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Twolined spittlebug (TLSB), Prosapia bicincta (Hemiptera: Cercopidae) was first detected in 2016 in the South Kona district of Hawai´i Island where it had damaged over 2,000 acres of rangeland. In 2017, four locations were selected for monthly monitoring of TLSB activity, population dynamics, and changes in plant community composition. Two monitoring sites were at the center of the infestation while the other two were located outside the northern and southern boundaries to estimate the rate of spread. At each location a series of 100-m long transects were established along elevational gradients between 500 and 1,850 m. Along each transect, ten 0.25-m2 quadrats were placed every 10 m on alternating sides of the transect. In each quadrat, data were collected on vegetative cover and height, plant species composition, live and dead grass cover (%), nymph and adult abundance, and nymph-plant associations. T he surveys revealed that between 2017-2023, TLSB expanded its range to over 72,183 ha, primarily in pastures of Hawai´i's most important forage grasses, Kikuyu (Pennisetum clandestinum) and pangola (Digitaria eriantha). TLSB was detected between ~500-1,700 m in elevation and activity was highest during the wet season (Apr-Oct). Mean TLSB densities (126 nymphs/m2) in pastures located between 1,000-1,300 m in elevation were significantly higher than in pastures between 500-999 m (64 nymphs/m2) and >1,300 m (20 nymphs/m2). Pastures with the highest TLSB densities experienced the greatest decrease in mean grass cover (30%) and greatest increase in mean forb (76%), bare ground (39%), and shrub (7%) cover. Landscape-level changes were observed in ran gelands damaged by TLSB as shown by the loss of forage grass cover and subsequent replacement by invasive weeds including Pamakani (Eupatorium adenophorum), wild blackberry (Rubus spp.), fireweed (Senecio madagascariensis), and Hilograss (Paspalum conjugatum). The establishment and spread of TLSB has devastating impacts on the ecosystem services Hawaii rangelands provide.