Biological Control

Biological control is the use of natural enemies of invasive plants to control their population. The agents used are typically native to the same regions as the problematic invasive but may also be indigenous to the area of protection as well. The biological control agents’ populations are typically monitored and sometimes augmented in continual releases. The agents may include insects, such as the often-touted milfoil weevil, Euhrychiopsis lecontei, which disrupts the plants transport of photosynthates, grazing vertibrates like the grass carp, Ctenopharyngodon idella, or mycoherbicides, which are fungi applied similarly to herbicides.

The most common type of biological control is the use of plant eating, or phytophagous, insects like the milfoil weevil. But animals of many types have been used globally. There has been a release of three hundred and fifty species worldwide to combat invasive weeds. It is important to note that of these programs only seventeen percent have been deemed successful. In order for a biological control program to be a success, there must have been a clear and measurable decrease in the target invasive’s population, with no sign of the control agent becoming a problematic species itself.

Euhrychiopsis lecontei, the milfoil weevil (here in larval form), feeding on eurasian milfoil.

As with many control options, time and money are a problem with biological control. It may take ten or more years of research to determine if a potential control agent is viable for use without presenting an invasion problem of its own. All of this research and development coupled with execution of the program (stocking, and management) typically costs in the three to eight million dollar range for the initial program. However, unless they are sterile triploids, as is the case with grass carp because of their voracious eating habits, then the population of the control agent is self-sustaining and should provide permanent control. The control agent’s population sustainability can make it very economical to pursue this method of control regardless of the high startup costs. Another drawback to this method in addition to the cost, is that it is less certain to be effective than mechanical or chemical control.

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