The ash whitefly {Siphoninus phillyreae) was first identified in California during 1988 and caused widespread defoliation to its primary hosts, ash (Fraxinus species) and ornamental pear (Pyrus species) trees. The ash whitefly caused higher levels of damage to trees in regions with hotter summers and lower damage to trees in regions with cooler summers. In 1990 a parasitic wasp, Encarsia inaron (=partenopea), was released into urban communities in California to control the ash whitefly infestation. By 1992 the wasp had reduced ash whitefly populations to undetectable levels and preserved the aesthetic benefits of the affected trees. The loss in aesthetic benefits due to ash whitefly damage was estimated using a standard tree appraisal technique, the Trunk Formula Method. The benefits were estimated as the change in the average appraised value of a susceptible tree due to ash whitefly damage times the number of each affected tree species for each region. The total benefits of the biological control program range from $324 million at wholesale values to $412 million at retail. The direct costs of the program were just over $1.2 million. The net benefits are between $323 million and $411 million. The respective benefit to cost ratios are $270:1 and $344:1.