Biocontrol Research
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:
- Acentria ephemerella (Denis
& Schiffermüller) (= Acentria nivea [Olivier])
- a naturalized pyralid moth (Painter and McCabe 1988, Creed
and Sheldon 1994b)
-
- Cricotopus myriophylli Oliver
- an indigenous chironomid midge (MacRae et al. 1990, Kangasniemi
et al. 1993)
-
- Euhrychiopsis lecontei (Dietz)
(= Eubrychiopsis lecontei Leng, error)
- an indigenous weevil (Kangasniemi 1983, Creed and Sheldon
1993a, 1995), the most promising agent.
All three taxa are present in Minnesota and Wisconsin (click
for chart) (Newman and Maher 1995).
Acentria ephemerella
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.
Additional information
Cricotopus myriophylli
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.
Additional Information
Euhrychiopsis lecontei
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:
Life history
Behavior and oviposition
choice
Control potential
Factors limiting populations
Factors influencing plant response
to weevils
