LAUSR

Abstract A 2-year field study (2014–2015) was conducted to determine the levels of Eustenopus villosus (Boheman) and Larinus curtus (Hochhut) infestation and seed consumption of the invasive plant Centaurea solstitialis L. (Asteraceae: Cardueae) in the steppe of southeastern Washington, USA, a marine-modified continental macroclimate. Mature terminal C. solstitialis flowerheads were collected and dissected to determine abundance of each weevil species and level of seed consumption by both species. Data were analyzed by year to assess climatic influences on insect prevalence and feeding activity. Results showed that E. villosus infested 34.2% of the flowerheads in 2014 and 29.9% in 2015. Larinus curtus infested 10.9% in 2014 and 13.7% in 2015. In 2014, 5.8% of the flowerheads were infested by at least one E. villosus and one L. curtus in the same flowerhead and 9.7% in 2015. Eustenopus villosus consumed a higher percentage of total seed production per flowerhead of C. solstitialis than L. curtus (71.4% and 59.8, respectively). Infestation levels of E. villosus and L. curtus in southeastern Washington are lower than in the Mediterranean macroclimates in California and northern Greece. Mean seed consumption was also lower in southeastern Washington compared to northern Greece, with no data specific to California. Lower levels of infestation and seed consumption in southeastern Washington suggests the efficacy of the insect biological controls may be less in more continental macroclimates than in climates with Mediterranean influences.