Estimated Ambrosia artemisiifolia raceme density per year at three study sites in Northern Italy in the presence and absence of Ophraella communa. CO = Corbetta; GR = Grugliasco; MA = Magnago. GR- and MA- indicate plots from which O. communa was excluded by spraying insecticides. At Corbetta permission for insecticide application was not obtained. Boxplots represent the variation across plots within treatment and site, with the black dot as the median, the boxes representing the quartiles, the whiskers 1.5 times the interquartile range and the yellow dots the data points.