Invasive organisms often exist at low densities at the beginning and end of eradication programs. As a consequence, such organisms are often difficult to find, particularly if they are dispersed long distances to unknown locations. In such circumstances, large amounts of money can be spent searching for invasive organisms without finding any. However, chance encounters between invasive organisms and private citizens can occur even when invasive organisms exist at low densities. Reports of these ‘passive detections’ may be a critically important source of information for public pest management agencies. Rates of reporting may be improved using bounty payments and increasing public awareness about the presence of the invader. To explore the importance of passive surveillance in general, and its interaction with active surveillance by pest management agencies, we developed a simulation model of the spread of an invasive species. Simulations conducted under alternative scenarios for detection rates and search effort applied demonstrate that even small increases in detection or reporting rates substantially reduced eradication costs and increased the probability of eradication. In circumstances where resources are insufficient to achieve eradication, the simulation model provides useful information on the minimum expenditure required to contain the invasion.