Biological control (biocontrol) is the use of parasitoid, predator, pathogen, antagonist or competitor populations to suppress a pest population (Van Driesche and Bellows 1996). The aim in biological control of weeds is not to eliminate the pest (and thus the control agent), but to suppress the pestpopulation to levels that are no longer a nuisance.
For more information on biological control in general click:
General: Cornell Biological Control; National Biological Control Institute; International Organization for Biological Control
Weeds: Cornell BioControl of Weeds; IOBC Working Group on Weeds; Biological Control of Non-Indigenous Plant Species
Insects: Insect Biocontrol Laboratory USDA;
Biological control has been used successfully for at least four genera of floating and emergent aquatic macrophytes, but work on submersed macrophytes such as Eurasian watermilfoil is recent and not yet operational. Grass carp have been used in some states but they are not very specific to Eurasian watermilfoil and will often eliminate native plants before controlling watermilfoil (see Madson 2000). Grass carp are not permitted in Minnesota but have been used in Washington State, although even there they are not recommended for control of Eurasian watermilfoil.
Three taxa have been considered for the biological control of Eurasian watermilfoil:
All three taxa are present in Minnesota and Wisconsin (click for chart) (Newman and Maher 1995).
The caterpillar of the moth Acentria ephemerella will eat many species of aquatic macrophytes (Buckingham and Ross 1981) and it was decided not to redistribute it to Florida as a classical biocontrol agent in the 1980's, due to lack of host specificity; however, it has been in North America since the 1920's and has expanded its range westward into the midwest (Newman and Maher 1995, Scholtens and Balogh 1996). Although the caterpillar has been associated with some milfoil declines, it does not appear to be a major factor in New England milfoil declines (Sheldon and Creed 1995) and has not attained high densities in Minnesota (Newman and Biesboer 2000). The caterpillar has been associated with declines in New York (Johnson et al. 1998, 2000) and is under more intensive investigation there. Currently, we are monitoring populations of Acentria ephemerella and another caterpillar, Parapoynx, in Minnesota and will intensify investigations of these agents if they are deemed important for control.
The midge, Cricotopus myriophylli, has been associated with milfoil declines in the Pacific Northwest (Kangasniemi and Oliver 1983, MacRae et al. 1990, MacRae and Ring 1993, Kangasniemi et al. 1993). Problems with mass rearing and lack of funding have inhibited further investigation there. The midge is present in the upper midwest and may be a factor in controlling milfoil, however it also has not reached high densities at our sites. Due to low densities and difficulties working with such a small agent we are not currently focusing on the midge, however, it is probably worthy of further investigation.
The weevil, Euhrychiopsis lecontei, appears to be the most promising agent. It has been associated with documented milfoil declines in New England (Creed and Sheldon 1995, Sheldon 1997), Wisconsin (Lillie 1996), Minnesota (Newman and Biesboer in press) and elsewhere (Creed 1998). It has been shown to control milfoil in the lab (Creed et al. 1992, Creed and Sheldon 1993, 1994b), in experimental tanks (Newman et al. 1996) and field enclosures (Creed and Sheldon 1995, Sheldon and Creed 1995). The weevil appears widespread across northern North America and occurs in many lakes in Minnesota and Wisconsin, lakes that have Eurasian watermilfoil and lakes with the native northern watermilfoil (Myriophyllum sibiricum Komarov) that have not yet been infested with Eurasian watermilfoil. Recent surveys in Wisconsin (Jester 1996) indicate that the weevil likely occurs in most lakes with northern or Eurasian watermilfoil. Experimental augmentations are being investigated in Minnesota and on a broad scale in Wisconsin. In the midwest, milfoil declines have been associated with the milfoil weevil in Fish Lake, Wisconsin (Lillie 1996) and a less well documented decline in McCullom Lake, Illinois (Creed 1998). In Minnesota, declines have been associated with the weevil in sites at four lakes: in one lake the milfoil recovered and increased to record highs (but has subsequently been suppressed); in the second lake, milfoil slowly declined and has maintained a stable population while the native plant community increased; in the third lake, milfoil was decreased by 75% in one year and has been suppress to <10% of the community for 5 years (Newman and Biesboer 2000, Newman et al. 2001); in the fourth lake, weevil populations were low from 1992-1999, but increased dramatically in 2000 and 2001, suppressing resurgence of milfoil (Newman et al. 2001). In several of the Minneapolis Chain of Lakes, milfoil weevil densities are very low and milfoil is not controlled. Experimental evidence suggested that a density of 200-300 weevils per square meter of bottom may be required to effect a decline (Newman et al. 1996). At many sites in Minnesota, weevil densities have failed to reach such levels, although 50-100 weevils per square meter (or less) may be adequate for declines (Newman et al. 1998; Newman and Biesboer 2000). We are currently investigating factors which influence weevil density; fish predation may be a factor at sites with low weevil populations (Sutter and Newman 1997). Harvesting can reduce weevil abundance (Sheldon and O'Bryan 1996b), but harvesting does not occur within our research sites.
Weevils affect the plant mainly by boring through the stem, consuming the cortex and thereby reducing plant buoyancy (Creed et al. 1992) and also reducing carbohydrate stores (Newman et al. 1996), which may reduce overwinter survival or competitive ability. Because milfoil declines have often occurred over winter and in early summer or have persisted over several years, it is likely that longer term effects, such as reduced overwinter survival or reduced competitive abilities are important to sustained control of Eurasian watermilfoil. Our observations suggest that plant community response, i.e., the ability of other species to occupy space left by damaged milfoil or the stress imposed by competition with other plants, is important to successful biological control (Newman et al. 1998).
Work on the operational use of Euhrychiopsis lecontei is continuing and incomplete. Although we know weevils can control milfoil, we are currently unable to predict when, where and by what means this will occur. More work is needed to determine what controls weevil populations and what role plant competition plays in successful control. More detail on the weevil and our work is available under the following topics:
Behavior and oviposition choice
Factors limiting populations
Factors influencing plant response to weevils