Many lake communities are seeking to control the growth of invasive Eurasian milfoil, one of the countries greatest ecological threats to our waterways. A program of using a native biocontrol agent, the Milfoil Weevil, could greatly relieve the use of short-term control methods such as mechanical harvesting and potentially harmful methods of chemical control. The Milfoil Weevil can provide an environmentally sensible solution, and at considerable cost savings.
LDFLA is working to implement a weevil program to address the lakes’ Eurasian milfoil problem. This is done either through setting up a volunteer culturing operation or through direct purchase of the weevils.
The North American weevil, Euhyrchiopsis lecontei (Dietz) has been found associated with declining populations of Eurasian water milfoil in northeastern North America. Euhrychiopsis lecontei has been found in numerous states living on both Eurasian watermilfoil and native northern milfoil (Myriophyllum sibericum) plants. Studies have shown that this native weevil appears to be a milfoil specialist and will not feed on other macrophyte species. Since the Milfoil Weevil is a native species distributed in much of the U.S., its use as a biocontrol agent is recognized as a viable biocontrol for Eurasian milfoil.
The milfoil Weevil can be raised in the laboratory and laboratory-reared weevils can be used to augment natural populations. The science behind weevil augmentation is that in order to be an effective biocontrol agent for the fast growing exotic, Eurasian milfoil, a higher reproductive threshold must be reached than that which is available in nature. To reach this level, laboratory cultured weevils are inoculated en masse to entire lakes or specific areas of lakes where Eurasian milfoil density is great. This artificially high augmented weevil level brings their populations to a point where the reproductive threshold can be reached and weevil population begins to increase (to balance the food resource provided by the Eurasian milfoil).
It appears that if augmentation is not used, the milfoil weevil cannot naturally reach high enough numbers to control the rapidly growing Eurasian milfoil. This is due to several ecological events, but in general the environmental resistance (other than available food) of the weevil's surroundings limits its population growth. For a weevil, and for any sexually reproducing animal, it is often difficult to find a partner to mate with when the population density is sparse. Augmentation from laboratory cultures makes it easier for the weevils to find one another that ultimately allows for their greater reproductive success.
The milfoil weevil density is reduced in tandem as the Eurasian milfoil is reduced. This balance thrusts the ecological interplay of weevil and milfoil into balance. Thus, the use of the milfoil weevil is considered a long-term biological control agent. How long is long-term? No one knows for sure since long-term research must be conducted to determine the effectiveness of the Milfoil weevil. For instance, it is know that sunfish and bluegills feed on the weevil thereby serving to suppress (environmental resistance) weevil populations.
Is the Milfoil Weevil a "silver bullet" for Eurasian milfoil control?
The short answer depends on who is doing the talking. There have been a few control programs that were highly successful cases, some with intermediate results and a few control attempts that have not been successful. So this pattern basically for a 'bell curve' distribution. Definitive research has been dogged by the complexity of working with highly variable ecological and lake use circumstances.
For example, because a long-term weevil control program will not produce immediate results when compared with the short-term gratification of mechanical harvesting or chemical treatment, lake communities must be willing to stop harvesting for several years until the Milfoil Weevil program takes hold. A Milfoil Weevil program is totally incompatible with mechanical harvesting since the latter removes the very part of the plant (the upper meter) that the weevils require to complete their life cycle. Another important aspect to understand is whether or not the Eurasian milfoil population is in a general expansion phase or has the population become fairly stable in the lake. The Milfoil Weevil program will likely be much more successful in areas where the Eurasian milfoil expansion has become fairly stable.
These are an example of one of many issues that the lake governance body will face.
Unfortunately, research has been unable to document many of the ecological interface issues that occur with regard the Eurasian milfoil/Milfoil Weevil/usage issues. This isn't a failure of the research (or the researchers) but rather is just a "given" in research that is trying to understand these very complex systems. The effectiveness of the weevil to inhibit Eurasian Watermilfoil over a sustained period of time will be studied for a very long time.