As indicated above, Eurasian watermilfoil declines have been associated with the occurrence of herbivorous insects at numerous locations. Declines of Eurasian watermilfoil associated with these insect have been documented in several lakes in Vermont (Creed and Sheldon 1995, Sheldon and Creed 1995, Sheldon 1997), lakes in Ontario (Painter and McCabe 1988), Fish Lake Wisconsin (Lillie 1996), lakes in New York (Johnson 1995a and 1995b), several sites in Minnesota (Newman, Ragsdale and Biesboer 1997), and several sites in Illinois (Creed, 1998). Creed (1998) gives a comprehensive list of declines and their relation to herbivore occurence. Smaller scale experimental manipulations, in field enclosures (Creed and Sheldon 1995, Sheldon and Creed 1995) and outdoor tanks (Newman et al. 1996; see below) have shown substantial and significant effects of these herbivores on Eurasian watermilfoil. These observations all indicate that the herbivores can control Eurasian watermilfoil, however, none is yet able to predict in advance when, where and how these herbivores will be effective. Newman et al. (1996) predicted that densities of 200-300 weevils per square meter (or approximately 1-3 weevils per stem) might be needed to effectively control Eurasian watermilfoil. More recent observations suggest that control may occur at lower densities (say half this density; Newman et al. 1998), however, control clearly does not consistently occur at these lower densities unless they persist throughout the summer. Several issues appear to be important. First, factors that limit weevil density and abundance must be investigated. Second, plant community response appears important. Lastly, the mechanism by which long term control occurs is not clear. For more information see:
Factors influencing plant response to control agents
Keep in mind that successful weed biocontrol aims to reduce pest populations to tolerable levels and that low densities of the pest are needed to maintain control agent populations. Succesful biocontrol of Eursaian watermilfoil would reduce its abundance, eliminate surface matting and promote healthy native plant communities.
Small scale controlled experiments:
A small scale tank experiment we conducted (Newman et al. 1996; abstract) illustrates the potential of the milfoil weevil under controlled conditions. Eurasian watermilfoil was allowed to root and grow in 16 outdoor tanks before adult weevils were stocked at densities of 0, 6, 12 and 24 per tank. Within a week weevils were reproducing and plants were damaged; reductions in plant density were noted in the higher treatments by 3 weeks. Milfoil in tanks with weevils failed to reach the surface, mat, and flower. Within four weeks, the end of the experiment, biomass of standing milfoil and milfoil roots was significantly reduced by weevil stocking density. These reductions were due to the extensive stem mining and reductions of viable milfoil rather than total consumption of all tissues because floating and sunken fragments increased with weevil density. Final weevil densities were similar for all three stocking densities (ca 300 per square meter), probably because weevils had eliminated suitable habitat (Eurasian watermilfoil) at the highest stocking densities. At the low stocking density, the average female weevil was able to produce 60-70 progeny during the four weeks of the experiment.
The extensive stem mining also resulted in reductions in stem and root carbohydrate concentrations and in the total stocks of non-structural carbohydrates (starches and sugars) which are needed for growth and reproduction. These reductions support Creed and Sheldon's (1995) hypothesis that stem mining results in the loss of ability to translocate materials and may reduce nutrient stores that are needed for successful overwinter survival and regrowth. The tank experiment illustrates the rapid and extensive potential for control with adequate densities of weevils. Successful control in the field requires that such densities and effects persist over time and and that longer term stress reduces Eurasian watermilfoil's ability to rapidly regrow during the next season. A better understanding of the factors that influence weevil densities and milfoil's response is the next step in being able to predict when, where and how control can be effected.