The main concern is a host shift of a biological control agent that results in the agent feeding on a desirable plant. Biological control agents are not available for all target pests. Some of the target weeds are closely related to plants that are desirable so the risk of introducing an agent is too great. Research time and money is needed to locate biological agents and screen them for host range before the agents are released. It generally takes years of research and testing before agents are released.
Biological control takes place slowly in most cases.
Localized weed problems may not be eliminated quickly enough to satisfy public opinion. Biological control relies on populations of the weed and the agent to maintain the system. Thus, weeds are not completely eliminated. Also, there is often highs and lows in population levels of the weed and the agent both in time and space. Thus, there are some situations where biological control does not seem to work well.