Natural enemies such as herbivores that are introduced to reduce invasive plants can spill over into nature, threaten indigenous species and impose significant costs. We develop a bioeconomic model to analyse the optimal control management strategy of an introduced herbivore that has spilled over from a managed system to a natural area. Cost-effective control strategies are analysed that reduce the spillover effects of herbivores on endangered plants species to reduce the risk of extinction and increase benefits obtained from the ecosystem. We consider two competing indigenous plant species as the representatives of the plant community. Only one of these species is consumed by introduced herbivore. We show that the optimal level of controlling herbivores is relatively high in the following circumstances: (a) the herbivore has a high attack rate on the non-target host, (b) the herbivore has a relatively low attack rate on the target species (i.e. the weed), (c) the costs of controlling herbivore are low, (d) the non-target species has a low density (e) the non-target host has a higher biodiversity value than does its competitor. Point (b) is particularly interesting in the view of previous literature that reaches opposite conclusions.