Monitoring and Impact of
Weed Biological Control Agents
Most weed biocontrol studies end with a report on the weed tissue destroyed and the depression of the weed population by the agent released. This information is not, however, directly related to the social and economic benefits such as increased forage yield, protection of a native plant community, or decreasing herbicide use, which are usually not identified as project goals. Indeed, weed reduction may not produce any benefits until it is below a threshold level. For example, cattle avoid grazing pasture with as little as 10 percent spurge cover ( Lym and Kirby 1987) because the spurge latex blisters their mouths, but at 5 percent cover they graze around the stems. Thus, reduction of spurge cover to 5 percent is the goal of the Canadian spurge biocontrol project. It is rash to start a biocontrol project that may cost $4 million and take 20 years without an explicit statement of such a goal. Without a specific goal, there can be no assessment of success, or of whether benefits are likely to be achieved; there is no end point to the project; and because expectations are likely to differ, disappointment is inevitable. Establishing a meaningful goal in terms of a threshold weed population requires a preliminary study, however, so it is usually not done.
Conventional wisdom says that the vegetation should be sampled at the time and place of agent release. Unfortunately, this has a low benefit-cost return since about a third of the species fail to establish and most of the releases of successful species also fail or do not have an impact on the weed within five to ten years of release. A better approach is to monitor, in sequence, the following steps, each with explicit goals: agent establishment, intensity of agent attack, agent impact, and project benefits ( Harris 1991). This process allows the selection of appropriate sampling sites and methods and avoids the collection of detailed data over long periods when there is no impact. If other major effects are noticed in the course of the program, the suggested approach is flexible enough to allow for investigation of them.