Defining success in biocontrol of weeds is usually subjective and highly variable. A project may be considered successful in an ecological sense when a biocontrol agent successfully establishes in an area and reduces the target weed’s population. However, the severity of damage inflicted by the biocontrol agent may not result in the level of control desired by lake managers, boaters and homeowners. Recently, a clear distinction has been made between “biological success” and “impact success.” Biocontrols can be biologically successful (they establish and maintain high population densities on the target weed), but may not realize impact success (they do not provide the desired level of control or impact on the weed).
Numerous studies have been conducted to evaluate the utility of native insect herbivores as potential biocontrol agents of Eurasian watermilfoil, but none have proven to be predictable or effective to date. Also, if native insects were able to effectively control introduced populations of Eurasian watermilfoil, new introductions of the weed would not result in population development and expansion to weedy proportions. Historical accounts of the introduction and spread of Eurasian watermilfoil suggest that this has not occurred.
The milfoil weevil . . . can be effective . . . if adequate densities can persist through the summer and among years. However, many of the sites investigated have failed to sustain sufficient herbivore [weevil] density to effect control. We currently cannot predict when and where herbivore populations will reach sufficient densities nor when or where declines and suppression will occur.10,29 Both adequate agent [weevil] densities and proper plant response are required for predictable control30... Further identification and prioritization of factors limiting herbivore populations is needed and methods to ameliorate these limiting factors must be developed before biological control of milfoil can be reliably applied on a large scale.