factor when choosing an insecticide, Boetel says. Asana XL should not be applied within 21 days of harvest, Sevin XLR within 28 days and Lorsban within 30 days. Both Lannate LV and Lannate SP can be applied up to within seven days of harvest. Follow label directions for intervals and rates.

Boetel says growers and crop scouts should keep a close watch on beet fields as other crops and broadleaf weeds begin to dry down to determine if treatment will be necessary. Beets in areas where other crops are stressed may be more at risk because the stressed plants may mature earlier than normal, causing tarnished plant bugs to search for more attractive food sources.