Most weed biocontrol studies end with a report on the weed tissue destroyed and the depression of the weed population by the agent released. The goal for monitoring biocontrol agents that become established is to determine the intensity of attack, that is, the proportion of the resource exploited by the agent, such as the degree of defoliation. A low level of attack is unlikely to control the weed, but a high level also does not necessarily result in any benefit and may merely indicate that an agent that attacks another part of the plant is necessary to achieve weed control. Weed control is an impact threshold determined by human values. Monitoring weed biocontrol projects is difficult because many parameters must be followed and the effects do not necessarily occur where and when they are expected. The impact of biocontrol agents on their target weed is not linearly related to the benefits of weed biocontrol, such as forage yield.