The Milfoil Weevil – the Long Term Solution for Milfoil Control

The milfoil weevil (Euhrychiopsis lecontei) is a natural plant predator of some types of milfoil and has been studied by researchers as a biological control for Eurasian watermilfoil for over two decades. Because weevils are commonly found in North American lakes having milfoil problems, the chances are good that they already live in your lake.  However, because milfoil grows so fast, natural populations of weevils cannot typically control it.  In essence, EnviroScience’s proprietary solution, Milfoil Solution^®,  is to simply boost the natural weevil population to sustainable levels high enough to effectively control the milfoil over the long-term.

What is a biocontrol?  Biological control, or biocontrol, is the practice of rearing and releasing natural enemies (predators and parasites) of other organisms that are considered pests (hosts).  A good biocontrol agent is very specific to one or a very few hosts and for this reason, these natural control agents are perfectly safe to humans and do not harm the environment. They simply reestablish the natural balance of nature.  Keep in mind, however, that biocontrol does not completely eradicate these pests; it keeps them at low levels where they aren’t considered a nuisance.

How does the weevil control milfoil?  Biological control organisms serve to suppress their host plants.  Not only does the weevil rely on the plant for food, but also completion of all its life stages. Much like a butterfly, the weevil’s life stages include an egg, larvae, pupa, and adult stage.  As adults, they eat the milfoil, but also seek out healthy milfoil tips (called the meristem) to lay eggs. Once the eggs hatch to larvae, the larvae tunnel down through the stem creating holes in the stem walls as it moves in and out and also hollows out the stem tissue. This internal damage disrupts the flow of nutrients throughout the plant and to the roots, where energy is stored for next years’ growth. In addition, holes in the stem created by the larvae allow gas to escape and cause the plant to lose buoyancy in the water.  As the stem is attacked by the weevil and dies, it begins to fall over and weigh down nearby stems, pulling them further from the lake surface.  This opens the plant canopy and allows desirable native plants to compete more effectively, and eventually replace the milfoil beds.

In general, three to four more generations of weevils will continue to reproduce and spread out around the lake in one season.  Over time, as the milfoil decreases in the treated lake, the beetle population gradually declines to a self-sustaining level.

What results can be expected using the milfoil weevil?  Overall, the goal is to establish a permanent population of the weevils that will continually control the milfoil for many years to come. With over 13 years of experience combating large-scale milfoil infestations, our biologists have developed specific weevil-stocking strategies to maximize success. This includes stocking relatively large numbers of weevils in multiple locations over 2-3 years on average, depending on the size of infestation.  We also know that stocking more weevils in the first year increases the rate of lake-wide control.

Expected control using the weevil typically takes between two to three years on average.  Control includes:

• Massive reduction in milfoil density, and in most cases elimination of nearly all milfoil beds;
• Maintenance of the remaining plants below the water’s surface at non-nuisance levels;
• Increase of native plant species and re-establishment of a diverse native plant community in areas once dominated by milfoil.