There have been some reports of lygus in sugarbeet in the RRV in the past week.  Treatment with an insecticide may be justified if an infestation exceeds 1 Lygus bug per plant (adults and nymphs combined) after checking 30 to 50 plants in a field. Significant economic loss is likely to occur if an infestation reaches 4 Lygus bugs per plant. Lygus bugs usually infested beets during August. Therefore consideration of pre-harvest interval may be a critical factor in choosing an insecticide.

Lygus concentrate in the crown of the plant, so this is the first place to look. Lygus adults are light to dark brown with either a white "V" or 3 white spots behind the head and nymphs are bright green, with 5 black spots on their blacks and they move very rapidly (Figure 1).  Lygus feeding causes damage to younger leaves in the crown and it is suspected this stimulates further vegetative growth (development of new leaves) rather than concentrating energy into sugar deposition in the root.  Damage can be seen as both crinkled and deformed younger leaves and black 'tarring' on all leaves (Figure 1).  Often associated with the lygus feeding injury are small shiny black beetles with 4 orange or yellow spots on their backs.  These are Sap Beetles (Figure 2) which preferentially feed on damaged fruits and vegetables, etc.  I doubt these beetle are directly damaging the plant and suspect they're feeding on the tarring (lygus 'honeydew'). These same beetles are found in corn and other crops where insect feeding creates a moist, decaying wound to the plant.

Careful consideration of insecticide pre-harvest intervals may be a critical factor in choosing a product to use because Lygus bugs usually infest beets late in the growing season. Currently, three insecticides labeled in sugarbeets that include Lygus under that crop heading include Dibrom (PHI= 2 days), chlorpyrifos (PHI= 30 days; Lorsban 4E, Warhawk, Yuma 4E), and Lannate LV (PHI= 21 days).

A number of insecticides approved for use on sugarbeets have activity against Lygus bugs; however, the species that attacks Red River Valley sugarbeet fields (Lygus lineolaris, the tarnished plant bug) is not listed as a target pest in the sugarbeet portion of those labels. These insecticides include Asana (PHI= 21 days), and carbaryl ((PHI= 28 days) several products). It is legal to apply one of these products to attempt protecting sugarbeets from Lygus feeding injury if it is labeled for use in the crop; however, if the specific target pest is not listed for that crop, efficacy is not implied by the manufacturer and growers that choose to use the product assume their own liability for any unsatisfactory performance.

Ian MacRae, Entomologist
U of MN NWROC, Crookston
And
Phillip Glogoza
Regional Extension Educators  -  Crops